Compressor

Compressor reduces the volume of louder sounds while leaving quieter ones untouched, so reducing the dynamic range of the audio (the difference between loud and soft). By default, the result is then amplified equally at all volume levels so that it sounds as loud as it can be without clipping. The sound is then perceived as louder than before, because compression has increased its average or RMS volume level. This can be useful for audio played in a noisy background, such as in a car stereo, or in speech for making a voice that sounds distant as loud as one that sounds much closer.

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

Controls

• Threshold: Sounds below this threshold will be left unchanged. Sounds above this threshold will cause the compressor to reduce the level of the audio. It sets the vertical position of the corner on the graph (the yellow horizontal line).??
• Ratio: The amount of compression applied to the audio once it passes 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. ///??define gain???///. If volume changes are slow, you can push this to a high value. Short attack times will result in a fast responses to sudden, loud sounds, but will make the changes in volume much more obvious to listeners.
• Decay Time: How soon the compressor starts to reduce the gain back to normal after the volume level drops below the threshold. A long time here will tend to loose quiet sounds that come after loud ones, but will avoid the gain being raised during short quite sections like pauses in speech.
• Normalize to 0 dB after compressing: Boost the entire signal after compression 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 overall effect will be to make the quiet sections of the audio louder. One of the main uses of compression is to make it possible to amplify the signal, so the whole recording sounds louder.///

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.