1、CCIR VOLUME*X-3 90 W 4855232 0503992 T Rec. 642-1 1 I5 SECTION 10B: FREQUENCY-MODULATION SOUND BROADCASTING .IN BANDS 8 (VHF) AND 9 (UHF) RECOMMENDATION 642-1 LIMITERS FOR HIGH-QUALITY SOUND-PROGRAMME SIGNALS (Questions 46/10, 19/CMT and 2O/CMT, Study Programme 46B/10) (1986-1990) The CCIR, CONSIDER
2、ING (a) to other transmissions; (b) may be raised by the application of pre-emphasis to the modulating signal; (c) lines); (d) that over-modulation of FM transmitters can cause distorsion of the programme material and interference that the level of some sound-signal components (most commonly those a
3、t the higher audio frequencies) that techniques exist to design low-distortion limiters without overshoot (for example, by the use of delay it is generally undesirable to subject sound signals to more than one limiting process, UNANIMOUSLY RECOMMENDS 1. more efficient use to be made of the available
4、 dynamic range; that low-distortion limiters should be employed to protect transmitters against over-modulation, and enable 2. this is described in Annexes I and II. that if pre-emphasis is applied, the limiter should take account of this. An example of a limiter which does 3. that the limiter be si
5、tuated at the interfaces between studios and sound-programme distribution circuits, so that each limiter may serve a relatively large number of transmitters and that in principle there may be no need to employ any subsequent limiting. Limiters so positioned may also provide a usefuld degree of prote
6、ction against overload for the sound-programme circuits which follow. ANNEX I VARIABLE-EMPHASIS LIMITERS Limiters are commonly employed at the programme inputs of frequency modulated sound-programme transmitters in order that the carrier deviation, and hence the signal-to-noise ratio at the receiver
7、 may be kept as high as is practicable, whilst avoiding over-deviation and the consequent risk of audible distortion or of causing interference. Similarly, limiters may be used at the sending ends of point-to-point sound-programme transmission circuits, analogue or digital, so that the signal-to-no
8、ise ratios at the receiving ends are optimized, by permitting the signal levels on the circuits to be kept high without risk of distortion caused by over-loading. In the above cases, the sound signals are commonly subjected to high-frequency pre-emphasis. However, programme levels are normally contr
9、olled using a meter (see Report 292) at a point in the circuit which is not subject to pre-emphasis. In consequence high-level, high-frequency, components are likely to cause overmodula- tion - even on properly controlled programmes - unless a limiter is employed to prevent this. CCIR VOLUME*X-1 90
10、M 4855212 0503993 L M 116 Rec. 642-1 In a conventional limiter, gain variations affect all sound signals equally, and limiter action brought about by high-amplitude high-frequency components may cause obvious and objectionable level fluctuations of low- and medium-frequency components in the reprodu
11、ced programme. This effect, commonly -known as “gain ducking” or “limiter-cut-back”, may be reduced by allowing a wide margin between the nominal maximum programme peaks and the limiting level. Such a practice is considered to be undesirable, as the listening signal-to-noise ratio would then be less
12、 than it otherwise could be. Tests have shown Manson, 1973, 19751 that the need for this wide margin may be avoided by the employment of a frequency-selective form of limiter. REFERENCES MANSON, W. I. I9731 Frequency-dependent limiters for f.m. sound transmitters. BBC Research Department Report 1973
13、/25. MANSON, W. I. 1975 Frequency dependent limiters for f.m. sound broadcasting: optimisation of attack period and of delay-1ength.h the variable de-emphasis stage. BBC Research Department Report 1975/22. ANNEX II DESCRIPTION OF A VARIABLE-EMPHASIS LIMITER FOR HIGH-QUALITY SOUND-PROGRAMME SIGNALS A
14、 block diagram illustrating one form of a variable-emphasis limiter which has been found to be suitable for use on 50 ys pre-emphasized signals (a different value of pre-emphasis time constant may be used if appropriate) is shown in Fig. 1. A A / B C D _CI L - - / dl Il Il Il G H J M - - N -+i- optimisation of attack period and of SHORTER, D. E. L., MANSON, W. I. and STEBBINGS, D. W. I9671 The dynamic characteristics of limiters for sound- delay-length in the variable de-emphasis stage. BBC Research Department Report 1975/22. programme circuits. BBC Engineering Division Monograph No. 70.
