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Патент USA US3026767

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March 27, 1962
R. J.
ZIEHLKE
3,026,757
TRANSIENT FILTER
Filed Nov. 24, 1958
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March 27, 1962
R. J. ZIEHLKE
3,026,757
TRANSIENT FILTER
Filed Nov'. 24, 1958
2 Sheets-Sheet 2
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INVENTOR.
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Unite States Patent
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1
3,026,757
Patented Mar. 27, 1962
2
3,026,757
control circuit having a manually operable control, actua- .
7 tion of which produces an objectionable transient into
Robert J. Ziehlke, Janesville, Wis, assignor to Gibbs
the tone representing signal, and a selectively operable
tremolo system including a variable impedance device
TRANSIENT FILTER
Manufacturing & Research Corporation, a corporation
‘of Wisconsin
Filed Nov. 24, 1958, Ser. No. 775,893
1 Claim. (Cl. 84-119)
This invention relates to an electronic musical instru
ment and more particularly to a novel circuit for an elec 10
tronic musical instrument, including ?lter means for re
ducing the relative amplitude of or eliminating objection
able transient effects.
This application is a continuation-in-part of copend~
ing Ziehlke application, Serial No. 745,564, ?led June 30,
1958, and assigned to the assignee of this application.
In an electronic musical instrument, as an electronic
organ, there are various devices in the signal handling
controlled by a sub-audio oscillator and introducing an
objectionable transient at the sub-audio rate.
Further features and advantages of the invention will
readily be apparent from the following speci?cation and
from the drawings, in which:
FIGURE 1 is a block diagram of an electronic musi
cal instrument embodying the invention;
FIGURES 2 and 2A are a schematic circuit diagram
thereof;
FIGURE 3 is a curve illustrating the transient char
acteristic of a lamp; and
FIGURE 4 is a curve illustrating a characteristic of
the transient ?lter.
In FIGURE 1 of the drawings, a block diagram of an
circuitry which may introduce objectionable transient sig
electronic musical instrument, here an organ, is illus
nals into the system, and it is extremely desirable that 20 trated, having a plurality of tone generators 10, the out
these transients be eliminated or reduced in amplitude
put of which is connected with a preampli?er 11. The
so. that they are not present to an appreciable degree
in the output of the instrument. For example, in a pre
amplitude of the output signal from preampli?er 11 is
controlled by means of swell control 12 with Which the
ferred form of tone generator, which utilizes a plurality
organist may control the ultimate volume of the gen
of selectively operable light bulbs or lamps in the genera 25 erated tones. The signals from the preampli?er 11 are
tion of the tone representing signals, the thermal or in
coupled to a further ampli?er 13, which is provided with
candescent characteristic‘ of the lamps may give rise to
a tremolo control 14 which may be rendered operative
a transient upon energization and deenergization of the
by the tremolo control switch 14a. The purpose of the
tremolo is to impart to the signal a tremolant quality,
lamps.‘ Particularly in an electronic organ, a manually
operable swell or volume control is provided; and in 30 i.e., a slight variation in amplitude, preferably above and
below the normal amplitude of the signal, at a low or
many swell control circuits, a transient is introduced
corresponding in frequency with the rate of operation
sub-audio rate. As will appear more fully below, certain
characteristics of the various elements of the system, in
of the manual swell control. Furthermore, in many elec
cluding tone generators and the swell and tremolo con
tronic organ systems, means are provided for producing
trol circuitry may introduce into the tone representing
a _“tremolo effect” in which the amplitude of the tone
signal a low frequency transient voltage which will have
representing signals is varied at a low or sub-audio rate,
an undesirable effect on the ?delity of the instrument.
introducing a transient at the sub-audio rate. These
In accordance with the present invention, a ?lter 15 is
transients have an undesirable effect if reproduced with
provided in the signal handling channel for eliminating
the tones generated by the instrument and should be
eliminated from the tone representing signal before re 40 or reducing the amplitude of the objectionable voltage
production.
with respect to the desired tone representing signal. The
output of the ?lter is coupled with power ampli?er 16,
A principal object of the present invention is the pro
and to a speaker system 17.
vision in an electronic musical instrument of a novel
Turning now to FIGURES 2 and 2A, a schematic cir
?lter means for reducing the amplitude of objectionable
cuit diagram of a musical instrument embodying the in
transients relative to the amplitude of the tone represent
ing signal.
vention will be described in some detail. The circuits of
FIGURES 2 and 2A are connected by terminals A-—A
‘One feature of the invention is the provision in an elec
and BJ-B. During the course of this description, tube
tronic musical instrument in which objectionable tran—
types and component values for many of the elements of
sients are introduced into the signal, at a frequency lower
than the lowest frequency of the tone representing sig 50 the system will be given. It will be understood that
this speci?c information is given for the purpose of mak
nal, wherein a high pass ?lter is connected to the output
ing a complete disclosure of an operative embodiment of
of a cathode follower for reducing the amplitude of
the invention, and the values given are not to be con~
the transients relative to the amplitude of the tone rep
sidered critical, as many changes will be apparent to those
resenting signal. > Another feature of the invention is the
skilled in the art.
?lter which includes a capacitor connected in series with
Lamps 21 are energized from a suitable source of elec
a winding of a coupling transformer, the capacitor and
trical energy 22, which may, for example, have an out
transformer forming a series resonant circuit at a fre
put voltage of ten volts DC. The connection of lamps
quency of the order of the lowest frequency of the tone
21 to the power source 22 is effected by key switches 23,
generating means. A further feature is that the ?lter has
which may be associated with the keys of a standard in
an attenuation characteristic with a slope of the order
strument keyboard, through a stop selector means 24 for
of twelve decibels per octave at frequencies below the
choosing the lamps associated with the desired tone pat
resonant frequency thereof.
terns. The photocells 20 for each of the tone generators
Still another feature is that the transient ?lter is used
in an electronic musical instrument having tone generat 65 10 (of which there may be twelve, one for each note in
an octave) are connected in parallel and ‘through a cur
ing means including a plurality of selectively operable
rent limiting resistor 25, l megohm, with a suitable B+
lamps, the light from the lamps being converted into an
supply source. The tone representing signals are con
electrical tone representing signal, and the thermal char
nected through capacitor 26, 0.01 ,uf. (microfarad), to
acteristics of the lamps introducing objectionable tran
the control grid of a cascade connected dual triode am
sients into the tone representing signal. Yet a further 70 pli?er '27, a l2AT7.
feature is that the transient ?lter is used in an electronic
musical instrument having circuitry including a swell
The characteristics of the ?lament of lamps 21 ‘are such
that the lamps do not shift instantaneously to a full in
3,026,757
3
candescent condition when the key switch is closed, nor
do they drop instantaneously to a dark condition when
the key switch is opened. As shown in FIGURE 3, a
shunt impedance presented to the signal channel, and
thus varying the relative amplitude of the tone represent
ing signal. Furthermore, as in the swell control circuit,
?nite’ period t1 is‘ required between the'instant the key is
the operation of the tremolo introduces transient signals
into the signal handling channel, here ‘basically at the
closed and the time at ,Whichithe average D.C. level of
the tone is achieved. Similarly, a ?nite period I2 is re
frequency of the sub-audio ‘oscillator. Again, a feedback
quired between the time the key is opened and the point
circuit including capacitor 48, 0.0l'uf., is connected be
where the output from the photocell due to light from the
lamp become insigni?cant.
tween the anode and the control grid of tube 45 in an
effort to reduce the transient effect, but it is not com
During these transition
periods, t1 and t2, transients are generated in the system 10
which are a result of the shift in D.C. level across photo
pletely eliminated.
cells 20. The frequencies of the transients is dependent
an output terminal 60 and a cathode 440 connected to
a terminal 61 by a resistor 62. The terminal 61 is con
,
The ampli?er 44 includes an anode 44b connected to
principally upon the thermal or incandescent characteris
tics of the ?laments‘ of lamps 21, but primarily they are
nected to the grounded negative supply terminal“. The
composed of low orsub~audio frequencies.
terminal 60 is connected to a positive supply terminal 64
_
The output of cascade connected ampli?er 27 is con
nected through a series resistor 28 and coupling capaci
tor 29 with the control grid of triode ampli?er 30, one
by resistors 65 and 66.
i
A bass boost circuit 50 is shunted across the input of
' ampli?er 44 and the output of the ampli?er is connected
half ofa 1-2AT7.
through capacitor 51 with the control grid 52a of cathode
Shunte‘d across the signal channel between ampli?ers 20 follower tube 52,;one-half of a 12AT7. The cathode
27 and 30 is a swell control circuit including a variable
impedance vdevice 31, one-half of a l2-AT7. The conduc
tion through device 31 is controlled by the swell pedal 12,
as will appear, varying the apparent impedance shunting
the. signal channel, and thus controlling the relative am
52b of the cathode follower tube is connected to ground
potential by a load resistance which is divided into two
portions, agrid bias resistor 53, 1000 ohms, and an out
put load resistor 54, 33,000 ohms, with the output po
tential being derived across resistor 54. The control grid
plitude of the tone representing signals transmitted through
is returned through resistor 55 to the juncture of the re
the channel. i
sistors 53 and 54 providing some degenerative feedback.
In‘ a preferred embodiment of, the system, swell con
The anode 520 of the cathode follower tube is connected
‘trol’ 12 actuates a mask 32' interposed between lamp 33
to the positive supply terminal 64 by the resistor 66.
and‘ a photocell 34 which is connected to resistor 35 in
The output of the cathode follower is coupled through
the circuit of the control grid of variable impedance de
capacitor 56 to the primary winding-57a of coupling trans
vice 31. With the swell control pedal fully raised, i.e.,
former 57. The secondary winding 57b of coupling trans- .
minimum volume, the full light from lamp 33 shines on‘
former 57 has a grounded center tap 57b’ and providesv
photocell 34‘ and it conducts heavily, applying a positive
two signals, 180° out of phase with each other, for push
potential through resistor 36 to the control grid of tube 35 pull driver ampli?ers 58a and 58b, the two halves of a
31.. In turn, tube‘ 31 conducts heavily and appears as a
twin triode, l2AU7. The output of drivers 58a and 58b
low impedance to the signal channel. As the control
is coupled to power ampli?er 1.6 and from there to
speaker
17.
pedal is depressed, mask 32 progressively cuts off the
light from lamp‘ 33 shining on photocell 34, reducing the
As is apparent from the foregoing discussion,_=the vari- ‘
positive potential applied to the control grid of tube 31 40 ousielements of the signal generating ‘and handling system
and increasing its apparent impedance. When the swell
have the incidental and undesirable effect of introducing
pedal is fully depressed, the current through tube 31 is
certain transients into the tone representing signal gen
at a minimum and its impedance is a maximum, 2. condi
erated and handled by the system. Furthermore, the
tion of maximum signal volume.
principal energy of these transient signals is in an ex—
As the swell pedal is operated and-the current‘ through 45 tremely low frequency range; and at least those transients
tube 31' changes, the D.C. potential of the anode also
generated by the tremolo circuit are primarily the sub‘
changes at a rate dependent upon the speed of movement
audio frequency of the tremolo oscillator 47.
of‘ ‘the control pedal. A transient signal thus produced
In accordance with the present invention, a ?lter net
is coupled through capacitor 37, 0.1 ,uf., to the signal
work is incorporated in the signal channel to reduce'the
channel; In an elfort to reduce the relative amplitude 50 relative amplitude of the undesirable transient frequencies.
of‘ the‘ transient signal, a feedback capacitor 38, 0.004
In the illustrated embodiment of the invention, capacitor
pf, is connected between'the anode and cathode tube 31,
56 forms a series resonant circuit in the input of coupling
providing a feedback signal. However, this feedback cir
transformer 57. The resonant frequency of the ?lter
cuit cannot‘ completely eliminate the transient effect, and
circuit‘v is preferably of the order of or slightly higher
a certain portion of the undesired transient is added to 55 than the frequency of the lowest generated tone.
the tone representing signal in the signal channel.
Returning now to a consideration of the primary signal
In a
speci?c embodiment of the system where lowest generated
tone has a frequency of 16 c.p.s., the resonant frequency
handling‘ channel, the output of ampli?er 30 is coupled
of the ?lter is 32. c.p.s. Forthis resonant frequency,
through capacitor 40 to a manually operable level control
capacitor 56 has a value of 1.25 ,uf, and the input of trans
potentiometer 41. The movable tap for potentiometer 60 former 57 an apparent inductance of the order of 20
41 is connected through series dropping resistors 42 and '
henries. The transfer characteristic of ?lter 15 is illus
trated in FIGURE 4, where the ratio of the output vol"?
Shunted across- the signal channel between series resistors
age to the input voltage of the ?lter is plottedas a func
42 and 43, is a selectively operable tremolo circuit includ
tion of frequency. It will be noted that the characteristic
ing a triode 45, one-half of a 12AT7, whose anode is 65 has a slight peak at the resonant frequency J‘ (here 32
43, to the control grid 44a‘of ampli?er 44, a 12AX7.
connected‘ with the signal channel through capacitor 46,
0.025 nf. The control grid circuit of tube 45 is selec
tively connectable with a sub-audio oscillator 47, through
c.p.s.), and then drops oif rather. rapidly. The slope of
the attentuation characteristic of the ?lter below the
resonant frequency f is preferably of the order of twelve
tremolo ‘control switch 14a. With switch 14a in the
decibels‘ per octave, so that the low transient frequencies
position shown in the drawing, connecting the sub-audio 70 are highly tattentuated. For example, the tremolo fre
oscillator with ‘the control grid. circuit of tube 45, to
quency from sub-audio oscillator 47 is about three octaves
shunt impedance of the stage is varied at the frequency
below the’ resonantv frequency of the ?lter, and is at
of the oscillator, which may be, for example, four or ?ve
tenuated 36 decibles below the level of the tone represent
cycles per second. The tremolo circuit operates in a
ing signals.
I
manner similar to the swell control circuit, varying. the 75 While I have shown and'describedlcertain- embodiment-s
3,026,757
of my invention, it is to be understood that it is capable
of many modi?cations. Changes therefore, in the con
struction and arrangement may be made without departing
from the spirit and scope of the invention as disclosed in
the appended claim.
I claim:
In a musical instrument of the type in which ?rst and
second tone signal terminals have applied thereto musical
tone signals extending over a range of several octaves to
gether with undesired transient noise substantially lower 10
in frequency than the lowest pitch tone signal and in
which a positive and a negative supply potential terminals
are connected respectively to the ?rst and second tone
signal terminals, in combination with the terminals, a
cathode follower including an electron discharge tube 15
having anranode connected 'to the positive supply potential
terminal, a grid connected to the ?rst tone signal terminal,
a cathode, and a load resistor connecting the cathode to
the second tone signal terminal; a voltage gain coupling
transformer having a primary winding and a secondary 20
6
winding; and a capacitor connected in series with the
primary winding and forming with the primary Winding
a series resonant circuit having a resonant frequency ap
proximately equal to the pitch of the lowest pitch musical
tone signal, the series connected capacitor and primary
winding being connected across the load resistor to induce
voltage ampli?ed tone signals in the secondary winding
and to substantially attenuate the low frequency transient
noise.
References Cited in the ?le of this patent
UNITED STATES PATENTS
1,930,898
Johnson _______ __> ____ __ Oct. 17, 1933
2,088,205
Jacobs _______________ __ July 27, 1937
2,142,580
2,513,109
2,535,323
2,570,701
2,817,708
Williams ______________ __ Jan. 3,
Roth ________________ __ June 27,
Skellett ______________ __ Dec. 26,
Martin ________________ __ ‘Oct. 9,
Fender ______________ __ Dec. 24,
1939
1950
1950
1951
1957
UNITED STATES PATENT OFFICE
CERTIFICATE OF CORRECTION
Patent No. 3,026,757
March 27, 1962
Robert J. Ziehlke
It is hereby certified that error appears in the above numbered pat
ent requiring correction and that the said Letters Patent should read as
corrected below.
.
Column 2, line 56i before "Lamps 21" insert the‘following:
A preferred form of tone generator 10
is indicated schematically in Figures 2 and
2A, each tone generator including a photo
cell 20 having a plurality of selectively
operable light sources or lamps 21 associated
therewith. The light from lamps 21 is utilized
to form tone signals in a well known manner.
column 3,
line 71, for "to" read -- the -—,
Signed and sealed this 10th day of July 1962.
(SEAL)
Attest:
ERNEST W. SWIDER ‘
‘DAVID L. LADD
Attesting Officer
Commissioner of Patents
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