Compressor

Bill: 28Nov09 Ready for Proofreading

The Compressor effect reduces the dynamic range of 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 the 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 environment such as in a car, or in speech, to make a distant voice sound as loud as a close one. Because the gain changes relatively slowly, a compressor does not distort the signal in the way that a Limiter or clipping would do.

Accessed by: Effect > Compressor...

Graph

The graph shows the input level along the bottom (horizontal axis) and the output level scale on the left (vertical axis) to illustrate the dynamic range compression effect. The graph will change as you adjust the Threshold and Ratio sliders, reflecting the settings made. The actual characteristics of the effect is also dependent on the other settings (not shown in the graph).

Controls

• Threshold: Compression is applied to sounds above this level.

• Noise Floor: This setting helps to prevent low level sound (background noise below this level) from being unduly amplified as a result of the processing.
• 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 compressed. The Ratio sets the slope of the blue line on the graph above the threshold.

• Attack Time: How soon the compressor starts to compress the dynamics after the threshold is exceeded. 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 release 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: Boosts the resultant audio after compression to a peak level of 0dB.

• Compress based on Peaks: The threshold and gain adjustment are based on peak values of the waveform rather than the average (RMS) value. The method used for compressing dynamics is different when this option is selected. When not selected, the compressor reduces the volume of louder sounds while leaving quieter ones untouched, but when "compress based on peaks" is selected, quieter sounds are amplified more than loud sounds. The result in both cases is to reduce the dynamic range of sounds above the threshold.

The illustration below compares characteristics of compression based on peak (red) and RMS (blue) levels (before make-up gain).

ToDo Gale: haven't followed the discussions on this, but needs explaining why RMS threshold line is not at -18 dB. How was the graph produced? The Audacity compressor graph can't produce an input/output at -60 dB with a -18 dB threshold.

Steve the Fiddle As is indicated on the graph, the x axis shows peak dB. A waveform that has an RMS level of -18dB will have a peak level somewhat higher than -18dB (as is indicated on the graph). The exact peak dB level is dependent on the waveform but for a sine wave is approximately 3dB higher than the RMS level.

When using RMS compression, the compression begins at a level several dB higher than the peak level that the user can observe. This impacts on the user and so has been included in the illustration.

For example, if the compressor threshold is set to -18dB and the effect is set to compress based on RMS, the effect on a sine wave will be that compression does not start to occur until the RMS level reaches -18dB which is equivalent to a peak level of -15dB. For square waves the peak and RMS levels are equal. For white noise the RMS level is approximately 4.5dB below the peak level. For white noise, while the compression begins when the RMS level reaches -18dB, the peak level will be around 13.5dB

The graph, as it says in the text directly above the graph, is an illustration. It does not represent the graph display in the compressor GUI, which as explained in the previous section is not an accurate plot of the actual input/output levels. The graph that is displayed in the GUI is based only on the Threshold and Ratio slider settings. It does not take account of any other settings such as make-up gain or if RMS based compression is selected (in which case downward compression is used).

Looking at this again I can see that the graphs are not quite correct. I'll have another go at this. Complete accuracy can not be attained with a simple 2 axis graph because the actual output is also dependant on the Noise floor setting and the time domain so we are just trying to indicate a general expectation of what will occur without getting bogged down in too much technical detail. (and the precise working of this compressor are very complex with peculiarities unknown in any other compressor).

Schematic example

Uncompressed: A simple sine wave that drops off by 6 dB half way through the selection, 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.