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Page 6
THAT4305 Pre-trimmed Analog Engine
sion ratios, while limiters have high ratios. In such
applications, the signal path has static gain so long
as the input signal remains below some threshold,
but gain is reduced when the signal rises above the
threshold. Compression ratio is defined as the num-
ber of dB the input signal increases for a 1 dB in-
crease in output signal.
Feedforward Topologies
To make a compressor or limiter with a 4305,
typically, the input signal is applied to both the VCA
and the RMS detector. The RMS output signal is fed
forward to the VCA's negative control port (EC-) via a
dc-coupled op-amp based stage. This stage has gain
above some dc level (the threshold), and no trans-
mission below that level. This path, called the
"sidechain," — from detector output to VCA control
port — determines the compression behavior of the
circuit. As signal level rises, the dc voltage at the
RMS' output rises.
Once the dc level exceeds the
threshold, the rms output signal is transmitted
through the sidechain and presented to the VCA con-
trol port, lowering the gain to signals passing
through the VCA. As a result, the output signal level
is reduced, or compressed, relative to rising input
signal levels.
Varying the threshold setting of the sidechain will
vary the point at which compression begins. Varying
the gain between the RMS output and the VCA con-
trol input varies the compression ratio.
Feedforward compressor topologies are espe-
cially versatile because they cannot become unstable
due to oscillation in the control loop. Unity gain in
the sidechain produces infinite compression (where
the output remains constant regardless of increases
in the input signal). With feedforward, negative com-
pression ratios are easily achievable. (Negative com-
pression occurs when the output signal decreases
as the input signal increases.) This approximates the
effect of playing music backwards, since the attack
is suppressed and the release is increased in vol-
ume.
Many other variations of the feedforward concept
are possible. These include implementing more than
one threshold, different ratios, additional time con-
stants, ac-coupling of some (or all) of the detector
output signal, and many more. See AN101A, The
Mathematics of Log-Based Dynamic Processors, for
more details of how the sidechain gain determines
compression ratios.
Feedback Topologies
An alternative configuration for compressor/lim-
iter design is to feed the output signal into the RMS
detector. The RMS output is fed back (dc coupled)
to the VCA's negative control port to reduce signal
levels.
Similarly as with a feedforward designs, a
threshold in the sidechain serves to stop the com-
pression action at low signal levels.
The feedback topology behaves somewhat differ-
ently from feedforward. First, reaching infinite com-
pression requires infinite gain in the feedback loop
from RMS output to VCA control port. Of course, in-
finite gain is impossible, so practical feedback com-
pressors are usually limited to ratios no greater than
20 or so. Additionally, the gain in the feedback loop
alters the effective time constant of the detector,
shortening the attack as the ratio becomes higher.
This may or may not be appropriate, depending on
the desired effect.
Expander (Gate) Configurations
By changing the sign of the sidechain in a
feedforward compressor, it is possible to arrange
signal gain to decrease along with signal level, thus
producing an expander.
This is typically applied
below a threshold (so, the threshold detector’s po-
larity is reversed from that of a compressor) to re-
duce noise or crosstalk during pauses in program
material.
This technique has long been used for
"cleaning up" individual drum tracks to reduce re-
verberation, interference from microphones picking
up adjacent drum sounds, and alter the attack/decay
characteristic of individual drum sounds.
Practical gates usually require very fast attack
times, and carefully programmable release times. In
a 4305, this is best accomplished by using the RMS
detector as a log rectifier with very short time con-
stants, and following the detector output with a
time-constant stage that applies the desired attack
and release behavior. This alters the 4305 detector’s
natural response characteristics to peak, rather than
rms, time constants. We intend to produce an appli-
cation note showing examples of these circuits. Un-
til that is available, see DN 100, which shows a noise
gate application using THAT's 4301 Analog Engine.
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