Universal Audio LA-610 Channel Strip - Manual (Page 13)

8
-30
-20
-10
0
+10
-30 -20 -10 0 +10
Input Level (dB B)
Output
Level (dB B)
Compression
Expansion
Perfect
amplifier
Figure 4 - Input/output characteristics of a compressor, an expander and a perfect
amplifier.
In contrast to an amplifier, whose job is to present a constant gain, a compressor varies its
gain in response to the level of the input signal. Large input signals result in less gain,
thus reducing or “compressing” the dynamic range of the signal. Referring again to the
line marked “compression” in Figure 4, we see that an input level of –30 dB results in an
output level of –20 dB, indicating a gain of 10 dB. Repeating this for input levels of –20
dB and –10 dB, we see that the compressor exhibits gains of 5 dB and 0 dB respectively.
From this, it is clear that the gain decreases as the input signal increases.
Referring to the diagram, we see that the compressor will increase its output level by 5
dB for every 10 dB that we increase the input level. The compression ratio is defined as
the ratio of these two numbers. In this case the compression ratio would be 10:5, which
can be reduced to 2:1.
As an aside, an expander is a device which increases the dynamic range of a signal. For
example, a 10dB change in the input signal might result in a 20 dB change in the output
signal, thus “expanding” the dynamic range.
There are several other terms related to compression that can be demonstrated by
referring to Figure 2. The amount of compression or gain reduction is typically given in
dB and is defined as the amount by which the signal level is reduced by the compressor.
Graphically, this can be understood by looking at the difference in levels between what
would have been the uncompressed (the output from an amplifier) output level and the
compressed output level. This value is what is displayed by the LA-610 meter when it is
switched to comp mode.