CX335 User Manual v1.0
10
Operating principles
Compression
Compression is an audio process that progressively reduces
the dynamic range of programme material once the signal
level has risen above a pre-set threshold. No treatment is
applied to signals below the threshold; the output of the
compressor will be the same as the input, and any change in
signal level will be reflected accurately at the output. If the
signal level exceeds the threshold, compression is applied;
the result of this is that changes in the input signal level are
reduced by an amount called the “compression ratio”, and
the overall output signal does not reach the same level it
would have done without compression.
Limiting is “extreme” compression; here, once the threshold
is exceeded, the output level will not rise any further.
Effectively, any compressor with its ‘ratio’ set higher than 20
to 1 can be considered to be a limiter.
These points are best illustrated graphically; the curve below
shows the “transfer characteristic” of a basic compressor/
limiter. (The numbers are artificial and have been chosen
simply to demonstrate the principle.)
The graph shows the action of a compressor with the
threshold set to -30 dB and the ratio set to 2:1. The
horizontal (x-) axis represents input signal level and the
vertical (y-) axis corresponding output signal levels. By
tracing vertically from any point on the x-axis to the transfer
characteristic, and then horizontally to the y-axis, we can see
what the output signal level will be for any input signal level.
Consider a quiet part of the programme material input signal
which only peaks at -35 dB (Input Level A); this is below the
threshold. All signals below the threshold are unaffected by
the compressor’s action, and this is indicated by the transfer
characteristic being 45° to both axes. Thus the output signal
level will be exactly the same as the input signal level: -35 dB.
Now consider a louder passage; one that peaks at -10 dB
(Input level B). This is 15 dB above the threshold, and so
compression is applied. However above the threshold the
transfer characteristic is closer to the horizontal, so the
corresponding output signal level in this case is only -22.5
dB. This is because the 15 dB of dynamic range above the
threshold is halved, by virtue of the 2:1 compression ratio.
(If the compression ratio is 3:1, then the output level will
be only 15/3 dB = 5 dB above the threshold, -25 dB). The
audible result is that the louder passages in the programme
material are less loud and the overall output level is lower.
If the compression ratio is increased further – to 20:1, say,
the transfer characteristic above the threshold becomes
virtually parallel to the x-axis. This means that no matter
how high the input signal becomes, the output level never
rises above the pre-set threshold. This principle can clearly
be used to provide excellent protection for the amplifiers
further along the signal chain, but the downside is that the
audible effect of extreme limiting can be rather tiring.
The examples given above all ignore the issue of gain
make-up. As the ultimate effect of compression will be to
reduce the volume of the sound that comes out of the
loudspeakers, most compressors include a gain control stage
after the compressor itself to compensate for any alteration
in overall level which is the consequence of compression. On
the CX335, this is the front panel GAIN control, which has a
range of ±12 dB.
Hard/Soft Knee
The transfer characteristic shown in the example above uses
a “hard knee”; the knee is the point in the characteristic
at the threshold itself. With a “soft knee” compressor,
the compression ratio is increased gradually from slightly
below the threshold to slightly above it, until the set ratio is
achieved. This is illustrated in the diagram below:
The use of a soft knee generally makes the audible effect of
compression on music somewhat less “aggressive”.
Normal/Slow Envelope
The rate at which a compressor takes effect once the
threshold is exceeded is called the
attack time
; similarly the
rate at which the compressor action ceases after the input
signal falls below the threshold is called the
release time
.
A compressor with the wrong attack time will sound bad,
