Universal Audio LA-610 Channel Strip - Manual (Page 14)

9
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 compressors, this transition is gentler than what is depicted in the
diagram.
Many modern compressors provide a control which adjusts the threshold directly. In the
case of the LA-610, the Peak Reduction knob controls both the threshold and the amount
of compression.
-30
-20
-10
0
+10
-30 -20 -10 0 +10
Input Level (dB B)
Output
Level (dB B)
Compression regioon
knee
2:1 Compressiion
10 dB of
compression
Figure 5 : Input/output curve of a compressor with a ratio of 2:1 and a threshold of -20
dB.
Gain Reduction Circuits
A brief overview of the operation will be provided here. The input transformer provides
isolation and impedance matching. After this the signal is fed into both the side-chain
circuit and the gain reduction circuit. The side-chain is comprised of a voltage amplifier,
a pre-emphasis filter, and a driver stage which provides the voltage necessary to drive the