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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 dy-
namic 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 that 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.
As mentioned previously, the compression ratio is defined as the ratio of the increase of the level of the input
signal to the increase in the level of the output signal. In this example, the input level is increased by 10 dB
while the output level only increases 5 dB. This would be a compression ratio of 2:1. Lower ratios such as 2:1
result in more gentle compression. (Note that a compression ratio of 1:1 is no compression at all).
Typically, compressors let you choose a threshold. This is the point at which gain reduction starts to take
place. When an audio signal is below this threshold the compressor acts like an amplifier and there is no gain
reduction. Above the threshold the slope becomes less than 45 degrees, indicating gain reduction and hence
compression.
The point at which a compressor transitions into compression is commonly called the knee. In practical com-
pressors, this transition is gentler than what is depicted in the diagram.
Many modern compressors provide a control that adjusts the threshold directly. In the case of the LA-3A, the
Peak Reduction knob controls both the threshold and the amount of compression.
INPUT/OUTPUT CURVE OF A COMPRESSOR WITH A RATIO OF 2:1 AND A THRESHOLD OF -20 DB.
