Companding

Virtually all wireless microphone systems use some form of companding to reduce noise. Low-level radio frequency interference and ambient RF noise introduce excess audio noise into wireless transmissions. Government regulations and the limitations of available circuitry also restrict the maximum signal-to-noise ratio (SNR) that can be achieved. Without companding, sometimes also referred to as compansion, the audio signal-to-noise ratio of wireless systems would be only 60-80 dB, too low for most professional applications. When companding is employed, SNRs of 100 dB or more are possible.

The term "companding" is a combination of the words "compressing" and "expanding" ("compansion" is a combination of "compression" and "expansion"). In a wireless system with companding, the audio signal is compressed in the transmitter and expanded in the receiver. The compression process reduces the dynamic range of the audio before it is transmitted. The original dynamic range of 100 dB or so is reduced to 50 dB in a "2:1" compressor. In the wireless receiver, a "1:2" expander increases the dynamic range of the received audio from 50 dB back to the original 100 dB.

The purpose of the companding process is to preserve the signal-to-noise ratio of the original audio. If the audio had been transmitted with its full original dynamic range, the SNR would have lowered to roughly 60 dB to 80 dB by the noise introduced by wireless transmission. However, because the dynamic range of the transmitted audio is reduced to only 50 dB, it is nearly unaffected by the transmission noise.

That is, the noise introduced by radio transmission is 20-40 dB higher than that of the original audio. With companding, however, the transmission noise is 10-30 dB lower than that of the compressed audio. When the audio in the receiver is expanded, the transmission noise is expanded along with the audio. Audio components that are 50 dB below full output will be expanded to 100 dB below full output, while transmission noise that is 60 dB below full output will theoretically be expanded to 120 dB below full output. However, 120 dB below full output is below both the noise in the original audio and the residual noise of the circuitry. Thus, the transmission noise will effectively have been eliminated.

Companding is also effective in reducing buzz, hiss and low-level audio tones caused by mild interference. For example, an audio tone only 50 dB below full output would be highly objectionable. With companding, the tone would end up 100 dB below full output, and while faintly audible, would likely not be objectionable.

Companding can improve the useful range of wireless systems as well. As range increases, the audio from a wireless system gradually becomes noisy. The SNR of a typical wireless system would drop to 80 dB at relatively short ranges without companding. With companding, the SNR of the compressed audio can drop to 40 dB before the output SNR falls to 80 dB. This represents anywhere from a roughly two-to-one to a ten-to-one improvement in range depending upon conditions.

Unfortunately, the companding process is not perfect. Compression and expansion of the audio unavoidably introduce small imperfections. In well-designed systems, these imperfections are barely noticeable even with careful listening. However, the quality of the audio is highly dependent upon the specific companding circuitry employed, the precision of the circuit components and the care with which the system is designed and manufactured. Because these factors do not appear on the product specifications, it is essential to listen carefully to the audio when evaluating wireless systems. There can be huge differences in the quality of audio from systems with similar specifications.

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