Companding refers to a technique for compressing and then expanding (or decompressing) an analog or digital signal. It is a combination of the words "compressing" and "expanding." This twin-sequential process is non-linear overall but linear over short periods of time. Data is compressed before being transmitted. Then, it is expanded at the receiving end using the same non-linear scale to restore it to its original form, but with reduced noise and crosstalk levels (meaning reduced disruption of, or interference with, signals in an adjacent circuit). This disruption or interference is commonly from alternating current (AC), direct current (DC) or other transmission lines. The electronic circuit responsible for companding is called the compandor. This term is also known as compansion.
Companding is used as a complement to the process of modulation and demodulation. In this process a voice signal is compressed, then changed from analog to digital, transmitted and converted back from digital to analog before it is expanded again. These processes are described in the ITU Standardization Sector for Telecommunications (ITU-T) recommendation G.711.For audio analog signals, the amplitude of weak signals is raised and the amplitude of strong signals is decreased, thereby altering (compressing and expanding) the dynamic range of the signals. The technique is used in AM, FM and single-sideband modulation radio and is helpful in improving the quality of amplified voice and musical instrument sounds. Dolby and dbx noise reduction also employ companding. Concert audio systems and noise reduction technologies such as dbx and Dolby use a triplet of amplifiers to accomplish this process, meaning a logarithmic amplifier, a variable-gain linear amplifier and an exponential amplifier.For digital audio signals, companding is used in pulse code modulation (PCM). The process involves decreasing the number of bits used to record the strongest (loudest) signals. In the digital file format, companding improves the signal-to-noise ratio at reduced bit rates. For example, a 16-bit PCM signal may be converted to an eight-bit ".wav" or ".au" file. Another application of companding involves professional wireless microphones, which have a larger dynamic range than is possible through radio transmission. By decreasing the amplitude of signals, the signals may be transmitted and then expanded at the receiver, where the original signals are reproduced by the receiving electronic equipment.
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