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JP2007110535

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DESCRIPTION JP2007110535
A noise cancellation headphone has difficulty in digitizing to realize a response frequency of 1
kHz due to the delay of an ADC and a DAC. An object of the present invention is to realize a
response frequency of 1 kHz by using a 1-bit signal processing technique, automatically adjust
gain variations of a feedback control loop, and obtain an optimum noise reduction effect.
SOLUTION: In a noise cancellation headphone, a speaker 44 and a microphone 46 are provided
close to each other, and whenever switching to the noise cancellation mode, sound is output from
the speaker 44 by a signal corresponding to a deviation signal of a music signal and a feedback
signal from the microphone 46. The open loop gain of the feedback control circuit is set in
advance each time the noise cancellation switch 62 is turned on, and the feedback control circuit
configured to output and feed back voice and noise from the speaker 44 by the microphone 46.
The apparatus is provided with an installation time variation adjustment means for adjusting the
target value when the headphones are installed. [Selected figure] Figure 1
Noise cancellation headphone and its variation adjustment method
[0001]
The present invention relates to a noise cancellation headphone, and more particularly, to a noise
cancellation headphone capable of reproducing high-quality sound by increasing the processing
speed of a digital signal processing circuit constituting a control loop of noise cancellation, and
control thereof The present invention relates to a method of adjusting variation of loop gain.
[0002]
08-05-2019
1
In recent years, noise cancellation headphones capable of canceling surrounding noise and
listening to music and the like have been commercialized by various companies.
These noise cancellation headphones input ambient noise from the microphone and cancel them
in reverse phase to realize low noise. There are two major methods for canceling noise in reverse
phase, that is, a closed loop (for example, Patent Document 1) and an open loop (for example,
Patent Document 2).
[0003]
FIG. 10 shows an outline of a control configuration in the prior art for realizing noise cancellation
by a closed loop. Control of noise cancellation by a closed loop will be briefly described with
reference to FIG. In the following description, sound is included and described as voice.
[0004]
In the noise cancellation control, not only passive noise control using a sound absorbing material,
but also electrically control to generate noise from the sound source from the sound source to
cancel noise, active noise that directly cancels the noise I have control. As a result, active noise
control can cancel low frequencies that can not be canceled only by passive noise control. Active
noise control of the conventional noise reduction control will be described with reference to FIG.
10 below.
[0005]
In FIG. 10, reference numeral 1 denotes a control circuit unit of the noise cancellation
headphone. Reference numeral 2 denotes an audio reproduction unit (hereinafter referred to as a
headphone) of the noise cancellation headphone. An audio signal reproduction apparatus 3
outputs an audio signal such as music as an analog audio signal. In FIG. 10, a silicon player
(portable audio apparatus whose storage medium storing music data is a semiconductor
memory) is used. . The control circuit unit 1 and the headphones 2 may be integrally configured
or separately configured. In addition to the headphone type, the headphone 2 may be an
earphone type.
08-05-2019
2
[0006]
The analog audio signal input from the audio signal reproduction device 3 is audio signal
processed by the control circuit unit 1 and output to the headphone 2 to be reproduced as audio
S.
[0007]
The control circuit unit 1 includes a correction circuit 4, a signal amplifier 5, an analog filter 6, a
changeover switch 17, a headphone amplifier 7, a microphone amplifier 8, a noise cancel switch
9, a power switch 10, a battery 11 and the like.
The headphones 2 are composed of a speaker 13, a microphone 14, a headphone housing 12, a
band 16, and the like. In addition, 15 is drawn by simplifying the listener's ear.
[0008]
The output of the audio signal reproduction device 3 is connected to the input of the correction
circuit 4. The output of the correction circuit 4 is connected to the positive input terminal of the
signal amplifier 5 and to the b terminal of the changeover switch 17. The output of the signal
amplifier 5 is connected to the input of the analog filter 6. The output portion of the analog filter
6 is connected to the a terminal of the changeover switch 17. Further, the changeover switch 17
is a changeover switch in which the terminal a or the terminal b is switchably connected to the
terminal c, and the terminal c is connected to the input portion of the headphone amplifier 7.
Further, an output portion of the headphone amplifier 7 is connected to the speaker 13. The
speaker 13 and the microphone 14 are mounted closely in the headphone housing 12. When the
user wears the headphone 2, the microphone 14 collects both the sound S from the speaker 13
and the ambient noise N that has reached the microphone 14 through the headphone housing
12. Here, since the noise reaches the ear space in the headphone housing 12 through the
headphone housing 12, the noise on the outside of the headphone housing 12 is expressed as N ',
and the noise on the inside is expressed as N.
[0009]
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3
The correction circuit 4 corrects the audio signal input from the audio signal reproduction device
3 so that the frequency characteristic of the audio S output from the speaker 13 in the audible
frequency range has a substantially flat characteristic. It is a circuit. The signal amplifier 5 is a
deviation amplifier that amplifies the difference between the audio signal input from the
correction circuit 4 to the positive input terminal and the feedback signal input from the
microphone amplifier 8 to the negative input terminal. Also, the analog filter 6 is a band pass
filter in which the gain is increased in the frequency band where noise is desired to be reduced.
Also, the headphone amplifier 7 amplifies the power of the input signal and supplies it to the
speaker 13. The microphone 14 collects the sound S and the noise N output from the speaker 13,
converts the sound into an electric signal, and supplies the electric signal to the microphone
amplifier 8. The microphone amplifier 8 amplifies the signal from the microphone 14 and feeds it
back to the negative input terminal of the signal amplifier 5 as a feedback signal.
[0010]
As described above, the audio reproduction system including the control circuit unit 1, the
headphones 2, and the audio signal reproduction device 3 has the signal amplifier 5, the analog
filter 6, the headphone amplifier 7, the speaker when the changeover switch 17 is switched to
the terminal a. A microphone 14 and a microphone amplifier 8 constitute a feedback loop of
negative feedback. At this time, the sound reproduction mode is the noise cancellation control
mode. Also, when the changeover switch 17 is switched to the terminal b, the output of the
analog filter 6 is cut off and the feedback loop is released, and the output of the correction circuit
4 is switched to the headphone amplifier 7 via the terminals b and c of the changeover switch
17. Connected to the input. At this time, the sound reproduction mode is a normal sound
reproduction mode in which the noise cancellation control is not performed.
[0011]
Now, consider a feedback loop in the noise cancellation control mode. The control circuit
configuration of the audio reproduction system of FIG. 10 is as shown in FIG. 11 in a block
diagram using a transfer function.
[0012]
The forward loop gain from the output portion x of the correction circuit 4 (= the positive input
08-05-2019
4
terminal of the signal amplifier 5 and the music signal I is input) to the output portion of the
speaker 13 (= the audio S is output) is G1. , The backward loop gain from the input portion y of
the microphone 14 (= the ear space portion of the listener and mixed sound S and noise N is
input) to the negative input terminal of the signal amplifier 5 is G2. . Also, noise N is input as
disturbance from the input point z to the listener's ear space y.
[0013]
In FIG. 11, 101 indicates a forward loop gain G1, 102 indicates a backward loop gain G2, 103
indicates a feedback signal adder, and 104 indicates a noise N addition point. The sound S from
the speaker 13 is a mixture of the sound by the music signal I and the sound by the noise N.
Further, in the ear space portion y, the sound S and the noise N from the speaker 13 are mixed.
[0014]
The closed loop gain H1 having the output portion x of the correction circuit 4 as an input point
and the input portion y of the microphone 14 as an output point is represented by the following
equation as apparent from the control theory (* means multiplication. And so on).
[0015]
H1 = G1 / (1 + G1 * G2) (1)
[0016]
Further, a closed loop gain H2 having the input portion z of the noise N as an input point and the
input portion y of the microphone 14 as an output point is as follows.
[0017]
H2 = 1 / (1 + G1 * G2) (2)
[0018]
The sound of the synthetic ear space y corresponding to the input of the music signal I and the
input of the noise N is a combination of the speech S and the noise N, and is as follows.
08-05-2019
5
[0019]
S + N = H1 * I + H2 * N = {G1 / (1 + G1 * G2)} * I + {1 / (1 + G1 * G2)} * N (3)
[0020]
The first term of the equation (3) represents the sound by the music signal I, and the second term
represents the sound by the noise N.
It can be seen that the second term should be reduced when canceling noise.
[0021]
The product of the gain G1 and the gain G2, that is, when the open loop gain G1 * G2 becomes
very large with respect to 1, the closed loop gain H1 from the output x of the correction circuit 4
to the input y of the microphone 14 is (1) From the formula,
[0022]
H1→1/G2 ・・・・・・・・・・・・・・・・(4)
[0023]
And the closed loop gain H2 from the input portion z of the noise N to the input portion y of the
microphone 14 is
[0024]
H2→1/G1*G2→0 ・・・・・・・・・・・・・・・・(5)
[0025]
となる。
Accordingly, it can be understood from equations (3) and (5) that noise is reduced when the open
08-05-2019
6
loop gain G1 * G2 is increased in the block diagram of the transfer function of FIG.
[0026]
Also, at this time, it is well known that the stability of the feedback control loop can be
determined by the open loop gain G1 * G2, which can be briefly described as follows using FIG.
[0027]
FIG. 12A shows gain characteristics of the open loop gain G1 * G2.
As illustrated, the cutoff frequency of the low band is 20 Hz to 30 Hz, and the cutoff frequency of
the high band is 800 Hz to 2 kHz.
Further, (b) of FIG. 12 shows the phase characteristic of the open loop gain G1 * G2.
According to the servo control theory, in order for the feedback control loop to be stable, there
should be about 30 ° or more of phase margin at the frequency where the gain characteristic is
0 dB and about 10 dB or more at the frequency where the phase characteristic is 180 °. Is
necessary.
If the phase margin or gain margin is smaller than this range, an oscillation phenomenon occurs
to cause howling and an abnormal state occurs.
[0028]
The above gain G1 is performed by adjusting the gain of the analog filter 6.
The gain G1 should be large for noise reduction, but if it is large, the stability of the feedback
control loop is lost and an abnormal state occurs.
08-05-2019
7
Further, the variation of the gain G2 includes a variation factor due to the hardware
characteristics of the microphone 14 and the microphone amplifier 8, and a variation factor due
to the wearing state of the headphone of the listener.
Conventionally, in order to ensure stability, the gain G1 is set smaller than necessary by adjusting
the gain of the variable volume of the analog filter 6 to obtain sufficient gain margin and phase
margin, and the headphone wearing condition of the listener Thus, even if the gain G2 changes
significantly, oscillation does not occur.
JP-A-6-343195 JP-A-11-196488
[0029]
In the above-mentioned prior art, the gain G1 is set to be smaller than necessary by adjusting the
gain with the variable volume of the analog filter 6 so that the oscillation phenomenon does not
occur even if the gain G2 largely changes depending on the headphone wearing state of the
listener. I was trying to get a margin and a phase margin. Considering the equations (4) and (5) in
combination, it is necessary to set the gain G1 as large as possible in order to obtain a large
effect of noise reduction. Conventionally, since it is not possible to cope with the fluctuation of
the gain G2 due to the headphone wearing state of the listener, the gain G1 is set smaller than
necessary to obtain a sufficient gain margin and phase margin.
[0030]
According to the present invention, the gain G1 is set as large as possible within the range in
which stability can be secured, and is automatically set in a state accurately adjusted so that the
influence of the dispersion of the gains G1 and G2 and the dispersion of the gain G2 in the
headphone wearing state is reduced. However, it is difficult to realize this by the method of
adjusting the variable volume of the conventional analog filter 6 which is analogically realized.
Therefore, in the present invention, it is considered to digitize and realize the adjustment
mechanism of the gain G1.
[0031]
08-05-2019
8
By the way, in the section of the background art mentioned above, in order to cancel noise, the
prior art not only passive noise control using a sound absorbing material but also active noise
which directly cancels noise by generating signals of opposite phase to noise from a sound
source. Although it has been described that the control is performed, the active noise control
reduces noise less than about 1 kHz by active noise control because the delay of operation of a
speaker or the like can not obtain high response, and the noise above that is passive noise. It is
because it is trying to reduce by control. For example, the operation delay of the speaker 13 is
the largest, about 100 μsec, and the response frequency as the audio reproduction system is
about 1 kHz or less by adding the delay of the other components.
[0032]
When digitizing the adjustment mechanism of the gain G1 and digitizing PCM music data, an
analog / digital converter (hereinafter referred to as ADC) for converting an analog feedback
signal from the microphone 14 into a digital signal and a digital audio signal are processed. Then,
at least one digital / analog converter (hereinafter referred to as DAC) is required to convert the
analog signal into an audio signal and output it to the speaker 13. Therefore, considering the
delay time of the ADC and DAC, the ADC has a delay time of about 340 μsec and the DAC has a
delay time of about 340 μsec. Therefore, if an ADC or DAC is used, the response drops to about
100 Hz or less and can not be put to practical use.
[0033]
As described above, it is difficult in the prior art to realize a response frequency of 1 kHz by
digital control, which is also the reason why digital filters are not adopted in the prior art.
[0034]
In view of the above problems, the present invention uses digital control technology to realize a
response frequency of 1 kHz, automatically adjusts the variations of G1 and G2, and wears
headphones every time the listener operates in the noise cancellation control mode. It is an
object of the present invention to adjust the gain G1 automatically according to the state, operate
the control system stably without obtaining the gain margin and the phase margin more than
necessary, and obtain the optimum noise reduction effect.
[0035]
08-05-2019
9
The gist of the invention according to claim 1 of the present invention is that in a noise canceling
headphone capable of inputting a music signal and listening to a voice whose ambient noise has
been canceled, a speaker and a microphone are provided in proximity to each other, Every time
the mode is switched to the noise cancellation mode, the speaker outputs a voice according to a
signal corresponding to the deviation signal of the music signal and the feedback signal from the
microphone, and the microphone collects the voice and the noise from the speaker. A feedback
control circuit configured to feed back as a feedback signal, and at the time of installation,
adjusting the open loop gain of the feedback control circuit to a preset target value when
wearing headphones, each time switching to the noise cancellation mode Noise cancellation head
characterized by having variation adjustment means It resides in the O emissions.
Further, according to a second aspect of the present invention, there is provided a noise
cancellation headphone capable of inputting a music signal and listening to a voice in which
surrounding noise is canceled, wherein a speaker and a microphone are provided in proximity to
each other. Each time the mode is switched to the noise cancellation mode, the speaker outputs a
voice according to a signal corresponding to the deviation signal of the music signal and the
feedback signal from the microphone, and the microphone collects the voice and noise from the
speaker. A feedback control circuit configured to feed back as the feedback signal, and a gain
adjustment value that makes the open loop gain of the feedback control circuit to a preset target
value before shipment is determined and stored in a non-volatile memory It is stored in the nonvolatile memory every time the power is turned on. Power-on-gain setting means for setting the
open loop gain of the feedback control circuit by the in adjustment value and the open loop gain
of the feedback control circuit every time when switching to the noise cancellation mode The
noise cancellation headphone according to the present invention is characterized in that it
comprises a wearing time variation adjustment means for adjusting as follows.
Further, according to a third aspect of the present invention, in the feedback control circuit, a
filter is provided between the deviation signal input unit and the speaker, and the mounting time
variation adjustment unit is in the noise cancellation mode. Each time switching is performed, the
gain of the filter is adjusted, and the open loop gain of the feedback control circuit is adjusted to
be a preset target value. Noise cancellation in the headphones. Further, in the gist of the
invention according to claim 4 of the present invention, the mounting time variation adjusting
means opens the loop of the feedback control circuit every time switching to the noise
cancellation mode to form a variation adjustment open loop. The gain of the filter is measured so
that the switching means and the gain of the variation adjustment open loop with respect to the
sine wave signal of the specific frequency are measured, and the preset value of the variation
adjustment open loop open loop gain adjustment target value is obtained. The noise cancellation
headphone according to any one of claims 1 to 3, further comprising gain adjustment means.
08-05-2019
10
Further, according to a fifth aspect of the present invention, in the noise cancellation headphone
according to the fourth aspect, the frequency of the sine wave signal is a specific frequency in a
frequency band to be noise-cancelled. It exists. Also, the gist of the invention according to claim 6
of the present invention is that the music signal is a 1-bit signal, the feedback signal is a signal
obtained by converting an analog signal from the microphone into a 1-bit signal, and the
deviation signal A 1-bit signal obtained by digitally subtracting a 1-bit signal of the music signal
and a 1-bit signal of the feedback signal, and the filter is a digital filter by 1-bit signal processing.
It exists in the noise cancellation headphones as described in any one of Claim 5. Further,
according to a seventh aspect of the present invention, there is provided a power cancellation
switch which turns on and off a control power supply in a noise canceling headphone which can
receive a music signal and listen to a voice in which a surrounding noise is canceled. Shipping
adjustment switch turned on at the time of shipment adjustment, noise cancellation switch for
switching between noise cancellation mode and normal mode, sine wave signal of specific
frequency as adjustment signal at dispersion adjustment A variation adjustment signal generating
circuit to be output, a speaker for reproducing sound by a signal according to an output signal of
the digital filter, and the speaker provided close to the speaker, the sound reproduced from the
speaker and noise from the surroundings are collected A microphone that produces sound by
converting it into an electrical signal, and a signal from the microphone are converted into a 1-bit
signal to be fed back A feedback circuit for digitally converting the music signal converted into a
1-bit signal and the deviation signal of the feedback signal converted into a 1-bit signal digitally
and input as a 1-bit signal; Of the digital filter of 1-bit signal processing for passing the deviation
signal of the signal in the noise cancellation frequency band, a level detection circuit for
detecting the signal level of the digital filter, and a level detection value detected by the level
detection circuit. The level adjustment circuit which obtains and outputs the gain adjustment
value of the digital filter based on the open loop gain adjustment target value, and stores the gain
adjustment value determined by the level determination circuit according to the ON operation of
the shipping adjustment switch. And the non-volatile memory according to the ON operation of
the power switch. In addition to the adjustment of the first gain adjustment means according to
the first gain adjustment means for adjusting the gain of the digital filter according to the gain
adjustment value stored in the memory, and the ON operation of the noise cancel switch, The
noise cancellation headphone according to the present invention is characterized in that it
comprises a second gain adjustment means for adjusting the gain of the digital filter with the
gain adjustment value determined by the level determination circuit.
The gist of the invention according to claim 8 is the variation adjustment method of noisecanceling headphones in which a music signal can be input and a voice whose ambient noise is
canceled can be listened to by a speaker. The feedback signal from the microphone is fed back
with negative polarity to the music signal to obtain a deviation signal, and a feedback control
loop controlled by the deviation signal is formed, and the noise cancellation mode is switched
every time when switching to the noise cancellation mode. The variation adjustment method of
08-05-2019
11
noise cancellation headphones is characterized in that the open loop gain of the feedback control
loop is adjusted to be a preset target value at the time of wearing the headphones. The gist of the
invention according to claim 9 of the present invention is a variation adjustment method of
noise-canceling headphones in which a music signal can be input and a voice whose ambient
noise is canceled can be heard by a speaker. A feedback control loop that feeds back the
feedback signal from the microphone with negative polarity to the music signal, and the open
loop gain of the feedback control loop is preset at the time of shipping adjustment. A gain
adjustment value which becomes a target value is obtained and stored, and each time the power
is turned on, the open loop gain of the feedback control loop is set by the stored gain adjustment
value, and whenever switching to the noise cancellation mode, When wearing headphones with
preset open loop gain of the feedback control loop It consists in dispersion adjustment method of
noise cancellation headphones and adjusts so that the target value. The gist of the invention
according to claim 10 of the present invention is characterized in that the feedback control loop
is provided with a filter for inputting the deviation signal to pass a frequency of a specific
frequency band, and the filter is switched every time switching to the noise cancellation mode
10. The noise according to claim 8, wherein the open loop gain of the feedback control loop is
adjusted to be a preset target value at the time of wearing the headphone by adjusting the gain of
the feedback control loop. Cancel The method of adjusting the variation of headphones. Further,
the gist of the invention according to claim 11 of the present invention is such that the feedback
control loop is opened to form a variation adjustment open loop each time the mode is switched
to the noise cancellation mode, and the variation with respect to a sine wave signal of a specific
frequency is formed. The gain of the filter is measured so that the gain of the adjustment open
loop is measured, and the gain of the variation adjustment open loop becomes a preset open loop
gain adjustment target value of the variation adjustment open loop. 11. The method of adjusting
variation of noise-canceling headphones according to any one of claims 8 to 10, characterized in
that:
Also, in the gist of the invention according to claim 12 of the present invention, the frequency of
the sine wave signal is a specific frequency within a frequency band to be noise-cancelled,
according to claim 11, characterized in that It exists in the variation adjustment method. The gist
of the invention according to claim 13 of the present invention is that the music signal is a 1-bit
signal, and the feedback signal is a signal obtained by converting an analog signal from the
microphone into a 1-bit signal, and the deviation signal 11. The method according to claim 10,
wherein the filter is a digital filter obtained by digitally subtracting the 1-bit signal of the music
signal and the 1-bit signal of the feedback signal, and the filter is a digital filter by 1-bit signal
processing. A method of adjusting variation of noise cancellation headphones according to any
one of claims 12 to 13.
[0036]
08-05-2019
12
According to the present invention, it is possible to realize a response frequency of 1 kHz using
digital control technology, and to automatically adjust the variations of G1 and G2. Further,
according to the present invention, the variation adjustment is automatically executed each time
the listener operates in the noise cancellation control mode, and the gain G1 is automatically
adjusted so as to correct the gain G2 which changes according to the wearing state of the
headphones. Adjusted. As a result, the control system operates stably without obtaining gain
margin and phase margin more than necessary, and an optimum noise reduction effect can be
obtained.
[0037]
Next, the best mode for carrying out the present invention will be specifically described with
reference to the drawings.
[0038]
First Embodiment FIG. 1 shows a first embodiment of the control configuration of the noise
cancellation headphone according to the present invention.
[0039]
First, the control configuration of the noise cancellation headphone will be described.
[0040]
In FIG. 1, reference numeral 31 denotes a control circuit unit of the noise cancellation
headphone.
In the case of stereo sound, there are left and right control circuit units having the same
configuration as the control circuit unit 31, but only one of the left and right control circuits is
shown, and the other is omitted.
Reference numerals 64 and 65 denote earphone type sound reproduction units.
08-05-2019
13
64 and 65 are not limited to the earphone type, and may be a headphone type. The following 64
and 65 are called headphones. In addition, the headphone 64 and the control circuit unit 31 can
be integrally configured or configured separately.
[0041]
Reference numerals 32 and 33 denote silicon players, and the silicon player 32 outputs PCM
digital music signals. Also, the silicon player 33 outputs an analog music signal.
[0042]
34 is a PCM / 1 bit converter which takes in a PCM digital music signal from the silicon player
32 and converts it into a 1 bit signal. An analog / 1-bit converter 35 takes in an analog music
signal from the silicon player 33 and converts it into a 1-bit signal.
[0043]
A selector 36 selects and outputs one of the input signals. Although the select signal is not
shown, the select signal may be generated and switched according to the connection state of the
silicon players 32 and 33.
[0044]
Reference numerals 37 to 39 denote selectors, which select and output any of the input signals.
The select signal is output from the switching signal generation circuit.
[0045]
A correction circuit 40 corrects the 1-bit signal from the PCM / 1 bit converter 34 selected by
the selector 36 or the 1-bit signal from the analog / 1 bit converter 35 and outputs the sound
output from the speaker 13 It is a correction circuit provided so that the frequency characteristic
08-05-2019
14
in the audio frequency range becomes a desired characteristic. Since the frequency characteristic
does not become flat in the audible range due to the presence of the feedback loop control circuit
for noise cancellation described below, this is corrected by the correction circuit 40 to be made
flat in the audible range. The correction circuit 40 can also be used as an equalizer function, and
at that time, the correction circuit 40 can be adjusted so as to obtain the desired characteristics
of the listener.
[0046]
Reference numeral 41 denotes a digital filter, which replaces the analog filter 6 shown in FIG. 10
and is provided to reduce noise. The digital filter 41 can change the filter characteristics by
changing the filter coefficients. The digital filter 41 stores a default value α0 for determining the
gain of the digital filter 41. When the power is turned on, the filter coefficient is determined first
by the default value α0 and the filter coefficient register is set. ing. Further, the value set in the
filter coefficient register is updated by a gain adjustment value input from the outside. The
default value α 0 may be stored in the ROM provided in the digital filter 41 or set as hardware
(for example, setting a signal of 1 or 0 using the power supply voltage and the ground potential)
It is also good.
[0047]
Reference numeral 42 denotes a switching amplifier, which is provided in order to equalize the
crest values of 1-bit pulses and to perform power amplification for outputting sound by the
speaker 46.
[0048]
Reference numeral 43 denotes a low pass filter, which is provided to filter high frequency
components of a pulse train of 1-bit pulses whose crest values are equalized by the switching
amplifier 42.
The 1-bit signal that has passed through the low pass filter 43 becomes an analog signal
according to the density of the pulse train. Then, the analog music signal of the low pass filter 43
is supplied to the speaker 44 (45) to reproduce the sound. The switching amplifier 42 and the
low pass filter 43 constitute a 1-bit signal D / A conversion unit that converts a 1-bit signal into
an analog signal.
08-05-2019
15
[0049]
A microphone 46 (47) collects the sound signal from the speaker 44 (45) and the ambient noise
to convert it into an electric signal. Reference numerals 48 to 51 denote signal lines. Also, 66 and
67 represent the listener's ear. () The written speaker (45) and microphone (47) indicate the
other of left and right speakers 44 and microphone 46 in the case of stereo sound. The basic
control operations of the one and the other are the same, so the description of the other is
omitted.
[0050]
A microphone amplifier 52 amplifies and outputs the electric signal from the microphone 46.
Reference numeral 53 denotes a 1-bit signal AD converter which converts an analog signal from
the microphone amplifier 52 into a 1-bit digital signal.
[0051]
Reference numeral 54 denotes a digital adder which digitally subtracts the 1-bit signal of the
correction circuit 40 and the 1-bit signal fed back from the 1-bit signal AD converter 53 and
outputs a deviation signal of the 1-bit signal as the difference. .
[0052]
Reference numeral 55 denotes a level detector which digitally detects the level of the music
signal from the output of the digital filter 41.
The level detector 55 detects the signal level using an up / down counter. The operation of the
level detector 55 will be described later.
[0053]
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16
56 is a level determination circuit (at the time of shipment), and the output signal (level detection
value for variation adjustment) of the level detector 55 is input in signal level adjustment at the
time of shipment, and the gain adjustment value of the digital filter 41 to the EEPROM 58 Output
As will be described in detail later, the closed loop circuit formed when operating the noise
cancellation function is an open loop because the signal is blocked by the selector 38 when the
level adjustment for this variation adjustment is performed. This open loop is called the variation
adjustment open loop). An open loop gain adjustment target value A0 (specifically, for example,
15 dB) of the variation adjustment open loop formed at the time of shipment adjustment is
stored in the level determination circuit (at the time of shipment) 56.
[0054]
Reference numeral 58 denotes an EEPROM, which is a rewritable non-volatile memory. The
EEPROM 58 stores the gain adjustment value of the digital filter 41 sent from the level
determination circuit (at the time of shipment) 56 in the dispersion adjustment operation at the
time of shipment.
[0055]
Reference numeral 57 denotes a level determination circuit (when worn), which receives the
output signal (level detection value for variation adjustment) of the level detector 55 when the
listener wears headphones and operates in the noise cancellation control mode. Based on this
level detection value, the gain adjustment value of the digital filter 41 is obtained and output to
the selector 37. As will be described in detail later, as in the case of shipping adjustment, an open
loop for variation adjustment is formed. An open loop gain adjustment target value A1
(specifically, for example, 20 dB) of the variation adjustment open loop formed at the time of
attachment adjustment is stored in the level determination circuit (in shipping) 57. Reference
numeral 59 denotes a variation adjustment signal generation circuit that generates a variation
adjustment signal, and in this embodiment, a 200 Hz generation circuit that generates a 200 Hz
sine wave signal.
[0056]
Reference numeral 60 denotes a power switch for turning on and off the control power from the
battery 63. Further, reference numeral 61 denotes a shipping adjustment switch for adjustment
08-05-2019
17
at the time of shipment, and reference numeral 62 denotes a noise cancellation switch for
switching to the noise cancellation mode. A switching signal generation circuit 68 receives the
ON and OFF signals from the power switch 60, the factory adjustment switch 61, and the noise
cancellation switch 62 to switch the selectors 37 and 38, and writes the digital filter Generate a
signal.
[0057]
When the shipping adjustment switch 61 is turned on, the selector 37 allows output data of the
EEPROM 58 from the switching signal generation circuit 68 to pass while the shipping
adjustment switch 61 is turned on, and when the noise cancellation switch 62 is turned on, the
selector 37 The output data of the level determination circuit (attachment) 57 is allowed to pass
for a predetermined time from when the noise cancellation switch 62 is turned on.
[0058]
When the shipping adjustment switch 61 is turned on, the switching signal generation circuit 68
outputs a signal for switching the selector 38 to the 200 Hz generating circuit 59 while the
shipping adjustment switch 61 is on.
[0059]
Further, when the noise cancellation switch 62 is turned on, the switching signal generation
circuit 68 outputs a signal for switching the selector 38 to the 200 Hz generation circuit 59 for a
predetermined time from when the noise cancellation switch 62 is turned on.
When the selector 38 is switched to the 200 Hz generation circuit 59 side, it passes a 200 Hz
sine wave signal.
[0060]
In addition, when the noise cancellation switch 62 is turned on, the selector 39 selectively passes
the signal from the selector 38 while the noise cancellation switch 62 is turned on.
[0061]
The digital filter 41 updates the value of the register that determines the filter coefficient
according to the write signal from the switching signal generation circuit 68.
08-05-2019
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[0062]
As a result, the gain adjustment value at the time of product shipment or the gain of the digital
filter 41 is automatically updated whenever listening in the noise cancellation control mode, and
the gain of the digital filter 41 always becomes an optimal value.
[0063]
Next, the delay time of 1-bit signal processing will be described.
[0064]
One of the components of the feedback loop that operates digitally and has the largest operation
time delay is the 1-bit signal AD converter 53.
Therefore, considering a 1-bit signal generated by a ΔΣ modulator (see, for example, Japanese
Patent Application Laid-Open Nos. 2003-318665 and 2005-151589) as 1-bit signal generation,
the 1-bit signal AD converter 53 is representative. In the example, conversion is completed in 7
clocks.
At this time, if 2.8 MHz is adopted as the clock frequency of the ΔΣ modulator, the elapsed time
until the conversion is completed is
[0065]
Td = 7 * 1 / 2.8 MHz ≒ 2.5 μsec
[0066]
となる。
The digital filter 41, the selectors 38 and 39, and the switching amplifier 42 can be ignored with
respect to the delay of the 1-bit signal A / D converter 53.
08-05-2019
19
[0067]
Considering the longest delay time including the analog circuit in the control loop, as described
above, the speaker delay of 100 μsec is the largest.
The delay 2.5 μsec of the 1-bit signal AD converter 53 by the 2.8 MHz clock Δ2.8 modulator is
small enough to be ignored with respect to the delay 100 μsec of the speaker.
The delay time of the 1-bit signal D / A conversion unit configured by the switching amplifier 42
and the low pass filter 43 can be made smaller than the delay time of the 1-bit signal AD
converter 53.
Therefore, if a 1-bit signal AD converter 53 using a 2.8 MHz clock ΔΣ modulator is used, it is
possible to realize a response 1 kHz that is about the same as the response 1 kHz using the
conventional analog filter 6.
[0068]
Next, the operation of the level detector 55 will be described.
[0069]
FIG. 2 shows a configuration example of the level detection circuit 55.
In FIG. 2, reference numeral 71 denotes an up / down counter which counts 1-bit signals. The
clock signal clock is input to the CK terminal, and each time the clock signal clock is input, the 1bit signal connected to the input terminal of the U / D is counted and output to the output
terminal Q. A maximum value detection circuit 72 detects the maximum value of the output Q of
the up / down counter 71. The reference numeral 73 denotes a maximum value output unit,
which holds the value of the maximum value detection circuit 72 until the next maximum value
detection timing, and adjusts the level determination circuit (at shipment) 56 and the level
determination circuit (at installation) 57 for deviation adjustment Output as the level detection
08-05-2019
20
value of
[0070]
FIG. 3 is a diagram for explaining the operation of the level detection circuit 55. (A) of FIG. 3
shows a pulse train of a 1-bit signal input to the up / down counter 71. As shown in FIG. FIG. 3B
shows the reset signal reset input to the up / down counter 71. The reset signal reset is output
from the reset signal generator 74 to the RES terminal of the up / down counter 71 every 1024
clocks based on the clock signal clock. (C) of FIG. 3 shows the final count value for each sampling
period of the up / down counter 71. The solid line (signal a) in (d) of FIG. 3 virtually represents
the count value of the up / down counter 71 in an analog manner. The dotted line (signal b) in (d)
of FIG. 3 virtually represents the value represented by the 1-bit signal as an analog value.
[0071]
A 1-bit signal is represented by a signal level "1" and a signal level "0". However, as a
characteristic of a 1-bit signal (for example, a 1-bit signal by a ΔΣ modulator), when the signal
level is large The number of signal levels "1" is large corresponding to the level, and when the
level of the signal is small, the number of signal levels "0" is large corresponding to the level.
Therefore, by counting the 1-bit signal by the up / down counter 71, the signal A proportional to
the signal B is obtained, and the signal level of the 1-bit signal can be detected by detecting the
maximum value of this signal. In the example of FIG. 3, the final value 560 of the count value of
the up / down counter 71 shown in (c) is the maximum value for the analog signal of -6 dB
shown virtually as the signal B by the maximum value detection circuit 72. It is detected. If the
count value of the up / down counter 71 is associated with the signal level (dB) represented by
the 1-bit signal in advance, the output of the maximum value detection circuit 72 can be used as
a level detection value. It can be done.
[0072]
When detecting the level for variation adjustment, the selector 38 selects the 200 Hz sine wave
signal from the 200 Hz generation circuit, and the output of the digital filter 41 is detected by
the level detection circuit 55. The 200 Hz sine wave signal may be selected from a typical
frequency within the frequency band of 20 Hz to 30 Hz to 1 kHz to 2 kHz of the noise to be
noise-cancelled. As described later, any value in the range of 50 Hz to 500 Hz can be selected,
08-05-2019
21
but when it is selected in the range of 100 Hz to 300 Hz, adjustment becomes easy. In this
embodiment, it is 200 Hz.
[0073]
Next, gain adjustment of the digital filter 41 at the time of variation adjustment will be described.
The gain adjustment of the digital filter 41 includes dispersion adjustment at the time of
shipment and dispersion adjustment when the listener wears headphones and operates a noise
cancellation function. Dispersion adjustment at the time of shipment aims at adjusting the
dispersion of the control loop gain inherent to headphones, and the dispersion adjustment
performed in the state where the listener wears the headphones is the dispersion of control loop
gain due to the headphone wearing state of the listener Aim to adjust the
[0074]
First, variation adjustment at the time of shipment will be described.
[0075]
When making adjustments at the time of shipment, attach the headphones to a dummy doll
called HATS.
Then, the selector 38 selects the 200 Hz sine wave signal from the 200 Hz generation circuit 59.
The 200 Hz sine wave signal is input to the speaker 44 via the switching amplifier 42 and the
low pass filter 43 and reproduced as sound S. This sound S is collected by the microphone 46,
but noise N is also collected at the same time. The sound S and noise N collected by the
microphone 46 are converted into analog electric signals and output to the microphone amplifier
52. The signal amplified by the microphone amplifier 52 is converted into a 1-bit signal by a 1bit A / D converter 53, and is output to the adder 54. The adder 54 obtains the difference
between the 1-bit signal from the correction circuit 40 and the 1-bit signal from the 1-bit A / D
converter 53 and outputs the difference to the digital filter 41. When the adjustment at the time
of shipment is performed, the value of the 1-bit signal from the correction circuit 40 is in a nonsignal state in order to eliminate the influence of the music signal. As a result, a signal having a
value obtained by inverting the polarity of the 1-bit signal from the 1-bit A / D converter 53 is
output to the digital filter 41 as it is.
08-05-2019
22
[0076]
The digital filter 41 has band-pass filter characteristics centered on 200 Hz. Therefore, the low
frequency component and the high frequency component of the input signal are cut off and
output. Due to the band pass filter characteristic of the digital filter 41, the 200 Hz sine wave
signal is passed, and the noise signals on the low band side and the high band side are attenuated
and cut off. As described above, the digital filter 41 passes a 200 Hz sine wave signal and
attenuates noise signals in other bands. Therefore, when the signal level of the 200 Hz sine wave
signal is detected by the level detection circuit 55, the noise N The effect of reducing the
influence of
[0077]
The closed loop circuit formed when operating the noise cancellation function is an open loop
because the signal is blocked by the selector 38 when this level detection is performed. This open
loop is called the variation adjustment open loop.
[0078]
The level adjustment value B 0 for variation adjustment detected by the level detection circuit 55
is input to the level determination circuit (shipment) 56. The level determination circuit (at the
time of shipment) 56 determines that the product is defective if the value of the received
variation adjustment detection level B0 does not fall within the predetermined range.
[0079]
FIG. 4 shows an operation flow of level adjustment value determination in variation adjustment at
the time of shipment.
[0080]
First, in step S1, for example, the open loop gain adjustment target value of the variation
adjustment open loop at the time of shipment is stored in advance in the level determination
08-05-2019
23
circuit (at the time of shipment) 56 as A0 (specifically, for example, 15 dB).
Further, since the gain of the digital filter 41 for causing the open loop gain of the open loop for
variation adjustment to be the open loop gain adjustment target value A0 can be obtained in
advance as a design value, this design value is used as a default value α0 to digitally It is stored
in the filter 41.
[0081]
Since the use condition after shipment is unknown, the open loop gain adjustment target value
A0 of the variation adjustment open loop gain at the time of shipment is preferably set somewhat
lower than the gain of the stability limit from the viewpoint of howling prevention. Further,
setting the open loop gain adjustment target value A0 to 15 dB is an example, and this target
value changes depending on the configuration of the control circuit, and more specifically, the
gain margin is set to be within the stability limit.
[0082]
In step S2, the shipping adjustment switch 61 is turned on. Then, in step S3, the selector 38 that
has received the signal from the shipping adjustment switch 61 selects the signal from the 200
Hz signal generation circuit.
[0083]
The 200 Hz sine wave signal generated from the 200 Hz signal generation circuit goes around
the open loop for variation adjustment, and the signal level is detected by the level detection
circuit 55 in step S4. The detected signal level is set as a level adjustment value B0 for variation
adjustment (for example, it is assumed that 10 dB is detected). The level detection circuit 55
detecting the level by one round of the 200 Hz sine wave signal means that the gain of the
variation adjustment open loop is measured.
[0084]
08-05-2019
24
The level detection value B0 detected by the level detection circuit 55 is input to the level
determination circuit (factory default) 56, and in step S5, it is determined whether or not the size
B0 is within an allowable range (for example, 10 dB to 20 dB). It is judged. This determination is
performed by the level determination circuit (at the time of shipment) 56. When it is determined
that it is within the allowable range of 10 dB to 20 dB, a ratio A0 / B0 to the open loop gain
adjustment target value A0 (15 dB) is determined, and the level determining circuit (at the time
of shipment) 56 calculates this ratio A0 / B0. It is output to the EEPROM 58 as a gain adjustment
value.
[0085]
In step S7, the gain adjustment value A0 / B0 output from the level determination circuit (factory
default) 56 is stored in the EEPROM 58. For example, if B0 = 10 dB for A0 = 15 dB, then A0 / B0
= 1.5 is stored. The value stored in the EEPROM 58 is retained even when the power is turned
off, and is set in the register of the digital filter 41 as the gain adjustment value of the digital
filter 41 when the listener turns on the power after shipment. .
[0086]
If it is determined in step S5 that the level B0 detected by the level detection circuit 55 is not
within the range of 10 dB to 20 dB, it is regarded as a non-adjustable defective product. When it
is determined that the product is defective, a buzzer sounds to notify that it is defective.
Alternatively, it may be output to the outside that it has been determined as a defective product,
and may be automatically sorted as a defective product. The level adjustment at the time of
shipment in this way means that the open loop gain of the variation adjustment open loop is
made equal to the open loop gain adjustment target value A0.
[0087]
Next, variation adjustment when the listener wears headphones will be described.
[0088]
When the listener actually wears headphones, the relationship between the speaker 44, the
08-05-2019
25
microphone 46, and the listener's ear 66 differs depending on the individual, so the magnitude of
the open loop gain of the variation adjustment open loop differs.
Therefore, the open loop gain of the open loop for variation adjustment when the listener wears
headphones will vary around the open loop gain adjustment target value A0.
[0089]
The basic concept of level adjustment when the listener wears headphones is the same as the
level adjustment at the time of shipment, but the specific adjustment method is different.
[0090]
For example, A1 (specifically, for example, 20 dB) is stored in advance in the level determination
circuit (at the time of wearing) 57 as an open loop gain adjustment target value of the open loop
for variation adjustment at the time of wearing the headphone.
When the power switch 60 is turned on, the value of the filter coefficient of the digital filter 41
becomes a value determined by the default value α0. In response to the signal that the power
switch 60 is ON, the switching signal generation circuit 68 sends a signal for selecting the
EEPROM 58 to the selector 37 after a predetermined time. As a result, the gain adjustment value
A0 / B0 at the time of shipment stored in the EEPROM 58 is output to the digital filter 41, and
the gain of the digital filter 41 is updated by A0 / B0. From this state, the variation adjustment at
the time of wearing the headphones is performed.
[0091]
The variation adjustment starts when the listener wears the headphones when the noise
cancellation switch 62 is turned on. When the noise cancellation switch 62 is turned on, the
switching signal generation circuit 68 outputs a signal for selecting the 200 Hz generation circuit
59 by the selector 38 for a predetermined time.
[0092]
08-05-2019
26
Therefore, the 200 Hz sine wave signal from the 200 Hz generation circuit 59 is selected by the
selector 38 similarly to the shipping adjustment, but the timing at which the 200 Hz sine wave
signal from the 200 Hz generation circuit 59 is selected by the selector 38 is shipping
adjustment. However, it is limited for a predetermined time after the noise cancellation switch is
turned on. Then, for a predetermined time, an open loop for variation adjustment is formed and
gain adjustment of the digital filter is performed, and after this predetermined time has elapsed, a
feedback circuit of noise cancellation control is formed so that it can be heard in the noise
cancellation mode. Ru.
[0093]
While the variation adjustment open loop is formed, the 200 Hz sine wave signal from the 200
Hz generation circuit 59 is input to the speaker 44 through the switching amplifier 42 and the
low pass filter 43 and reproduced as the sound S. This sound S is collected by the microphone
46, but noise N is also collected at the same time. The voice S and the noise N are converted into
analog electric signals by the microphone 46 and output to the microphone amplifier 52. The
signal amplified by the microphone amplifier 52 is converted into a 1-bit signal by a 1-bit A / D
converter 53, and is output to the adder 54. The adder 54 obtains the difference between the 1bit signal from the correction circuit 40 and the 1-bit signal from the 1-bit A / D converter 53
and outputs the difference to the digital filter 41. As in the case of adjustment at the time of
shipment, the value of the 1-bit signal from the correction circuit 40 is in a non-signal state in
order to eliminate the influence of the music signal. As a result, a signal having a value obtained
by inverting the polarity of the 1-bit signal from the 1-bit A / D converter 53 is output to the
digital filter 41 as it is.
[0094]
The digital filter 41 filters and outputs the differential signal of the 1-bit signal according to its
characteristic. The output signal of the digital filter 41 is detected by the level detection circuit
55 as a level detection value B1. The level detection value B1 detected by the level detection
circuit 55 is input to a level determination circuit (when mounted) 57.
[0095]
08-05-2019
27
Since the open loop gain adjustment target value A1 of the variation adjustment open loop
formed at the time of attachment adjustment is stored in the level decision circuit (at attachment)
57, the level decision (at attachment) 57 is an open loop. A ratio A1 / B1 between the gain
adjustment target value A1 and the level detection value B1 detected by the level detection
circuit 55 is obtained and output to the selector 37 as a gain adjustment value. Therefore, the
value of the filter coefficient of the digital filter 41 is updated by the gain adjustment value A1 /
B1. After all, the final gain of the digital filter 41 in the noise cancellation mode is α0 * (A0 / B0)
* (A1 / B1).
[0096]
Hereinafter, with reference to the flowchart of FIG. 5, the operation of adjusting the variation
when the listener wears headphones will be described.
[0097]
First, in step S11, the level determination circuit (at the time of mounting) 57 stores in advance
the open loop gain adjustment target value A1 of the variation adjustment open loop.
For example, specifically, for example, 20 dB is stored. Note that setting the open loop gain
adjustment target value A1 to 20 dB is an example, and it can be selected arbitrarily within a
range where the gain margin is within the stability limit.
[0098]
Next, in step S12, the power switch 60 is turned on. When the power switch 60 is turned on, the
process proceeds to step S13, and the digital filter 41 is set to a default value α0 as a gain
setting value. Further, in response to the ON signal of the power switch 60, the switching signal
generation circuit 68 sends a signal for selecting the EEPROM 58 to the selector 37 after a
predetermined time. As a result, the factory-set level adjustment value A0 / B0 stored in the
EEPROM 58 is output to the digital filter 41, and the gain of the digital filter 41 is updated by A0
/ B0. At this time, the gain of the digital filter 41 is updated to A0 / B0 times α0.
[0099]
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28
In step S14, headphones are worn by the listener. Since the level adjustment in the noise
cancellation mode is to correct the variation due to the wearing state of the listener, the level
adjustment is always performed in the state where the headphones are attached.
[0100]
When the noise cancellation switch 62 is turned on in step S15, the selector 38 which receives
the signal from the noise cancellation switch 62 selects the signal from the 200 Hz signal
generation circuit in step S16.
[0101]
In step S17, the level detection circuit 55 detects the level detection value B1 of the 200 Hz sine
wave signal that has made one round of the variation adjustment open loop.
The level detection value B1 detected by the level detection circuit 55 is input to a level
determination circuit (when mounted) 57.
[0102]
In step S18, a ratio A1 / B1 to the open loop gain adjustment target value A1 (20 dB) stored in
advance is determined, and this ratio A1 / B1 is used as a gain adjustment value to determine the
level from the circuit (when mounted) 57 to the selector 37 Output to
[0103]
When the noise cancellation switch 62 is turned on, the switching signal generation circuit 68
outputs a signal for selecting the level determination circuit (when mounted) 57 to the selector
37 for a predetermined time in step S19.
During this time, the gain adjustment value A1 / B1 output from the level determination circuit
(when mounted) 57 is input to the digital filter 41 via the selector 37. The digital filter 41
updates the gain of the digital filter 41 based on the input gain adjustment value A1 / B1. At this
time, the gain of the digital filter 41 is α0 * A0 / B0 * A1 / B1. As a result, the open loop gain of
08-05-2019
29
the variation adjustment open loop becomes A1 of the open loop gain adjustment target value.
[0104]
In step S20, a noise cancellation control loop is formed, and the listener can hear the noisesuppressed voice in the noise cancellation mode.
[0105]
The timing at which the noise cancellation control loop is formed can be a predetermined time
after the above process is completed after the switch 62 is turned on and the open loop gain
adjustment target value A1 is set, but the open loop gain adjustment target is The processing
until the value A1 is completed is completed in a short time.
[0106]
Here, the listener will hear a 200 Hz voice during this adjustment.
This 200 Hz sound can be heard while the noise cancellation mode is set and the variation is
being adjusted.
Therefore, this 200 Hz voice can be used as a beep to notify the listener that the noise
cancellation mode has been entered and that the variation adjustment has been started. In order
to make the listener effectively recognize this beep sound, it is preferable to set it for a long time,
for example, about 200 ms. This time may be set to any value, not limited to 200 ms, as a time
for the listener to effectively recognize the beep sound.
[0107]
Second Embodiment Next, the characteristic setting of the digital filter 14 capable of effectively
performing level detection in the above-mentioned variation adjustment will be described.
[0108]
When adjusting the variation, the characteristic of the digital filter 14 may be the characteristic
of the digital filter 14 at the time of listening, but since the microphone 46 also collects the noise
N, the noise detected to the value detected by the level detection circuit 55 The influence of
08-05-2019
30
[0109]
Therefore, in the present embodiment, when performing level adjustment of variation adjustment
using a specific frequency, for example, a 200 Hz sine wave signal, the characteristic of the
digital filter 14 is modified so as to reduce the influence of noise.
[0110]
FIG. 6 shows the open loop gain characteristics when the characteristics of the digital filter 14
are modified to change the open loop gain characteristics of the feedback loop in the noise
cancellation mode and the open loop characteristics when performing variation adjustment.
There is.
[0111]
In FIG. 6, 201 is an open loop gain characteristic of the feedback loop when listening in the noise
cancellation mode.
Reference numeral 202 denotes an open loop gain characteristic of the variation adjustment
open loop when the level for variation adjustment is detected.
[0112]
The open loop gain characteristic 201 of the feedback loop when listening in the noise
cancellation mode has the characteristic of a band pass filter that matches the frequency band to
be reduced in noise.
In the present embodiment, the cutoff frequency of the low band is 20 Hz to 30 Hz, and the
cutoff frequency of the high band is 800 Hz to 2 kHz.
On the other hand, the open loop gain characteristic 202 of the open loop for variation
adjustment when detecting the level for variation adjustment has a width between the low
frequency side and the high frequency side cut-off frequency as shown in the figure. It is
08-05-2019
31
narrowed centering on 200Hz which is the adjustment frequency.
[0113]
With such characteristics, the detection signal of the level detection circuit can be detected as a
value with less influence of noise on the low frequency side and the high frequency side with the
adjustment frequency of 200 Hz as the center.
[0114]
In order to modify the characteristics of the digital filter 14, the filter coefficient for the open
loop gain characteristic 201 of the feedback loop when listening in the noise cancellation mode
and the variation adjustment opening when the level for variation adjustment is detected Both
filter coefficients for the open loop gain characteristic 202 of the loop are stored in the digital
filter 14 and selected using, for example, the switching signal to the selector 37 and the OR
signal of the ON / OFF signal of the adjustment switch 61 at shipment. Just do it.
An example of a time sequence of this switching signal is shown in FIG. 7 and FIG.
[0115]
FIG. 7 shows the modified state of the characteristics of the digital filter 14 in the adjustment of
variations at the time of shipment.
Here, the open loop gain characteristic 201 of the feedback loop when listening in the noise
cancellation mode is a wide band, and the open loop gain characteristic 202 for variation
adjustment when the level for variation adjustment is detected is a narrow band. I will call it.
[0116]
The signals shown in FIG. 7 are, from top to bottom, ON and OFF signals of the power switch 60,
ON and OFF signals of the adjustment switch 61 at shipment, ON and OFF signals of the noise
cancel switch 62, and select signals of the selectors 37, 38 and 39, The OR signal of the selector
37 and the factory adjustment switch 61, the gain adjustment value write signal WR1 to the
EEPROM 58, the gain adjustment value write signal WR2 to the digital filter 41, and the gain
08-05-2019
32
state of the digital filter 41 are shown.
[0117]
The gain adjustment value write signal WR1 to the EEPROM 58 is output from the switching
signal generation circuit 68 to the EEPROM 58 when a predetermined time .tau.0 elapses from
the time the adjustment switch 61 is turned on at the time of shipment. The value is written
[0118]
The characteristics of the digital filter 41 are switched by the OR signal of the selector 37 and
the factory adjustment switch 61. FIG. 7 shows that the factory adjustment switch 61 is wide
band before it is turned on and narrow band when it is on. ing.
When the power switch is turned on, the gain state of the digital switch 41 becomes α0.
The register of the filter coefficient of the digital filter 41 is updated every operation clock (the
operation clock frequency is, for example, 2.8 MHz), so when the power is turned on, the register
value of the filter coefficient is updated to obtain the gain. Is set to be the default α0.
[0119]
The select signals of the selectors 37, 38 and 39 shown in FIG. 7, the OR signal of the selector 37
and the factory adjustment switch 61, the gain adjustment value write signal WR1 to the
EEPROM 58, and the gain adjustment value write signal WR2 to the digital filter 41 are The
switching signal generation circuit 68 generates the signal based on the ON / OFF signal of the
power switch 60, the ON / OFF signal of the adjustment switch 61 at shipment, and the ON / OFF
signal of the noise cancellation switch 62.
[0120]
FIG. 8 shows the modified state of the characteristics of the digital filter 14 in adjusting the
variation when the headphones are worn.
08-05-2019
33
The signals in FIG. 8 are the same as in FIG. When the power switch is turned on, the gain state
of the digital switch 41 becomes α0 which is the default. That is, since the register of the filter
coefficient of the digital filter 41 is updated every operation clock, the register value of the filter
coefficient is updated when the power is turned on, and the gain first becomes the default α0. Is
set as
[0121]
Next, when a predetermined time τ1 has elapsed from the power supply switch 60 being turned
on, a gain adjustment value write signal WR2 to the digital filter 41 is output. At this time, since
the selector 37 selects the EEPROM 58, the factory-set level adjustment value stored in the
EEPROM 58 is written. Thus, the digital filter 41 updates the gain from the default value α0 to
A0 / B0 times. At this time, the characteristic of the digital filter 41 is wide band.
[0122]
Next, when the noise cancel switch 62 is turned on, the gain adjustment value write signal WR2
to the digital filter 41 is output after the predetermined time τ2 has elapsed. At this time, since
the selector 37 selects the level determining circuit (at the time of mounting) 57, the gain
adjustment value A1 / B1 from the level determining circuit (at the time of mounting) 57 is
written to the digital filter 41 via the selector 37. As a result, the digital filter 41 updates the gain
so that the previously updated α0 * A0 / B0 is further multiplied by A1 / B1 to be α0 * A0 / B0
* A1 / B1. At this time, the characteristic of the digital filter 41 is narrow band since the variation
adjustment using the 200 Hz sine wave signal is being performed. Since the characteristic of the
digital filter 41 is an OR signal of the selector 37 and the switch 61 at the time of shipment,
when the selector 37 becomes 0 after the predetermined time τ 3, switching to the wide band is
performed.
[0123]
Here, the predetermined time τ3 is a period in which the above-mentioned beep sound can be
heard by the listener. In order to make the listener effectively recognize this beep sound, it has
already been mentioned that it should be performed for a relatively long time, for example, about
200 ms.
08-05-2019
34
[0124]
As described above, the characteristic of the digital filter 41 switches to the narrow band during
variation adjustment using the 200 Hz sine wave signal, and becomes the wide band at the time
of normal listening and in the noise cancellation mode. As described above, when listening in the
noise cancellation mode, the characteristics of the digital filter 41 having the bandwidth desired
to be noise-cancelled can be obtained, and the characteristics of narrower bandwidth than usual
can be obtained during the dispersion adjustment using the 200 Hz sine wave signal. Therefore,
noise can be effectively reduced when listening in the noise cancellation mode, and level
detection can be performed without being affected by noise during the adjustment of variation
using a 200 Hz sine wave signal.
[0125]
In FIGS. 7 and 8, the characteristics of the digital filter 41 are the same as those of the first
embodiment unless they are switched to the narrow band.
[0126]
FIG. 9 shows an example of data obtained by measuring the open loop gain characteristics of the
variation adjustment open loop.
The low frequency cutoff frequency is 10 Hz, and the high frequency cutoff frequency is 1 kHz.
また、100Hz∼200Hzで20dBのゲインとなっている。 This characteristic is an
example, and causes variations due to various factors. As a result of analyzing a large amount of
data, it has been found that the frequency range of 50 Hz to 500 Hz is preferable in order to
detect the level detection values B0 and B1 accurately by the level detection circuit 55 when
adjusting the variation. It has been found that detection can be performed more accurately if the
frequency range of 100 Hz to 300 Hz can be obtained as a large value with little variation in
gain. In the present invention, as the above embodiment, the signal of the 200 Hz generation
circuit is generated as 200 Hz. Of course, it is possible to adopt one in the range of 50 Hz to 500
Hz as this frequency, and in order to detect more accurately, it may be 100 Hz to 300 Hz.
[0127]
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Although the digital filter 41 has a band pass characteristic in the description of the above
embodiment, the digital filter 41 may have a low pass characteristic or a high pass characteristic.
That is, since the speaker 44 and the microphone 46 have a high pass characteristic and the low
pass filter 43 is provided as an element in the feedback loop, the digital filter 41 can be a low
pass characteristic or a high pass by using these characteristics. A band pass characteristic can
be obtained as a total as a characteristic. In this case, by adjusting the gain characteristics and
the phase characteristics of the digital filter 41 having low pass characteristics or high pass
characteristics, the control system is stabilized as in the above embodiment. Although the present
invention has been described above by the specific embodiment, it goes without saying that the
present invention is not limited to the above embodiment, and can be modified and implemented
without departing from the scope of the present invention.
[0128]
The present invention can be widely used not only for noise cancellation headphones but also for
devices intended to suppress noise. For example, the present invention can be applied to a device
for noise control such as an expressway.
[0129]
FIG. 2 is a control circuit diagram showing a first embodiment of a noise cancellation headphone
according to the present invention. It is a detailed block diagram of the level detection circuit of a
1st embodiment by the present invention. It is operation | movement explanatory drawing of the
level detection circuit of 1st Embodiment by this invention. It is a flowchart of dispersion
adjustment at the time of shipment of a 1st embodiment by the present invention. It is a
flowchart of the dispersion | variation adjustment at the time of the headphones mounting of 1st
Embodiment by this invention. It is a figure which shows an example of the modification of the
digital filter at the time of dispersion adjustment of 2nd Embodiment by this invention. The
timing chart at the time of dispersion adjustment at the time of shipment of a 2nd embodiment
by the present invention is shown. The timing chart at the time of dispersion adjustment at the
time of the headphones wearing of a 2nd embodiment by the present invention is shown. An
example of the gain characteristic of the variation adjustment open loop according to the present
invention is shown. It is a figure which shows the example of the control circuit structure of a
noise cancellation headphone by a prior art. 11 is a block diagram of a transfer function of the
prior art control circuit of FIG. It is a figure explaining stability of feedback control of a noise
cancellation headphone.
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36
Explanation of sign
[0130]
1, 31 ... Control circuit unit 2, 64, 65 ... Headphones (audio reproduction unit) 3, 32, 33 ... Silicon
player (audio signal reproduction device) 4 ... Correction circuit 5 ... Signal Amplifier 6: Analog
filter 7. Headphone amplifier 8, 52: Microphone amplifier 9, 62: Noise cancel switch 10: Power
switch 11, 63: Battery 12: headphone housing 13 , 44, 45: Speaker 14, 46, 47: Microphone 15,
66, 67: Ear 16: Band 17: Switch 34: PCM / 1 bit converter 35: Analog / 1 bit converter 36 to 39
... selector 40 ... correction circuit 41 ... digital filter 42 ... switching amplifier 43 ... low pass filter
48 ~ DESCRIPTION OF SYMBOLS 1 ... Signal line 53 ... 1 bit signal AD converter 54 ... Digital
adder 55 ... Level detector 56 ... Level determination circuit (at the time of shipment) 57 ... Level
determination circuit (attachment At the time) 58 ... EEPROM (nonvolatile memory) 59 ... 200 Hz
generation circuit (variation adjustment signal generation circuit) 60 ... power supply switch 61 ...
shipment adjustment switch 68 ... switching signal generation circuit 71 ... Up / down counter 72
... Maximum value detection circuit 73 ... Maximum value output section 74 ... Reset signal
generator 101 ... Forward loop gain G1 102 ... Backward loop gain G2 103 ... Feedback signal
addition unit 104 ... addition point
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