Compressor

This page need a major make-over. The Compressor plugin now has another slider - Noise Floor - that needs to be documented. 'Normalize to 0 dB after compressing' now reads 'Make-up gain for 0 dB after compressing'. There is a checkbox for 'Compress based on peaks'. The blue line in the graph is now pinned to the upper-right instead of the lower-left. This is confusing for audio engineers as they're used to seeing compressor graphs with the curve pinned to the lower-left. The old graph was better in that regard. The new graph display is, I suppose, appropriate when the 'Make-up gain' option is chosen. It would be nice if the graph would change where it's pinned based on that option.

• Gale: I will raise this on -devel list - I don't recall any reason for the pinning axis changing, but I will ask. I agree Cool Edit Pro pins as you suggest.

All that aside, I guess we need to document what the Compressor currently does and how it behaves. I may have a go at describing how the graph works and what it means, after I un-learn 35 years of using compressors.
Does anybody know what the 'Noise Floor' slider does? - Bill W

• Gale: I had asked the contributor who made these changes to document them. I have some very basic information I can add based on the -devel list discussions. I think it's better I do that and you ask questions after that if you have suggestions for improvement.
  

Bill: I've gone ahead and documented this, as it seems unlikely to be fixed in time for 2.0.

Compressor reduces the volume of louder sounds while leaving quieter ones untouched, so reducing the dynamic range of the audio. One of the main purposes of reducing dynamic range is to permit the audio to be amplified further without clipping than would be otherwise possible. Therefore by default Compressor amplifies audio as much as possible after compression. The resultant increase in average or RMS level can be useful for audio played in a noisy background, such as in a car stereo, or in speech, to make a distant voice sound as loud as a close one.

Accessed by: Effect > Compressor...

ToDo Old text "Compressing the louder sounds gives more headroom for amplifying the fainter sounds without causing clipping" and "Because the gain changes relatively slowly, a compressor does not distort the signal in the way that a Limiter or clipping would do." should be worked in or not, as thought best, and section written for graph describing what the axes mean. Then probably work in the two images at the bottom. Gale

Graph

The graph shows the input level along the bottom (x-axis) and the output level along the left (y-axis). Note that the graph always goes through the (0,0) point at the upper left - this is consistent with the "Make-up gain for 0 dB after compressing" option. The graph will change as you adjust the sliders.

Controls

• Threshold: Sounds above this threshold will cause the compressor to reduce the level of the audio. It sets the horizontal position of the corner on the graph.
• Noise Floor: Sets the level below which make-up gain will not be applied. This prevents the compressor from amplifying noise.
• Ratio: The amount of compression applied to the audio once it passes the threshold level. The higher the Ratio the more the loud parts of the audio will be turned down. The Ratio sets the slope of the graph above the corner (above the yellow horizontal line).
• Attack Time: How soon the compressor starts to reduce the volume level after it rises above the threshold. If volume changes are slow, you can push this to a high value. Short attack times will result in a fast response to sudden, loud sounds, but will make the changes in volume much more obvious to listeners.
• Decay Time: How soon the compressor starts to increase the volume level back to normal after the level drops below the threshold. A long time value will tend to lose quiet sounds that come after loud ones, but will avoid the volume being raised too much during short quiet sections like pauses in speech.
• Make-up gain for 0 dB after compressing: Boost the resultant audio after compression equally at all volume levels, to be as loud as possible without distortion. Because the compressor works by making loud sections quieter, you will usually want to do this. If this option is selected, the effect will be both to make the quiet sections of the audio louder, and increase the overall loudness.
• Compress based on Peaks: The threshold setting will react to the peak values of the waveform rather than the average value.

When "Compress based on Peaks" is checked, make-up gain is always applied regardless of the setting of the make-up gain checkbox.

Schematic example

Uncompressed: A simple sine wave that drops off by 6 dB at half time, to demonstrate how some compressors handle signals.
After: The attack part of where the compressor is working is clearly visible at the start of the audio.The release part still affects some audio that is beneath the threshold as the compressor gain change slowly ebbs out and the material fades back to normal level.