Note: Descriptions are shown in the official language in which they were submitted.
~1~90~
- . Background of the Invention
High quality audio signals, such as those generated
in the course of many live musical performances or those
produced on playback from carefully processed and recorded
material, may have a dynamic range of as much as one hundred
decibels. A dynamic range of 100 dB constitutes a difference
of 100 d~ between the quietest sounds discernible and the
loudest undistorted sounds represented by the signal.
Another instance of a broad dynamic range occurs in the case
of sound reinforcement microphones, which are required to
handle a dynamic range within the limitations imposed by room
background noise and by loud talking in close proximity to
the microphones. Here, the dynamic range may exceed 90 dB at
the microphone.
Preservation of the full dynamic range for a given
audio source, whether in radio broadcasting, in operation of
a sound reinforcement system, or in a residential or
commercial playback system, is not always desirable and
frequently is inordinately expensive. For such situations,
compression/limiter amplifiers have been used to develop an
audio signal having a restricted dynamic range from an input
signal with a substantially broader dynamic range. Thus,
broadcast radio stations have used compression/limiter
amplifiers to increase the average level of the signal
received by listeners. Low level signals in the source
material are amplified enough to be heard even if the
listener is in a high background noise environment. The
compression/limiter amplifier prevents high level signals in
the source material from over-modulating the transmitter.
Similarly, sound reinforcement systems ideally should
compensate for level variations among talkers and for
variations in microphone talking distances by providing a
0~1~
more constant and comfortable level in the loudness of the
sound reproduced for listeners. A compression/limiter
amplifier, adjusted in threshold level, can provide full gain
for soft talkers while at the same time reducing the gain for
loud talkers.
Another application for compression/limiter
amplifiers in audio systems pertains to consumer records and
tapes. The dynamic range for a record or a tape is limited
by the noise and distortion characterics of the recording
medium. A compression/limiter amplifier can be used to
restrict the dynamic range prior to recording. In this way,
low level sounds are amplified above the recording noise and
high amplitude inputs are limited to levels below the
recording distortion level.
The reduced dynamic range for the output of a
compression/limiter amplifier results in low level signals
not falling as low as before the amplifer and high level
signals not being as high as before the amplifier. The key
to effective design of a useful audio system utilizing a
compression/limiter amplifier is to provide a dynamic range
restriction having the least audible side effects.
Two different forms of audio systems using
compressor/limiter amplifiers have been most commonly
employed. In a feed forward compressor/limiter amplifier
system, it is customary to employ a voltage-controlled
amplifier having a gain characteristic that decreases with
increasing DC voltage on its control input. The gain control
~ircuit for an amplifier of this kind converts an incoming
audio signal to a DC level proportional to the incoming
signal amplitude. This gain control voltage is fed forward
to the voltage controlled amplifier so that if the audio
signal input decreases in level, the amplifier gain
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increases. If the audio input signal increases in level, a
control signal of greater amplitude is supplied to the
voltage-controlled amplifier and th~ gain is reduced.
Consequently, the output level tends to remain constant
despite changes in input level. The other commonly employed
compression/limiter amplifier circuit is a feedback control.
In this type of system~ increases in output level from the
voltage controlled amplifier increase the feedback control
voltage and reduce the amplifier gain, thereby decreasing the
output level.
To achieve a flat limiting response in a
compression/limiter amplifier of the feedback type, a high
gain must be incorporated in the feedback control loop.
Consequently, after a transient level change, a feedback
limiter amplifier approaches final value in an oscillatory
manner that is undesirable for a high quality audio system.
A feed forward compression/limiter amplifier, on the other
hand, is free of control loop feedback, and hence can provide
limiter action without oscillation or "hunting", but a
controlled "mild" compression characteristic (e.g. 2:1
compression) is difficult to achieve. Conversely, a feedback
compression/limiter amplifier is usually stable for a mild
compression characteristic with only limited gain in the
feedback loop.
Some attempts have been made to combine the feed
forward and feedback controls in a compression/limiter
amplifier, but the results have been generally
unsatisfactory. These combination circuits have been based
upon a summation of the control signal outputs from the feed
forward and feedback controls and the result tends to
erratic operation of the gain level in the controlled
amplifier.
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Summary of the Invention
It is a principal object of the present invention,
therefore, to provide a new and improved loudness control
system using a gain-controlled audio amplifier of the
compression/limiter type that retains the principal operating
advantages of previously known feed forward and feedback
controls in a unified circuit.
A specific ob~ect of the invention is to provide a
new and improved audio loudness control system that combines
the flat limiting characteristic and stable gain control of a
feed forward compression/limiter amplifier with the easily
implemented mild compression action of a feedback type
compression/limiter amplifier.
Another ob]ect of the invention is to provide a new
and improved audio loudness control system, using a
compression/limiter amplifier, that incorporates a feed
forward gain control circuit that eliminates adverse effects
of transient feed signals and which provides for rapid update
without needless "hunting" of the control voltage so that a
subjectively smooth gain control action is afforded.
Accordingly, the invention relates to an audio
loudness control system for use with a broad dynamic range of
audio input signals, comprising a gain-controlled audio
amplifier having an audio signal input, a gain control input,
and an audio signal output, feed forward control signal
generator means, having an input coupled to the audio signal
input of the amplifier, for generating a feed forward control
signal representative of excursions of the average audio
input signal above a predetermined loudness threshold,
feedback control signal generator means, having an input
coupled to the audio signal output of the ampli~ier, for
generating a feedback control signal representative of
8~
~ . transient excursions of the audio output signal above the
predetermined loudness threshold, and selector gate means,
having inputs coupled to the feed forward and feedback
control signal generator means and an output coupled to the
gain control input of the amplifier, for selecting the
control signal instantaneously representative of the larger
excursion above the loudness threshold and applying the
selected control signal to the gain control input of the
amplifier.
Brief Description of the Drawings
Fig. 1 is a block diagram of a preferred form of an
audio loudness control system constructed in accordance with
the present invention;
Fig. 2 is a generalized illustration of certain
operating characteristics of the audio loudness control
system of the invention; and
Figs. 3 and 4 are detail circuit diagrams for a
specific embodiment of the circuit of Fig. 1.
Description of the Preferred ~mbodiments
The audio loudness control system 10 illustrated in
the block diagram of Fig. 1 has an audio input terminal 11
connected to an over-range limiter circuit 12. The output VN
of limiter circuit 12 is supplied to a gain-controlled
amplifier 14. The output VO of amplifier 14 is connected to
an output amplifier 15 in turn connected to the audio output
terminal 16 of the loudness control system 10.
Loudness control system 10, Fig. 1, further
comprises a feed forward control signal generator 17 and a
feedback control signal generator 1~. The feed forward
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control 17 generates a feed forward control signal that is
representative of excursions of the average audio input
signal above a predetermined loudness threshold, based on an
input to control 17 of the audio input signal VN from limiter
12, the same signal that is supplied to the gain-controlled
amplifier 14. The feed forward control signal output FF
from control 17 constitutes one input to a magnitude selector
gate 19. Feedback control 18, on the other hand, which has
an input comprising the audio output signal VO from amplifier
14, generates a feedback control signal FB representative
of transient excursions of the audio signal VO above the
predetermined loudness threshold. That feedback control
signal FB is also supplied as an input to selector gate 19.
Gate 19 selects the control signal input that is
instantaneously representative of the larger excursion above
the loudness threshold and supplies the selected control
signal GC to the gain control input of amplifier 14.
Feedback control 18 is a relatively simple circuit
of a kind often used in compression/limiter amplifiers of the
feedback type. It may include an envelope detector ~1,
usually a full wave rectifier, and a peak detector
(integrator) or integrator circuit 22. The output of
detector 22 is the feedback control signal FB.
The feed forward control 17 is more complex. Thus,
the feed forward control t7, starting at its input, may
comprise an equalizer circuit 23 that weights incoming signal
strength in accordance with an approximation of the frequency
sensitivity of the human ear, preferably at the seventy phon
loudness level. The output of equalizer 23 is supplied to an
envelope detector 24, usually a fu1l wave rectifier, followed
by a peak detector 25. Detector 25 should have an attack
time that is substantially shorter than its release time to
osa
simulate the loudness growth and decay characteristics of the
human ear. For example, the attack time of peak detector 25
may he approximately one hundred milliseconds and the release
time about five hundred milliseconds. Thus, the output
signal FC from peak detector 25 is a loudness signal that
exhibits frequency and time characteristics generally
corresponding to the selective loudness values of the audio
input signal VN.
The loudness signal FC from peak detector 25 is
supplied as an input to a slew filter 26 and to a history
comparator circuit 27. Slew filter 26 also has an input
derived from the history comparator 27 and another input
derived from a gain limit set circuit 28. The output of
filter 26 is the feed forward control signal FF.
In considering the operation o loudness control
system 10, it is convenient to start with the assumption that
the system has been energized but that there is no initial
audio input signal at terminal 11. In these circumstances,
of course, the audio input VN to amplifier 14 is effectively
zero and so is its output signal VO. With no available
signal to detect, the feedback gain control signal FB from
feedback control 18 is zero. Similarly, with no input signal
VN, feed forward control 17 does not generate a loudness
signal FC at the output of the equalizer and detector
circuits 23-25 constituting the initial stages of feed
forward control 17. For these conditions, the output of slew
filter 26, the feed forward gain control signal FF, is
maintained within a predetermined range established by the
gain limit set circuit 28. Gate 19 selects the feed forward
gain control signal FF and supplies that as the control
signal GC to amplifier 14. Of course, with no output VO from
amplifier 14, there is no output signal at terminal 16
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08~
. It may next be assumed that an audio input signal
within the normal dynamic range oE system 10 is applied to
terminal 11. This signal may be originated by someone
speaking into a microphone, by pickup from a live musical
performance, or from virtually any other source. This audio
input passes through limiter 12, which has no substantial
effect, and appears as the input signal VN to amplifier 14
and feed forward control 17. As previously noted, the feed
forward control is an averaging device having its operation
based upon relatively long-term effects; consequently, the
feed forward gain control signal FF does not change
immediately following initiation of an audio input to system
10. If signal VN is low enough so that the output signal VO
from amplifier 14 is below the system threshold (set by
circuit 28) the gain of amplifier t4 remains unchangedn
On the other hand, if the input signal VN has a
sufficient amplitude so that the output signal VO from
amplifier 14 exceeds the system threshold loudness level, the
feedback gain control signal FB developed by feedback control
18 exceeds the effective amplitude of the feed forward gain
control signal FF and hence is selected by gate 19 and
supplied as the gain control signal GC to amplifier 14.
Th~s, on start-up, feedback control 18 may control the gain
of amplifier 14 for a limited interval, determined by the
averaging time for the feed forward contLol 17, with system
10 initially functioning as a compression/limiter amplifier
of the feedback type.
Assuming that the audio input signal at terminal 11
extends in duration for an appreciable time, the feed forward
gain control signal FF assumes a significant role in control
of system 10. Thus, the audio input signal VN is weighted,
in eguali~er circuit 23, which constitutes a seventy phon
~ ~90B~
filter, producing an equalized audio signal, supplied to
detector 24, having an amplitude corresponding to the
frequency sensitivity of the human ear. This assures that
the control voltage FF ultimately developed by the feed
forward control 17 reacts in proportion to the loudness value
of the incoming signal rather than simply to its overall
amplitude.
The equalized audio output signal from filter 23 is
envelope detected in circuit 24, producing a DC envelope
level signal having an amplitude proportional to loudness of
the audio input. This envelope level signal is further
detected in peak detector 25 to develop the loudness
signal FC. As previously noted, peak detector 25 preferably
has a relatively short attack time, approximately one hundred
milliseconds, and a substantially longer release time, about
five hundred milliseconds, to simulate the loudness growth
and decay characteristics of the human ear. Accordingly, the
loudness signal FC exhibits the fre~uenc~ and time
characteristics of the subjective loudness level of the audio
input.
The loudness signal FC is supplied as an input to
filter 26; any increase or decrease in the loudness signal FC
tends to cause a corresponding increase or decrease in the
output signal FF from the slew filter. Slew filter 26,
however, controls the slew rate for its output signal ~F in
accordance with a predetermined rate of change
characteristic. Moreover, operation of the slew filter is
controlled so that its output FF is maintained within
predetermined limits established by gain limit set circuit
28. In the preferred construction described more fully
hereinafter in connection with Figs. 3 and 4, slew filter 26
is constructed to enforce a logarithmic rate of change on its
output signal FF relative to changes dictated by the input
signal FC. By maintaining a logarithmic rate of change
characteristic in the operation of slew filter 26, feed
forward control 17 is effective to eliminate needless
"hunting" in its output signal FF, the feed forward gain
control signal, and affords a subjective smooth gain control
in the operation of amplifier 14 because the human ear
responds logarithmically to loudness changes. Slew filter 26
also acts to smooth out transient fluctuations of the
loudness signal FC in the generation of gain control signal
FF.
In the operation of feed forward control 17, it is
desirable to prevent small "noise" in the audio input signal
VN, unrelated to actual audio or program information, from
affecting the gain control signal FF that is the output of
slew filter 26. It is also desirable to limit the operation
of feed forward control 17 so that it functions as an
averaging control; that is, transient excursions of the audio
input signal VN are better left to the control of feedback
control 18. These purposes are served by history comparator
circuit 27, which effectively blocks the operation of slew
filter 26 to prevent change in its output signal FF for small
noise and transient low amplitude excursions occurrinq in the
audio input signal VN. Comparator 27 has a further effect on
the operation of slew filter 26 and its feed forward gain
control output signal FF, once system operation is carried
forward for an appreciable interval (e.g., more than one
second). Thus, if the audio input signal VN is interrupted
for a relatively brief period, as when a pause occurs in a
live pickup from one or more microphones or when the music
stops in a pickup from a musical performance, comparator 27
effectively blocks the operation of slew filter
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26 and prevents an undesirable change in the level of the
feedbac~ gain control signal FF.
Once the initial brief period of actual feedback
control as descr bed above has transpired, the gain control
signal GC supp~ied to amplifier 14 from gate 19 is that input
signal, either the feed forward gain control signal FF or the
feedback control signal FB, having the greater amplitude. In
general, for most audio input signal variations, the control
is exercised by feed forward control 17 and its output signal
FF. For any high level audio excursions of relatively
transient nature, however, gain control reverts to feedback
control circuit 18 and its output signal FB. 8ecause
~eedback control 18 is utilized to control transient output
level changes, the equalization applied to the feed forward
control by circuit 23 is not used. This permits feedback
control 18 to control the gain of amplifier 14 in a manner
that prevents system overload for frequencies not normally
within the range of sensitivity of the human ear, as may
result from microphone handling noise in some audio systems.
In practical applications of loudness control
system 10, the total change of gain provided for amplifier 14
is limited such that for low audio signal levels a maximum
gain limit is set and for high signal levels a minimum gain
level is set. Thus, a threshold must be exceeded before the
gain of amplifier 14 is reduced through operation of either
feed forward control 17 or feedback control 18. The system
thresholds are determined by gain limit set circuit 28. In
the preferred system 10, as shown in Fig. 1, over-range
limiter 12 is provided with a feedback connection from
detector 22 in feedback control 18 to prevent overload of the
system from excessively high level inputs that might occur at
terminal 11.
~9~3080
- . Some of the generalized operating characteristics
of audio loudness control system 10 are illustrated in Fig.
2. As shown therein, the feedback control characteristic,
line 31, corresponds to a relatively "mild" compression
control at a ratio of approximately 2:1. The feed forward
control charcteristic begins with line 32, determined by the
higher gain threshold established by gain limit circuit 28,
with no appreciable change in the gain of amplifier 14 until
point 33. At point 33, the input loudness level exceeds the
lower threshold set by circuit 28; for the next portion 34 of
the feed forward control characteristic, the system functions
as a flat limiter~ with no increase of output loudness
despite an appreciable increase of input loudness. The
signal range indicated by portion 34 of the feed forward
control characteristic ends at point 35. The input loudness
range between points 34 and 35 may, for example, be of the
order of ten dB. From point 35 on, the feed forward control
chracteristic continues with a segment 36 that has the same
slope as the initial segment 32 but has been displaced for an
effective reduction in loudness of the output signal relative
to the input. Segment 36 of the control characteristic ends
at a point determined by the operational characteristic of
over-range limiter 12, which produces the final segment 37 of
the operating characteristic for the overall system.
From the foregoing description of Figs. 1 and 2, it
is seen that audio loudness control system 10 utilizes the
flat limiting and stable gain characteristics afforded by
feed forward amplifier gain control in combination with the
easily implemented mild compression action of a feedback
control. Thus, in system 10, the feedback control exercised
by circuit 18 and exemplified by curve 31 corresponds to a
relatively mild compression action at a ratio of about 2:1.
Q1~30
Of course, this ratio can be varied in dependence upon the
circuit parameters and operating characteristics utilized for
detectors 21 and 22 in feedback control 18. The flat
limiting characteristic of a feed forward compression/limiter
amplifier, exemplified by portion 34 of the feed forward
operating characteristic, is also provided by system 10. The
two gain control signals FF and FB are selected on the basis
of magnitude by gate 19 and only one of them is supplied as
the gain control signal GC to amplifier 14. This avoids the
erratic operating characteristics of arrangements in which
feed fowrard and feedback gain control signals are summed or
otherwise combined for use as gain control signals, an
arrangement that almost inevitably leads to over-control with
too frequent and excessive swings in the gain of amplifier
14. Stated differently, the audio loudness control system
10, unlike previously known systems that combine feedback and
feed forward control voltages, switches control between the
two without disturbing the value of either.
Figs. 3 and 4 togther afford a detailed schematic
diagram for the operating circuits of a preferred embodiment
of audio loudness control system 10 of Fig. 1. Fig. 3
includes specific circuits for the over-range limiter 12 and
the complete feed forward control 17 including equalizer 23,
envelope detector 24, peak detector 25, history comparator
27, slew filter 26, and gain limit set circuit 28. Fig. 4,
on the other hand, shows specific circuits for
gain-controlled amplifier 14, output amplifier 15, the
envelope detector 21 and peak detectoL- 22 that constitute
feedback control t8, and selector gate 1~. In each of these
3~ figures, specific values are given for all capacitors ~nd
resistors. All diodes are Type lN4148 unless otherwise
- 13 -
o~
designated. ~perational amplifiers identified by the symbol
(1) are Type 4741 and those identified by the symbol (4~ are
Type TL074. The analog switches S1 and S1 in slew filter 26
are Type 4016.
In Fig. 3, the over-range limiter circuit 12 is a
generally conventional attenuator in which the attenuation
level is controlled by an FET 41. To avoid distortion that
might be created because the FET is closer to turning on for
negative peaks in the audio input signal than for positive
peaks, AC feedback is supplied to the gate electrode of the
FET. This maintains the resistance in the control channel of
the limiter much more constant during the input cycle than
would otherwise be the case. In limiter 12, the AC signal is
also supplied to the source electrode of the FET, with the
result that there is hardly any voltage difference anywhere
on the FET, reducing distortion to a minimum. The limiting
level for circuit 12 (see characteristic 37 in Fig~ 2) is
determined by a feedback signal LC derived from the feedback
peak detector 22 (Fig. 4) and applied to the operational
amplifier 42 connected to the gate electrode of the FET.
Limiter 12, in Fig. 3, is shown as including an
input level adjustment potentiometer P1 that is mechanically
ganged to a similar potentiometer P2 in output amplifier 15,
Fig. 4. This arrangement is utilized to allow for adjustment
of the input signal level of system 10 while maintaining
constant overall gain in the system.
The circuit shown in Fig. 3 for the seventy phon
filter, equalizer 23, is essentially conventional. Equalizer
23 affords an operating characteristic that rises relatively
rapidly from very low frequencies up to about 100 Hz, with a
more gradual increase up to about 2000 Hz and a relatively
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- . rapid decline from about 5000 Hz, simulating the frequency
response of the human ear. ~s previously noted, the output
of equalizer 23, on line 43~ constitutes an equalized audio
signal that is detected in envelope detector 24 to develop,
on line 44, a DC signal essentially proportional to loudness.
Peak detector 25, as shown in Fig. 3, comprises an
operational amplifier A1 with a feedback circuit including a
diode D1 and a resistor R20 The loudness signal FC
constituting the output from detector 25 is derived from a
circuit comprising a resistor R1 and a capacitor C1. With
the values given, peak detector 25 has an attack time of one
hundred milliseconds and a release time of five hundred
milliseconds. Thus, the signal FC exhibits the frequency and
time characteristics desired, corresponding to the subjective
loudness Gf the audio onput.
Slew filter 26, in the circuit shown in Fig. 3,
constitutes a novel constant logarithmic slew filter formed
by a comparator CP2 having its output connected to one input
of an aplifier A2. The output of amplifier A2, which
constitutes the feedback gain control signal FF, is fed back
to another input of the same amplifier through a circuit that
includes an electronic switch S1 actuated by the output from
comparator CP2. This feedback circuit further comprises
resistors R5, R6A and R6B, and a capacitor C3.
Slew filter 26 functions to slew the level of its
output signal FF up and down at a rate determined by the time
constant of the series resistors R6A and R6B and the
capacitor C3. The slew direction is determined by comparator
CP2, based on its input FC~ When the voltage on capacitor C3
exceeds that on capacitor C1 (the level of signal FC)
comparator CP2 opens switch S1, thereby allowing exponential
0~
decay of any charge on capacitor C3 through resistors R6B,
R6~, and R5, assuming switch S2 to be closed~ On the other
hand, if the loudness level indicated by the charge on
capacitor C1 exceeds the control voltage on capacitor C3,
comparator CP2 closes switch S1 and amplifier A2 applies a
voltage to capacitor C3 through series resistors R6A and R6B
so that capacitor C3 is charged exponentially. The gain of
amplifier A2 is preferably approximately equal to two, so
that the decay rate for capacitor C3 is essentially equal to
the charge rate and both proceed in accordance with the same
exponential (logarithmic) characteristic. This allows slew
filter 26 to smooth the functions of the loudness signal at
capacitor C1 into a controlled rate of change signal at
capacitor C3. Use of a logarithmic rate of change allows for
rapid update without needless hunting in the control voltage
FF, with a subjectively smooth gain control because the human
ear responds logarithmically to loudness changes.
If the gain of amplifier A2 is made greater or less
than two, charge rate of capacitor C3 will exceed or be less
than the decay rate. Thus, adjustment of the amplifier gain
can be made to modify the operating characteristic of slew
filter 26, as by providing a more rapid attack to high gain
for system 10 for "soft" talkers without a correspondingly
fast attack to low gain for loud talkers.
The history comparator or activity gate circuit 27
controls a switch S2 in slew filter 26. In the circuit
arrangement of ~ig. 3, circuit 27 comprises an operational
amplifier CP1 functioning as a comparator with one direct
input constituting the loudness signal FC and the other input
derived from the loudness signal FC through a circuit
comprising a series resistor R3, a shunt capacitor C2 and a
shunt resistor R4 that is connected to a fixed threshold
o~
voltage. Resistor R3 and capacitor C2 have values selected
to average the input signal FC; an appropriate time constant
is one second. For comparator CP1 to supply an output signal
to switch S2 that will close that switch, the voltage on
capacitor C1 in peak detector 25, representative of
subjective loudness, must be greater than the threshold set
by the reference voltage to which the comparator is connected
through resistor R4. This prevents small noises unrelated to
actual audio or other program information from being sampled
into the control voltage FF by slew filter 26. Furthermore,
to close switch S2 the signal FC must be greater than a
fraction, determined by resistors R4 and R3, of the recent
(one second) average for the loudness signal. This forces
peak loudness values to be sampled into the feed forward
control voltage FF. For a brief interruption in audio signal
input, history comparator circuit 27 maintains switch S2
open, precluding any substantial change in the charge on
capacitor C3, the basic determinate of forward feed gain
control signal FF.
Gain limit set circuit 28, as shown in Fig. 3, is
basically a clamp circuit for capacitor C3. It sets both
upper and lower threshold levels for the voltage on capacitor
C3 and thus determines the operational thresholds for the
system.
As shown in Fig. 4, the envelope detector 21 in
feedback control 18 may utilize the same circuit as envelope
detector 24 in feed forward control 17 (Fig. 3). The audio
output signal VO is rectified in detector 21 and supplied to
peak detector 22, which comprises an amplifier A3, a diode
D2, the resistors R7 and R8, and a capacitor C4. Peak
detector 21, like detector 25, has an attack time of one
hundred milliseconds and a release time o~ five hundred
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080
milliseconds to simulate the growth and decay in subjective
loudness of signal VO~
The feedback gain control signal FB, derived from
the charge on capacitor C4, is supplied as one input to
selector gate 19. The other input to gate 19 is the feed
forward gain control signal FF from slew filter 26 (Fig. 3).
Gate 19 includes an operational amplifier A4 and a diode D3
which, in combination, afford an ideal diode action. The
combination of an operational amplifier A5 and a diode D4
serves the same purpose. In gate 19 whenever the feed
forward signal FF exceeds the feedback signal FB, the output
signal GC is the signal FF because diode D4 is driven
nonconductive. Similarly, if signal FB exceeds signal FF,
then diode D3 is driven nonconductive and the output signal
GC .s the feedback gain control signal FB. Thus, gate 1
applies the larger control voltage to gain-controlled
amplifier 14 and can instantly change between feed forward
and feedback control with each control mode remaining
independent of the other.
The preferred form of gain-controlled amplifier 14
shown in Fig. 4 comprises an analog multiplier circuit 46
connected in a feedback circuit for an operational amplifier
A6. Multiplier 46 functions in a manner analogous to a
voltage-controlled resistor having a variable resistance
value that determines the gain of amplifier A6. An increase
in the DC gain control voltage GC supplied to multiplier 46
results in an increase in the feedback signal to amplifier
A6, corresponding to a decrease in the effective resistance
in the feed~ack circuit provided by multiplier 46 so that the
result is a decrease in gain of amplifier A6. Conversely, a
reduction in the gain control signal GC causes an increase in
the gain of amplifier A6. With the illustrated circuit,
8~
amplifier 14 has an operating characLeristic such that a one
dB increase in the gain control voltage ~C results in a one
dB reduction in gain for the amplifier. It will be
recognized that other forms of gain-controlled amplifier
could be used for circuit 14, but the illustrated circuit has
been found satisfactory.
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