better yet read a manual for any compressor on the sw or hw market (easy enough to obtain)
however the nuts and bolts are easy enough to identify
- threshold: the signal level at which the compressor starts to work
- attack time: how long after the signal crosses the threshold the compressor starts to work
- release time: how long after the signal falls below the threshold the compressor stops working
- ratio: two numbers X:Y (X in the ratio to Y - mathematically this means a fraction is applied, see below)
- left hand number is how many dB of signal goes into the compressor
- right hand number is how many dB of signal comes out as a result of the compressor working
when we put a signal into a compressor it can be above, at or below the threshold. If above the threshold then it begins to act by subtracting the threshold from the signal then reducing the gain of the signal by the ratio
signal in -> threshold -> above: compress difference signal by ratio; at or below: let signal pass through unchanged -> output (compressed signal)
best way to understand is
if the signal is above the threshold then output = threshold+(difference * Y/X),
otherwise output = input;
where difference is the amount of signal above the threshold i.e. signal level-threshold level
(e.g a threshold of -20dB results in a difference of 1dB if the signal level is -19dB i.e. -19dB--20dB = -19dB+20dB = 1dB)
e.g.
in each of the 3 following examples the compressor has threshold of -20dBfs
example 1
ratio is set at 2:1 which means for each rise of 2dB above the threshold we only get a rise of 1dB out
input | difference | compressed | output |
---|
-30dBfs | -10dBfs | - | -30.00dBfs |
-20dBfs | 0dBfs | - | -20.00dBfs |
-19dBfs | 1dBfs | 0.50dBfs | -19.50dBfs |
-18dBfs | 2dBfs | 1.00dBfs | -19.00dBfs |
-17dBfs | 3dBfs | 1.50dBfs | -18.50dBfs |
-16dBfs | 4dBfs | 2.00dBfs | -18.00dBfs |
-15dBfs | 5dBfs | 2.50dBfs | -17.50dBfs |
-14dBfs | 6dBfs | 3.00dBfs | -17.00dBfs |
-13dBfs | 7dBfs | 3.50dBfs | -16.50dBfs |
-12dBfs | 8dBfs | 4.00dBfs | -16.00dBfs |
-11dBfs | 9dBfs | 4.50dBfs | -15.50dBfs |
example 2
ratio is set at 4:1 which means for each rise of 4dB above the threshold we only get a rise of 1dB out
input | difference | compressed | output |
---|
-30dBfs | -10dBfs | - | -30.00dBfs |
-20dBfs | 0dBfs | - | -20.00dBfs |
-19dBfs | 1dBfs | 0.25dBfs | -19.75dBfs |
-18dBfs | 2dBfs | 0.50dBfs | -19.50dBfs |
-17dBfs | 3dBfs | 0.75dBfs | -19.25dBfs |
-16dBfs | 4dBfs | 1.00dBfs | -19.00dBfs |
-15dBfs | 5dBfs | 1.25dBfs | -18.75dBfs |
-14dBfs | 6dBfs | 1.50dBfs | -18.50dBfs |
-13dBfs | 7dBfs | 1.75dBfs | -18.25dBfs |
-12dBfs | 8dBfs | 2.00dBfs | -18.00dBfs |
-11dBfs | 9dBfs | 2.25dBfs | -17.75dBfs |
example 3
ratio is set at 10:1 which means for each rise of 10dB above the threshold we only get a rise of 1dB out
input | difference | compressed | output |
---|
-30dBfs | -10dBfs | - | -30.00dBfs |
-20dBfs | 0dBfs | - | -20.00dBfs |
-19dBfs | 1dBfs | 0.10dBfs | -19.90dBfs |
-18dBfs | 2dBfs | 0.20dBfs | -19.80dBfs |
-17dBfs | 3dBfs | 0.30dBfs | -19.70dBfs |
-16dBfs | 4dBfs | 0.40dBfs | -19.60dBfs |
-15dBfs | 5dBfs | 0.50dBfs | -19.50dBfs |
-14dBfs | 6dBfs | 0.60dBfs | -19.40dBfs |
-13dBfs | 7dBfs | 0.70dBfs | -19.30dBfs |
-12dBfs | 8dBfs | 0.80dBfs | -19.20dBfs |
-11dBfs | 9dBfs | 0.90dBfs | -19.10dBfs |