1、BRITISH STANDARD BS 6840-8: 1988 IEC 268-8: 1973 Sound system equipment Part 8: Methods for specifying and measuring the characteristics of automatic gain control devices IEC title: Sound system equipment. Part 8: Automatic gain control devices UDC 621.396 + 681.84:621.395.61:621.317.6BS6840-8:1988
2、This British Standard, having been prepared under the directionof the Electronic Equipment Standards Committee,was published underthe authority of the BoardofBSI and comes intoeffect on 29 February 1988 BSI 07-1999 The following BSI references relate to the work on this standard: Committee reference
3、 EEL/32 Draft for comment73/24288DC ISBN 0 580 16212 5 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Electronic Equipment Standards Committee (EEL/-) to Technical Committee EEL/32, upon which the following bodies were represented: Asso
4、ciation of British Theatre Technicians Association of Sound and Communications Engineers Limited Audio Engineering Society British Educational Equipment Association British Kinematograph, Sound and Television Society British Radio and Electronic Equipment Manufacturers Association British Telecommun
5、ications plc Cable Television Association of Great Britain Consumer Standards Advisory Committee of BSI Consumers Association Federation of British Audio Independent Television Companies Association Institution of Electronic and Radio Engineers Radio Electrical and Television Retailers Association R
6、oyal Institute of British Architects Royal National Institute for the Deaf Society of Cable Television Engineers Training and Educational Systems Testing Bureau Amendments issued since publication Amd. No. Date of issue CommentsBS6840-8:1988 BSI 07-1999 i Contents Page Committees responsible Inside
7、front cover National foreword ii 1 Scope 1 2 Object 1 Section 1. Conditions for specification and measurement 3 General conditions 2 4 Rated conditions and normal working conditions 2 Section 2. Characteristics to be specified and the relevant measuring methods 5 General characteristics 2 6 Gain cur
8、ve 2 7 Reference output level 2 8 Lower AGC-limit 3 9 Reference AGC-level 3 10 Upper AGC-limit 3 11 Upper AGC-limit for programme material 3 12 Temperature limited source e.m.f. 3 13 Reference input range 3 14 Reference output range 3 15 Reference gain reduction 4 16 Reference compression ratio 4 17
9、 Maximum AGC-input range 4 18 Maximum AGC-output range 4 19 Maximum gain reduction 4 20 Attack time (gain reduction time) 4 21 Recovery time (gain restoring time) 4 22 Attack-transient ratio (gain reduction transient ratio) 5 23 Distortion in amplifiers with AGC-circuits 5 Section 3. Classification
10、of the characteristics to be specified 24 General 5 25 Classification 6 Figure 1 Gain curve (Clause 6) 7 Figure 2 Attack time (Clause 20) and recovery time (Clause 21) 8 Figure 3 Attack transient (Clause 22) 9 Publications referred to Inside back coverBS6840-8:1988 ii BSI 07-1999 National foreword T
11、his British Standard has been prepared under the direction of the Electronic Equipment Standards Committee and is identical with Publication 268-8:1973 “Sound system equipment” Part 8: “Automatic gain control devices” published by the International Electrotechnical Commission (IEC). The text of this
12、 Part of BS6840 is unchanged from the text of BS5428-7:1979 which is now withdrawn. All Parts and Sections of BS 5428 will be published as Parts of BS 6840 in order to achieve consistency of numbering between the corresponding Parts of the IECPublication and the British Standard. BS6840 consists of
13、the following Parts: Part 1: Methods for specifying and measuring general characteristics used for equipment performance (Supersedes part of BS5428-1); Part 2: Specification for general terms and calculation methods (Supersedes part of BS5428-1) 1) ; Part 3: Methods for specifying and measuring the
14、characteristics of sound system amplifiers (Supersedes BS5428-2) 1) ; Part 4: Methods for specifying and measuring the characteristics of microphones (Previously BS5428-3); Part 5: Methods for specifying and measuring the characteristics of loudspeakers (Supersedes BS5428-11 and Part11: Supplement 1
15、) 1) ; Part 6: Methods for specifying and measuring the characteristics of auxiliary passive elements (Previously BS5428-4); Part 7: Methods for specifying and measuring the characteristics of headphones and headsets (New) 1) ; Part 8: Methods for specifying and measuring the characteristics of auto
16、matic gain control devices (Previously BS5428-7); Part 9: Methods for specifying and measuring the characteristics of artificial reverberation, time delay and frequency shift equipment (Previously BS5428-8); Part 10: Specification for programme level meters (Supersedes BS5428-9) 1) ; Part 11: Specif
17、ication for application of connectors for the interconnection of sound system components (Supersedes BS5428-5.2) 1) ; Part 12: Specification for application of connectors for broadcast and similar use (Supersedes BS5428-5.3) 1) ; Part 13: Guide for listening tests on loudspeakers (New); Part 14: Gui
18、de for circular and elliptical loudspeakers: outer frame diameters and mounting dimensions (Previously BS5428-5.1); Part 15: Specification for matching values for the interconnection of sound system components (Supersedes BS5428-10) 1) ; Part 16: Guide lo the RASTI method for the objective rating of
19、 speech intelligibility in auditoria (New) 1) . Terminology and conventions. The text of the international standard has been approved as suitable for publication as a British Standard without deviation. Some terminology and certain conventions are not identical with those used in British Standards;
20、attention is drawn especially to the following. 1) In preparation.BS6840-8:1988 BSI 07-1999 iii Wherever the words “These recommendation(s)” appear, referring to this standard, they should be read as “This British Standard”. The Technical Committee has reviewed the provisions of IEC65, to which refe
21、rence is made in the text, and has decided that they are acceptable for use in conjunction with this standard. A related British Standard to IEC65:1976 is BS415:1979. NOTEIn note 1 to clause 20.2 the word “be” should be inserted after “preferably”. A British Standard does not purport to include all
22、the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-references International standard Corresponding British Standard IEC268-1:1985 BS6840
23、Sound system equipment Part 1:1987 Methods for specifying and measuring general characteristics used for equipment performance (Identical) BS5428 Methods for specifying and measuring the characteristics of sound system equipment IEC268-3:1969 a Part 2:1979 Amplifiers (Technically equivalent) IEC268-
24、10:1976 a Part 9:1981 Programme level meters (Technicallyequivalent) a These Parts of BS5428 will be withdrawn when revisions of IEC268-3 and IEC268-10 are published and implemented as Parts of BS6840. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, page
25、s 1 to 10, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.iv blankBS6840-8:1988 BSI 07-1999 1 1 Scope 1.1 These recommendations apply to autom
26、atic 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. a) Limiting action is mainly used as a safeguard to prevent overloadin
27、g of equipment such as tape recorders, power amplifiers, transmitters, etc. b) 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 s
28、ucceeding 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 no
29、t include: a) 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. b) devices in which the gain in a sound channel is controlled, not by the signal fed
30、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 ma
31、y be specified and measured in a similar way. c) 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. 2 Object 2.
32、1 These recommendations relate to the characteristics to be specified and the relevant measuring methods for automatic gain control devices. In general, the methods of measurement recommended are those which are considered to be the most directly related to the characteristics. This does not exclude
33、 the use of other methods, which will give equivalent results. 2.2 Attention is drawn to the fact that there is no defined distinction between limiters and compressors. Normally, the gain reduction curve of a limiter has a sharp bend, whereas the curves of the compressors may have a great variety of
34、 slopes depending upon the purpose for which the device is intended. The gain control circuit of a limiter is activated when the input signal has reached a value slightly below the lower AGC-limit. For a further increase of the input level, the output level will remain practically constant. The gain
35、 control circuit of a compressor is also activated at an input signal slightly below the lower AGC-limit. By further increase of the input level, the output level will rise at a lower rate than the input level until the reference AGC-level is reached. Beyond this level, the device will operate in mo
36、st cases as a limiter. The distinction between a limiter and a compressor comes more from the mode of operation than from the electrical design. With a limiter, the input level is set to such a value that the AGC-circuit only occasionally is in action. The dynamic range of the signal is scarcely inf
37、luenced by this procedure. With a compressor, the input level is set to such a value that the AGC-circuit is almost continuously operating. The amount of reduction of the dynamic range depends largely upon the recovery time. A short recovery time results in a drastic change of the ratio peak power t
38、o longtime average power. This must be borne in mind if the compressed signal is fed into devices which are sensitive to thermal overloading such as power amplifiers. 2.3 In amplifiers with more than one input channel, the automatic gain control circuit is in most cases incorporated in the summing c
39、hannel. For these amplifiers, the input signal that has the highest level in the summing channel will cause equal gain reduction for all input signals. The characteristics for such devices can be given in accordance with this recommendation. 2.4 This recommendation is to be read in conjunction with
40、the following parts of IEC Publication268, Sound System Equipment: Part 1: General; Part 3: Amplifiers.BS6840-8:1988 2 BSI 07-1999 Section 1. Conditions for specification and measurement 3 General conditions Reference is made to IEC Publication268-1, Sound System Equipment Part 1: General, as concer
41、ns: units and system of measurement; frequencies of measurement; quantities to be specified and their accuracy; marking and symbols for marking; ambient conditions; filters, networks and measuring instruments for noise specification and measurement; individual specification and type specification; g
42、raphical presentation of data; scales for graphical presentation of data; personal safety and prevention of spread of fire. 4 Rated conditions and normal working conditions For convenience in specifying how automatic gain control devices are to be set up for measurement or verification, certain sets
43、 of conditions have been defined in this recommendation under the titles of rated conditions and normal working conditions, respectively. For these conditions, reference is made to IECPublication268-3, Sound System Equipment Part 3: Sound System Amplifiers, Sub-clause 3.2 Section 2. Characteristics
44、to be specified and the relevant measuring methods 5 General characteristics 5.1 Characteristics to be specified For the characteristics to be specified and the methods of measurement for sound system amplifiers, see IEC Publication268-3. If rated conditions are referred to, this does not apply to t
45、he source e.m.f. If not stated otherwise, the AGC-circuit shall be connected. If the AGC-device is provided with controls with which its characteristics can be adapted to different kinds of use, this shall be stated. The position of these controls for which the characteristics listed below are valid
46、 shall be stated. NOTEAn AGC-circuit is normally designed to effect no clipping of the signal. If the amplifier is equipped with an additional clipping circuit, the clipping level shall be stated by the manufacturer. 6 Gain curve 6.1 Characteristic to be specified The graph showing the relation betw
47、een output voltage and source e.m.f. with the AGC-device connected and adjusted for rated conditions without changing other adjustments or connections. Unless otherwise specified, the curve shall refer to a sinusoidal test signal with a frequency of 1kHz. The curve shall have the source e.m.f. as ab
48、scissa and the output voltage as ordinate, both expressed in decibels on linear scales of identical sizes. The tolerances of the curve and the 0 dB reference level shall be specified. If the gain curve is dependent on the source impedance, the rated source impedance shall be specified (seeFigure 1,
49、page7). NOTEThe curve shall be understood to be valid for a pseudostatic variation of the source e.m.f. In the gain curve of an AGC-device, three different parts may be distinguished. Below the lower AGC-limit, the ratio between the increase in source e.m.f. and the increase in output voltage both expressed in decibels is unity. Above this limit, the curve bends down to a part having a smaller slope used for compression. This part is followed by a nearly flat part of the curve, used for limiting. The flat
