Originally published by Paul Stenning in Electronics - The Maplin Magazine, July 1996

• Diagrams and Schematics

Dynamic Range
The dynamic range of an audio signal is the difference between the quietest discernible part of the signal and the loudest undistorted part of the signal. It is normally expressed in decibels (dB).

With Compact Disc (CD) the possible dynamic range is often quoted at around 110dB. In practice this sort of range is rarely used on commercial recordings, but the recording medium no longer imposes a restriction to the usable range.

The dynamic range available with cassette recording depends on the quality of the tape and the equipment. The limit is usually imposed by tape hiss at the lower end and saturation at the higher end. Noise reduction systems such those designed and licensed by Dolby offer a useful increase in the dynamic range achievable, often at the expense of some higher frequency signals at lower levels.

With a cheap ferric cassette and no noise reduction system, the dynamic range can be 20dB or possibly even lower. At the other end of the scale, a high quality metal tape in a hi-fi cassette deck with Dolby S noise reduction can achieve 70dB. A typical domestic system with Dolby B will give a dynamic range of perhaps 50dB.

The range available with vinyl records depends primarily on the spacing between the grooves. With wider spacing it is possible to achieve a higher recording level because the groove width itself can be greater. The lower limit is dictated by surface noise, which is dependent on the quality of the vinyl and pressing equipment used.

This explains the popularity of 12" singles in nightclubs - the wide groove spacing allows a higher recording level to be achieved and hence an increased dynamic range, up to about 60dB. The sound quality obtained from such records on a good quality playing system is often not far removed from that on CD.

On the other hand, albums where the record company have attempted to cram as much music as possible onto each side, have a very limited dynamic range, sometimes as low as 10dB. This is sometimes referred to as "groove jamming" and was common on the compilation albums issued by companies like K-Tel and Ronco in the seventies and early eighties.

Dynamic Range Processor
The Dynamic Range Processor presented here allows the dynamic range of a signal to be increased (expansion) or decreased (compression) by a variable amount up to 3:1. The unit was developed for use when recording music from various sources.

For example, when recording CD's onto cassette for use in a car it is helpful to use some compression so that the quieter sections are not drowned out by the engine noise. Also it is useful to employ some expansion when recording tracks from poor quality records or pre-recorded cassettes. Suggestions for using this unit and processing music from various sources are given later.

Obviously a system such as this could never be truthfully described as Hi-Fi, since any form of audio effect unit, by definition, "distorts" the original signal. However the unit has been subjected to extended listening tests and the design has optimised to produce the best possible results on a wide range of music.

Circuit Description
The circuit is based on the SSM2120 dynamic range processor IC. This is available from various suppliers including Maplin and RS, and costs about £10. The IC contains two level detectors and two voltage controlled amplifiers, making it ideal for processing stereo signals. The circuit diagram for the two channels is virtually identical, so this discussion will concentrate on the left channel.