Audio Noise Reduction Systems

The upper end of the dynamic range of an audio system is normally the level at which distortion occurs because of overload. The lower end of the range is set by noise: tape hiss, ‘thermal’ noise in electronic components, mains hum, and so on.

The effects of such noise can be reduced very significantly by noise reduction systems, which are commonly used, particularly in analogue recording. (Digital systems are almost totally unaffected by the noise problems mentioned above.) Most noise reduction systems make use of some form of companders—that is, the dynamic range is reduced before recording or transmission etc. and expanded afterwards. Unfortunately simple compressors and expanders generally produce audible side effects. The more successful arrangements manage to avoid these effects.

Dolby A

This was the original system produced by the Dolby organisation and was intended to reduce the effects of noise on professional tape recording and also on long-distance audio transmission systems. The increased use of digital recording and transmission has made Dolby A less important although it is still widely used in multitrack analogue machines. A brief account of it is worth giving as the other Dolby systems are based on it. The essential point is that it operates only on low-level signals, the argument being that high-level signals mask the effects of noise. Also it divides the audio frequency range into four bands which work independently of each other. This means that, for example, reasonably high-level low-frequency signals are not processed when the higher frequency end does contain low levels and is therefore processed. The Dolby A frequency bands are shown on the opposite page.

Because the Dolby A system operates only on low-level signals, line-up of ‘send’ and ‘receive’ (e.g. record and replay) is rather critical.

Other systems

● Dolby B. A low-cost method of reducing tape hiss on cassettes. It operates only on the higher frequencies and can result in about 10 dB of reduction of tape hiss.

● Dolby C. A more advanced form of Dolby B. It operates on two frequency bands and can give about 20 dB of noise reduction. It is frequently found on professional and the better domestic cassette decks.

● Dolby SR (‘SR’ = spectral recording). This is relatively recent. It is claimed that recording levels can be increased by 10 dB and there is around 24 dB reduction in tape noise.

● dbx. This consists of a compressor-expander pair working with a compression ratio of 2:1. It can provide up to 24 dB of noise reduction and does not need careful line-up. What noise does get through, though, is likely to vary with programme level and might in some circumstances be noticeable.

● Telcom c4. This is a sort of combination of dbx and Dolby A. It can give about 15 dB of noise reduction and does not need accurate line-up. It is relatively expensive.

A simple companding system