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JP2002291088

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DESCRIPTION JP2002291088
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
audio output circuit used when a speaker is shared by an audio device and another audio output
device.
[0002]
2. Description of the Related Art In recent years, for example, in a car, one of a plurality of
speakers provided for a car audio system is also used to reproduce an audio signal from a car
accessory such as a car navigation system. An example of the configuration of the audio output
circuit for sharing the speaker in this way is shown in FIG.
[0003]
In FIG. 2, in the on-vehicle audio unit 2 to which power is supplied from the on-vehicle battery 1,
it is general to use the BTL power amplifier IC 3 in order to meet the requirements for large
output, high efficiency, miniaturization and the like. (Note that although four channels are
normally used for the BTL power amplifier IC 3, only one channel is shown in the figure).
Similarly, in the car navigation device 4 supplied with power from the battery 1, SEPP (Single
Ended Push) can be directly connected to the speaker 5, which is a low impedance load, without
passing through the output transformer for amplification of the audio signal. It is common to use
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a power amplifier 6 comprising a (Pull) circuit. However, when using this type of power amplifier
6, it is necessary to cut the DC component, so an output capacitor 7 is interposed between the
power amplifier 6 and one of the audio signal output terminals 4 a of the car navigation device 4.
ing. The other audio signal output terminal 4b of the car navigation device 4 is connected to the
ground terminal.
[0004]
Further, two relay switches 8 and 9 are provided to switch the pair of audio signal input
terminals 5a and 5b of the speaker 5 to the on-vehicle audio unit 2 side or the car navigation
device 4 side. In these relay switches 8 and 9, the common contacts c1 and c2 are connected to
the audio signal input terminals 5a and 5b, respectively, and one movable contact b1 and b2 are
respectively connected to the pair of output terminals 2a and 2b of the car audio unit 2. The
other movable contacts a1 and a2 are connected to the pair of output terminals 4a and 4b of the
car navigation device 4, respectively. Resistors 10 and 11 are respectively connected between
the movable contacts a1 and a2 of the relay switches 8 and 9 and between b1 and b2.
[0005]
In the above-described conventional audio output circuit, since the switching operation of the
audio signal is performed by the relay switches 8 and 9 having mechanical contacts, the product
characteristics are uneven. Then, it can not be denied that the timing of the contact switching
operation of the relay switches 8 and 9 may be shifted in time. In this case, no problem occurs in
the state in which the contact switching operation of the relay switches 8 and 9 is simultaneously
performed, but if the switching operation is temporally deviated, problems as described below
occur. That is, from the state in which the relay switches 8 and 9 are on between the contacts
(c1-b1) and (c2-b2), that is, from the state where the audio signal from the on-vehicle audio unit
2 is given to the speaker 5, When switching between contacts (c1-a1) and (c2-a2) to the ON
state, relay switch 9 is the first contact while relay switch 8 is still ON between contacts (c1-b1).
When the state between (c2-a2) is switched on, a bias output voltage is applied from the BTL
power amplifier IC3 to the output terminal 2a, and the output terminal 4b of the car navigation
device 4 is at the ground potential. Due to the level relationship, a relatively large short circuit
current flows through the speaker 5. For this reason, there arises a problem that a relatively loud
noise (so-called "pop noise") is generated due to this. Such a problem is not only unpleasant for
car users, but also impairs the luxury of the on-vehicle audio unit 2 and the car navigation
apparatus 4, so it is desirable to suppress it as much as possible.
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2
[0006]
The present invention has been made in view of the above circumstances, and its object is to
make the noise sound generated when switching the sound source of the speaker extremely
simple when the speaker is shared by the audio device and another audio output device. An
object of the present invention is to provide an audio output circuit that can be effectively
suppressed by the configuration.
[0007]
The means described in claim 1 can be employed to achieve the above object.
According to this means, an output capacitor for DC component cutting necessary for an audio
driver circuit on another audio output device side sharing an audio device and a speaker, a third
for outputting an audio signal from the audio driver circuit, and A configuration in which, among
the fourth output terminals, it is interposed between the fourth output terminal conventionally
connected to the ground terminal and the ground terminal, and a resistor is connected between
the third and fourth output terminals. In the state where the third and fourth output terminals
are separated from the audio signal input terminal of the speaker, the potential level of the fourth
output terminal is equal to the level corresponding to the output bias voltage of the audio driver
circuit. It will be lifted. Therefore, the potential difference between the fourth output terminal and
the first and second output terminals for outputting the audio signal from the BTL circuit on the
audio device side is the conventional configuration (the fourth output terminal is grounded
Smaller than the connected configuration).
[0008]
For this reason, when the first output terminal and the fourth output terminal are connected via
the speaker due to a temporal shift in the timing of the contact switching operation of the two
relay switches. However, since a large short-circuit current as in the prior art does not flow
through the speaker, it is possible to effectively suppress the occurrence of a loud noise (socalled "pop noise"). In this case, since the configuration is extremely simple to change the
connection position of the output capacitor, the cost does not increase.
[0009]
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3
BEST MODE FOR CARRYING OUT THE INVENTION An embodiment in which the present
invention is applied to an on-vehicle audio output circuit will be described below with reference
to FIG. In FIG. 1, an on-vehicle audio unit 22 supplied with power from an on-vehicle battery 21
is a BTL (Balanced Transformer) for amplifying an audio signal from an audio device (not shown)
such as a CD player, an MD player, or an AM / FM tuner. -Less) A power amplifier IC23
(corresponding to the BTL circuit in the present invention) is provided. The BTL power amplifier
IC 23 is a four-channel one for driving, for example, four sets of speakers 24 (only one set is
shown) installed in a car. Here, a car navigation device 25 (others described later) is used. Only
one channel shared with the audio output device is shown.
[0010]
This BTL power amplifier IC 23 is a well-known device including, for each channel, two single
ended push pull (SEPP) circuits 23a and 23b and a phase inverting circuit 23c for driving the
SEPP circuits 23a and 23b in reverse phase. The output terminals of the SEPP circuits 23a and
23b are connected to a pair of output terminals 22a and 22b (corresponding to a first output
terminal and a second output terminal) of the in-vehicle audio unit 22, respectively. In this case,
the output bias voltage of the SEPP circuits 23a and 23b is generally set to about 1/2 of the
output voltage B of the battery 21.
[0011]
The car navigation device 25 is also supplied with power from the battery 21, and incorporates a
pair of output terminals 25a and 25b (corresponding to a third output terminal and a fourth
output terminal) for giving an audio signal to the speaker 24. ing. The car navigation apparatus
25 is provided with a power amplifier 26 (corresponding to an audio drive circuit) including an
SEPP circuit for amplifying the audio signal. However, when this type of power amplifier 26 is
used, it is necessary to cut the DC component unlike the BTL circuit. Therefore, in the present
embodiment, the output capacitor 27 is interposed between one output terminal 25b of the car
navigation device 25 and the ground terminal, and the output end of the power amplifier 26 is of
the car navigation device 25. It is directly connected to the other output terminal 25a. Further,
the output bias voltage of the power amplifier 26 is also set to be about 1/2 of the output voltage
B of the battery 21 like the SEPP circuits 23a and 23b in the BTL power amplifier IC 23.
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[0012]
Furthermore, in the car navigation system 25, two relay switches 28 and 29 are provided to
switch the pair of audio signal input terminals 24a and 24b of the speaker 24 to the on-vehicle
audio unit 22 side or the car navigation system 25 side. Provided. In these relay switches 28 and
29, the common contacts c1 and c2 are connected to the audio signal input terminals 24a and
24b of the speaker 24, respectively, and one movable contact b1 and b2 are a pair of output
terminals 22a of the on-vehicle audio unit 22 The other movable contacts a1 and a2 are
connected to the pair of output terminals 25a and 25b of the car navigation device 25,
respectively.
[0013]
The switching operation of the relay switches 28 and 29 is performed by a command from a
control circuit (not shown) in the car navigation apparatus 25. In normal operation, between the
contacts (c1-b1) and (c2-b2) Is switched to the first state of turning on, but only when the car
navigation device 25 outputs voice guidance (for voice navigation, guidance of operation method,
etc.) between the contacts (c1-a1), (C2-a2) is switched to the second state where it is turned on.
Further, between the movable contacts a1 and a2 of the relay switches 28 and 29 and between
b1 and b2, the resistors 30 and 31 for suppressing noise generation are respectively connected,
but the resistor 31 on the BTL power amplifier IC 23 side is It is not necessary to provide in
particular.
[0014]
According to the circuit configuration of the above-described embodiment, the following actions
and effects can be obtained. That is, the audio signal input terminals 24a and 24b of the speaker
24 are the output terminals 22a and 22b of the in-vehicle audio unit 22 when the contact points
(c1-b1) of the relay switches 28 and 29 and (c2-b2) are on. The speaker 24 is driven by the audio
signal from the BTL power amplifier IC 23 to be in the first state connected to each. In addition,
when the contact (c1-a1) of the relay switches 28 and 29 and (c2-a2) are turned on from this
first state, the audio signal input terminals 24a and 24b of the speaker 24 are in the car
navigation system 25. The speaker 24 is driven by the audio signal from the power amplifier 26
because it is switched to the second state connected to each of the output terminals 25a and 25b.
Incidentally, even when the audio signal from the BTL power amplifier IC 23 is not supplied to
the speaker 24 by such switching operation, the audio signal is continuously supplied to the
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other speakers (not shown). There is no interruption in the playback sound by.
[0015]
Here, in the present embodiment, the output capacitor 27 for DC component cutting necessary
for the power amplifier 26 on the car navigation device 27 b side is connected to the output
terminal 25 b of the car navigation device 25 (conventionally to the ground terminal In addition
to being interposed between the output terminal 25b) and the ground terminal, the resistor 30 is
connected between the output terminal 25b and the output terminal 25a corresponding to the
output terminal of the power amplifier 26, The potential level of the output terminal 25 b
corresponds to the output bias voltage of the power amplifier 26 in a state in which the contact
points (c1-b1) and (c2-b2) of the relay switches 28 and 29 are turned on. (Where B is the output
voltage of the battery). Further, the potential level of the output terminal 22a on the side of the
car audio unit 22 is also B / 2 which corresponds to the output bias voltage of the SEPP circuit
23a in the BTL power amplifier IC23.
[0016]
Therefore, when switching the relay switches 28 and 29 from the on state between the contacts
(c1-b1) and (c2-b2) to the on state between the contacts (c1-a1) and (c2-a2), A state in which the
relay switch 29 first turns on the contacts (c2-a2) while the relay switch 28 is still on between
the contacts (c1-b1) due to the characteristic variation of the relay switches 28 and 29 (Ie, when
the output terminals 22a and 25b are connected via the speaker 24), the potential difference
between the output terminals 22a and 25b is reduced. It is possible to prevent such a situation
that a large short circuit current flows and a large noise noise (so-called "pop noise") is
generated. Therefore, as in the conventional configuration, there is no risk of giving a feeling of
discomfort to a car user or impairing the sense of quality of the on-vehicle audio unit 22 and the
car navigation device 25. Moreover, since the above-described effects can be obtained by a very
simple configuration that only changes the connection position of the output capacitor 27, there
is no increase in cost.
[0017]
The present invention is not limited to the above-described embodiment, and the following
modifications or extensions are possible. Car navigation devices were mentioned as an example
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of audio output devices other than audio devices, but Mayday system or vehicle information
(information showing replacement time of engine oil and oil filter, information showing
abnormality of vehicle, etc.) Audio output devices for such systems can also be targeted. Of
course, the present invention can be applied not only to audio output circuits for vehiclemounted audio devices and other audio output devices, but also to audio output circuits for home
or business audio devices and other audio output devices. Although the power amplifier 26
including the SEPP circuit has been described as an example of the audio drive circuit, the
present invention can be applied to any power amplifier that requires an output capacitor for
cutting a DC component.
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