For a stand-alone compressor/limiter that only processes one signal, tracking and gain vs. Tracking is also important for noise reduction and other 'companding' applications, because expansion has to be exactly complementary to compression to restore the original dynamic range.ĭistortion is always important, but tracking only becomes an issue with stereo compressor/ limiters or expanders (aka expandors), where the relative levels of left and right (or multiple) channels must be maintained within close limits. The latter is only important for volume controls and multi-channel compressors, where the relative channel-channel balance must be preserved. The most difficult problems to solve are distortion, control voltage feedthrough and tracking from one unit to the next. They were also at the heart of many early noise reduction systems, such as those used by Dolby (over several generations of noise reduction systems), as well as simpler 'level dependent filters' that were once fairly common in consumer applications. VCAs are also used to provide mixing desk automation, noise gates, 'de-essers' (sibilance reduction), duckers (that automatically reduce background music when someone speaks into a microphone), in synthesisers and even as volume controls. All available circuits have limitations, and this article looks at the various techniques used. 10.0 Pulse Width Modulation (Switching)Ī VCA (voltage controlled amplifier/ attenuator) is the heart of any compressor or peak limiter circuit, and there are quite a few different approaches.5.2 Operational Transconductance Amplifiers.