Once you've built it, you probably want to know whether it is compressing properly, and to what degree. This can be done using a fairly simple procedure.
The compression ratio describes the ratio of the amount of change between the input and output signals after the input signal has crossed a particular threshold.
The input control really acts as a threshold control, the higher it is (more CW) the lower the threshold before compression takes effect.
The output control, on the other hand, is used for makeup gain following compression.
First we set the input control to midway. Set the release control to fully CW and the attack control to 'off' (fully CCW).
Now feed a 0dBu signal into the input and adjust the output control until 0dB is measured at the output.
Now we are going to perform a measurement of the output for each of the ratio positions for a number of of input signal levels. This should allow us to plot a curve of compression (input vs output) and see where the threshold was set to and what the ratio settings actually are.
Switch the attack pot on to enable the GR circuitry. Start at the lowest ratio setting and measure the output signal level. Perform this measurement for each ratio setting, keeping the input level constant and not touching the input or output controls.
Adjust the input signal level down by 3dB steps and repeat this measurement down to about -15dB, or until there is no appreciable signal level change when compression is turned on (this means our signal level is now below the threshold so no compression is taking place). Remember to allow 10 seconds or so for the output level to settle after switching compression ratios.
You should be able to plot this and see a set of gain reduction curves and the threshold where the ratio returns to 1:1. If you use the attached spreadsheet simply fill in the green area with your measurements (in dB). You will also probably notice the soft knee as the threshold is approached.
To measure the ratio (also in the spreadsheet) I have taken the first 6dB of gain reduction (to avoid the nonlinearities as the compression knee softens) and calculated the relative change in output for a corresponding relative change in input.
Here's a curve for one of the channels of my unit.
The ratios of this channel work out as: