Dynamic Range Control Technology
The dynamic range of an audio system is the range of input variation, from the lowest to highest signal levels, that can be faithfully reproduced.
Dynamic range is limited at the low signal end by random circuit noise in analog systems, or quantization noise in digital systems. In either case, beyond a certain point, small signals are overwhelmed by noise.
Dynamic range is limited at the high signal end of the range by the voltage and/or current limitations of an analog circuit, or by the maximum numeric value in a digital system. In either case, if the dynamic range is exceeded at the upper limit, the signal peaks are clipped, resulting in highly objectionable distortion.
A linear system applies uniform gain (the ratio of output level to input level) regardless of signal levels. If the system gain is 2, for example, a 10 millivolt input signal will produce a 20 millivolt output signal, or a 2.5 volt input signal will produce a 5 volt output signal.
Dynamic range control is accomplished by varying the system gain in accordance with input signal level, so that quiet signals are subject to different gain than loud signals.
Compression & Expansion
Compression reduces the dynamic range of the signal, lifting the level of softer passages and/or lowering the level of louder passages. Compression can be used to help prevent softer passages from being overwhelmed by ambient noise in the environment, to help prevent loud material from causing distortion, or simply to increase average signal energy.
While less frequently used, expansion increases the dynamic range of the signal such that softer passages are further reduced in level and/or louder passages are further increased. This can restore dynamic interest to previously-compressed material, or help to reduce noise that is inherent in the signal itself.
Limiting is essentially a severe form of compression that is especially valuable for preventing distortion. In this application, a limiter is configured such that the very highest signal levels are subjected to a significant gain reduction, preventing signal peaks from exceeding the limits of the system.