Limiters, compressors, expanders and gates are all devices which control the level of audio signals and therefore the dynamic range of a track or piece of music. They affect the loudness of a sound just as a fader or volume control does. In a dynamics device, however, the device according to parameters set up by the engineer controls the level automatically. In a traditional analogue device this would have been an electrical circuit element that would first sense the level of the signal and then adjust the output as required. Analogue compressors are still regularly used at the input stage, during recording, but in digital systems most dynamics control is now achieved through plug-ins which emulate the analogue world in terms of both performance and parameters. Below is a diagram to show how a traditional analogue dynamics unit would work.
As we noted earlier on, the useful dynamic range of the human ear is from OdB SPL (the threshold of hearing) to 115dB SPL (the threshold of pain). Digital sound equipment is now able to cover this dynamic range with 24bit recording, which has a range of 144dBs (theoretically anyway as the real limit of even the best Digital to Analogue converter is 122dB SPL of dynamic range). CD audio is a 16-bit system and has a dynamic range of 96dBs, which is more than adequate for most music and speech recording. Classical music is most renowned for having very quiet and very loud passages — a wide dynamic range. Even if reproduced in a Hi-Fi environment this would equate to about 65dBs difference between the loudest and quietest parts. Most pop and rock formats area designed to have a much smaller dynamic range since they need to be intelligible in a wide variety of situations - such as cars, factories, kitchens, railway stations etc - where ambient noise might be considerable. Under such conditions the signal must be compressed so that the quietest level is audible above the background noise without the loudness of reproduction being unacceptable for the given situation. Background music, for example, must be reproduced quietly yet should at all times be audible enough to be intelligible. The key to a successful commercial mix lies in an ability to engineer loudness and impact into a mix that is heard on a transistor radio. The limiter/compressor is a tool that makes this possible.
Common applications for automatic gain control
Protecting a system from being overloaded
In order to optimise the signal-to-noise ratio and maintain a high average record level without the fear of accidental overload, a limiter can be inserted to operate just prior to the onset of significant distortion
Changing the sound by making it either "denser (by reducing the dynamic range) or lighter (by increasing it).
Reducing background noise, "spill", cross-talk and improving the signal-to-noise ratio of analogue tape recorders and other noisy signal paths by the use of elaborate systems such as the Dolby ones.
Extreme "s" and "t" in vocals and other similar problems with the use of de-essers.
Voice-over and ducking (as used by disc-jockeys), dynamic equalisers such as the Opti-Mod for increasing loudness in radio broadcasts and a myriad of other inventive applications that we shall describe later.
Cover photo by Pedronchi