Stereo Compressor.pdf

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i AN AUDIO COMPRESSOR IS AN
sential tool for audio processing
and it is commonly found in re-
cording
studios.
Most recording
studios have a selection of com-
pressors, each chosen for some
specialized feature or nuance.
Your home may not be a profes-
sional recording studio, but you
do need our Stereo Compressor,
so read on.
As the device’s name implies,
the Stereo Compressor com-
presses or limits the dynamic
range of an audio signal. (Make
sure you read the sidebar How
Compressors Work.) In a record-
ing studio, compressed audio
prevents tape-recorder overload
during loud passages without
losing quiet passages in the
noise floor (hiss). Before com-
pressors were invented, the re-
cording engineer would ride the
gain by manually adjusting the
signal level. Compressors are
commonly used to record vo-
calists and acoustic instru-
ments, especially those that
have large dynamic ranges.
Compressed audio can also be
used to create artificial effects
while recording, such as in-
creasing the apparent sustain
of an instrument or completely
squashing the level of a signal.
We will learn how to accomplish
these audio effects later.
Compressors can also be
abused. Have you ever won-
dered why TV commercials
sound louder than regular TV
programming? Well, the pro-
ducer of the offending TV com-
mercial has used a large
amount of compression on the
audio tracks and then boosts
the gain to just below distortion
level. The end result is an appar-
ent increase in loudness to the
listener. This works because
modern TVs and home theaters
have a large dynamic range for
the audio signal.
Film and TV producers take
advantage of the dynamic range
available to them by recording
normal conversation and back-
ground sounds 6 to 12 dB below
the maximum signal level. Then
for dramatic effect, music and
sound effects such as explo-
sions can be that much louder
than the dialogue track. This is
very noticeable when watching
ES-
BUILD THIS
Designed to lower the audio blasting
that goes with TV-commercials,
the unit also serves as a studio sound
manipulator!
a movie on a high-fidelity video
tape recorder. In fact, for late-
night viewing,
when
you are try-
ing to be quiet, you have to turn
the TV up just to hear the di-
alogue and then quickly turn it
down when the car chase scene
comes crashing into your living
room. But, with the Stereo
Compressor, you don’t need to
constantly ride the gain. It will
automatically do that for you!
When you patch the Stereo
Compressor between your hi-fi
VCR and your stereo sound sys-
tem, you can enjoy listening to
movies late at night in full
fidelity, at low volume, without
being blasted out of your chair
when Rambo raids your living
room. You can also use the unit
to prevent tape (recording) over-
load while making home audio
recordings.
How
it is done
The block diagram of the Ster-
eo Compressor is shown in Fig
1. The input signal is fed to a
voltage-controlled amplifier
3
(VCA) that has a nominal gain of
unity Some of the output signal
is fed to a precision rectifier fol-
lowed by a logarithmic convert-
er circuit. The output of this
block is a DC voltage propor-
tional to the log of the average
level of the input signal. By
sending some of this DC control
voltage to the VCA we automat-
ically reduce the gain of the VCA
when the input signal exceeds a
user-determined threshold
level. It is important to note that
we determine the signal level
after the VCA and not before.
This allows the output level to
increase and sound normal, but
not increase as much as the in-
put signal does. By varying the
amount of feedback we adjust
the compression ratio, which in
conjunction with the
T H R E S H
control, determines the operat-
ing characteristics of the com-
pressor.
The optional
sidechain
jacks
permit external processing of
the audio signal or substituting
a completely different audio sig-
nal as the control signal. This
add-on circuitry lets the user
experiment and achieve some
useful audio effects.
How it works
Figure 2 is the schematic di-
agram of the Stereo Com-
pressor. The device has two
independent channels of com-
pression, right and left. The
heart of the circuit is the
SSM2120 dual dynamic range
processor integrated circuit
lC3. The SSM2120 features two
complete dynamic range pro-
cessors. Each one consists of a
voltage-control amplifier, a log-
arithmic converter, and a preci-
sion rectifier. The chip also fea-
tures a dynamic range of 100 dB
at only .0l-percent total har-
monic distortion at + 10 dB in-
put. The remainder of the
Stereo Compressor circuit con-
sists of input and output buff-
ers, the control circuitry and
the comparator.
Since the electronics for both
channels is identical, we will
look at the right channel in de-
tail. The input audio signal
from Jl goes to an inverting
buffer circuit consisting of R40
and R44 and ICl-b. Its output
signal (ICl-b, pin 7) is coupled
to the signal input on lC3, pin
8, via C4, which blocks any DC
component of the input signal.
The SSM2120 chip is actually
looking for a current input
source, and R43 provides the
proper amount of current to the
chip. The RC filter formed by C8
and R18 eliminates any stray RF
interference.
The output signal from the
internal VCA of lC3 (pin 4) is
actually a current signal. It is
restored to a voltage signal by
current-to-voltage converter
ICl-a and its feedback resistor
R46. The signal from ICl-a, pin
1 is routed to output stage am-
plifier lC5-a via R34. The output
signal from lC5-a, pin 1, is cou-
pled via R35 and C9 to jack J3.
OUTPUT
potentiometer R55 lets
you adjust the unit’s gain.
The output signal is also sent
to the rectifier input (lC3, pin 9)
via R36 and C7 either directly,
or via the optional side-chain
jack circuitry including R50.
The side-chain jacks consist of
a standard l/4-inch open-cir-
cuit
OUT
phone jack (J7) and a
l/4-inch
IN
phone jack (J8) with
a normally-closed switch built
into them. These jacks form a
normalized patch point for ad-
ditional audio processing. With
nothing plugged into J8, the
signal path is uninterrupted.
By inserting a phone plug into
the side-chain
IN
jack (J8), the
normal signal path is broken
and either the processed origi-
nal signal is sent to the rectifier
input, or a completely different
(new) signal is sent to the rec-
tifier input. We will later see how
to use this feature for some
really powerful audio process-
ing.
But first, the control side of
the house. Resistor R22 pro-
vides a reference current to the
log-averaging circuit within IC3
via pin 2. It also forms an RC
timing circuit with C5. This RC
circuit determines the response
time for the compressor. The
time constant is set so that the
compressor will respond rapidly
without distorting.
Potentiometer R51 along with
other resistors, develops the
threshold level signal. The volt-
age from the wiper of R5 1 is sent
to the threshold input of IC3
(pin 1) via R42. Another resistor
R41 across the threshold pin 1
and the control output pin 3 es-
tablishes the internal gain of
the control stage. The control
output signal from pin 3 goes to
a voltage-divider network cen-
tered around potentiometer
R53 then to the inverting VCA
input of IC3 (pin 7). A positive
voltage on this input reduces
the VCA gain, which is what we
need to make the compressor
work. Note that both the invert-
ing and non-inverting control
inputs (pins 5 and 7) are tied to
ground via R5 and R7. The con-
trol inputs must remain close to
ground potential for proper op-
eration. A 6-millivolt change in
voltage at these control pins
causes a 1 dB change in VCA
output. Diodes D6 and D7 en-
sure a unipolar control voltage.
Potentiometer R53 is the com-
pression
RATIO
control. It gives

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