BS 6840-8:1988

National foreword
Committees responsible
1. Scope
2. Object
Section One - Conditions for specification and
              measurement
3. General conditions
4. Rated conditions and normal working conditions
Section Two - Characteristics to be specified and the
              relevant measuring methods
5. General characteristics
6. Gain curve
7. Reference output level
8. Lower AGC-limit
9. Reference AGC-level
10. Upper AGC-limit
11. Upper AGC-limit for programme material
12. Temperature limited source e.m.f.
13. Reference input range
14. Reference output range
15. Reference gain reduction
16. Reference compression ratio
17. Maximum AGC-input range
18. Maximum AGC-output range
19. Maximum gain reduction
20. Attack time (gain reduction time)
21. Recovery time (gain restoring time)
22. Attack-transient ratio (gain reduction transient
     ratio)
23. Distortion in amplifiers with AGC-circuits
Section Three - Classification of the characteristics
                to be specified
24. General
25. Classification
Figures

Devices for limiting and/or compressing properties are included, but expanders, clipping devices, and gain control using an external source are not included.

1.1 These recommendations apply to automatic gain control circuits (AGC-circuits) used for different purposes in sound systems.

1.2 These recommendations include devices which have limiting and/or compressing properties with respect to the envelope of the input signal.

1. Limiting action is mainly used as a safeguard to prevent overloading of equipment such as tape recorders, power amplifiers, transmitters, etc.
2. Compression is used to reduce the dynamic range of speech and music programmes. The object of introducing compression is to match the dynamic range of the programme material to the available signal-to-noise ratio of the succeeding transmission chain or to the difference in level between the programme sound level allowed and the ambient noise level in the surroundings where the programme is radiated.

   Compression is also used to achieve special effects when reproducing sound programmes.

1.3 These recommendations do not include:

1. expanders, as such devices for the time being are only used in highly specialized professional equipment. The definitions in this recommendation can, however, be extended to also include such devices.
2. devices in which the gain in a sound channel is controlled, not by the signal fed through the channel but from an external source, e.g. when the gain in a paging channel is varied in accordance with the ambient noise level in the listening area or for “override limiters?? where the gain in a music channel is reduced when the announcer speaks. The characteristics of such devices may be specified and measured in a similar way.
3. clipping devices, as such devices are non-linear devices used for cutting off the peaks above a certain level; such devices differ basically from limiters and compressors which, in a steady state, preserve the waveform of the input signal.