Companding

From Academic Kids

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A waveform before and after the compression stage of non-linear companding

In telecommunication, signal processing, and thermodynamics, companding (occasionally called compansion) is a method of reducing the effects of a channel with limited dynamic range. It is a combination of compressing and expanding (the word companding therefore is a portmanteau word), and is a variant of audio level compression.

While the compression used in audio recording and the like depends on a variable gain amplifier, and so is a locally linear process (linear for short regions, but not globally), companding is non-linear and takes place in the same way at all points in time. The dynamic range of a signal is compressed before transmission and is expanded to the original value at the receiver.

The use of companding allows signals with a large dynamic range to be transmitted over facilities that have a smaller dynamic range capability. Companding reduces the noise and crosstalk levels at the receiver.

This is sometimes used in digital systems, compressing before input to a analog-to-digital converter, and then expanding after a digital-to-analog converter. This is equivalent to using a non-linear ADC. This is also used in digital file formats for better signal-to-noise ratio at lower bit rates. For example, a linearly encoded 16-bit PCM signal can be converted to an 8-bit WAV or AU file while maintaining a decent signal-to-noise ratio by compressing before the transition to 8-bit and expanding after a conversion back to 16-bit. This is effectively a form of lossy audio data compression.

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