Dynamic Range Control
The process of automatically adjusting the level of audio is called dynamic range control and is used in many different applications, including television, radio, music production, live music, etc. It is called dynamic range control because the ratio between the largest (i.e., loudest) and smallest (i.e., faintest) sound is in effect controlled in the process. For example, if faint sounds are amplified more than loud sounds, then the dynamic range has been decreased. This is, in fact, what a compressor does. The basic principle of dynamic range control is that the level of the audio signal is measured every so often and then a gain is applied depending on the measured level. This gain typically also depends on what the gain was previously so as to get a smooth transition from one gain to another. In this chapter, we will present some common dynamic range control algorithms, namely, the compressor, expander, limiter, and gate. We will also look into how the level can be measured in different ways and how the gain can be applied in a way that sounds good.
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