Talk:Compressor

Bill 28Nov09: I'd like to keeps these graphs and discussion here for a while. False starts, old page text, and text that made it to the main page have been removed.

When "Compress based on peaks" is not checked
The following image shows the input-output characteristic of the Compressor effect when "Compress based on peaks" is not checked.
 * The light blue line shows the characteristic when "Make-up gain for 0 dB after compressing" is not checked.
 * In this mode the Compressor effect behaves like a traditional hardware compressor.


 * The dark blue line shows the characteristic when "Make-up gain for 0 dB after compressing" is checked, and corresponds to the line shown on the graph in the Compressor dialog.
 * In this mode the Compressor effect behaves like a traditional hardware compressor with a "smart" output gain control.

Note that the line from the threshold point to the left of the graph always has a slope of 1.
 * [[image:CompressorIOgraphRMS.png]]

This is implied by the graph on the effect, but the graph is wrong. After make up gain to 0dB has been applied the slope below the threshold is no longer 1. This is easy to see when you consider the case of silence (-inf dB) - both input and output are silence. Steve the Fiddle Bill 29Nov09: Steve, try this. Generate chirp 440 Hz, 5 secs, 0.01 to 0.1 (-40 to -20 dB). Duplicate track. Apply Compressor to first track Threshold -26 dB, Ratio 4:1, RMS, make-up gain ON. Apply Compressor to second trach same except make-up gain OFF, then Normalize. They are virtually the same. And the bit at the beginning that started at -40 dB is now at about -20 dB.

When "Compress based on peaks" is checked
The following image shows the input-output characteristic of the Compressor effect when "Compress based on peaks" is checked.
 * The red line shows the characteristic when "Make-up gain for 0 dB after compressing" is not checked.
 * The orange line shows the characteristic when "Make-up gain for 0 dB after compressing" is checked and the input signal had a maximum peak value of less than 0 dB.

Note that the line from the threshold point to the left of the graph no longer has a slope of 1.

When "Compress based on peaks" is checked the Compressor effect applies "upward compression" to the input. This is very different from the behaviour of a traditional hardware compressor. Note that when the volume of the input is below the threshold the volume is "stretched" to fit the new post-compression range. Sounds above the threshold still receive downward compression. The result is that very quiet sounds are not significantly amplified.


 * [[image:CompressorIOgraphPeak.png]]

Here's my suggestion: Steve the Fiddle

Then Steve's suggestion made it to the main page unaltered.

Old page text removed.

Steve the Fiddle Compression graph on page updated. I believe that it is finally correct. Perhaps a bit large - I have the original SVG file so can easily provide a resized version if required.