1、CCIR VOLUMEUX-3 90 4855232 0503947 5 Rec. 639 69 RECOMMENDATION 639 * NECESSARY BANDWIDTH OF EMISSION IN LF, MF AND HF BROADCASTING * The CCIR, (Question 44/10, Study Programme 44A/10) (1986) CONSIDERING (a) that in amplitude modulated double-sideband (AM-DSB) sound broadcasting, the bandwidth of em
2、ission is twice the audio-frequency (AF) bandwidth; (b) that for quality reasons, the AF bandwidth should be as high as possible; (c) that adjacent-channel interference is determined by, among other factors, the bandwidth of the modulating signal, and that some sound processing of the audio programm
3、e may significantly increase the higher frequency audio components; (d) that the bandwidth of the complete AM-DSB transmission system (system bandwidth) is determined by the combined effect of the bandwidth of emission and the receiver bandwidth; (e) that in most practical cases the bandwidth of emi
4、ssion considerably exceeds the receiver bandwidth, although receivers with wider or double bandwidths are becoming more prevalent in some parts of the world; (f) that, among other factors, efficiency of spectrum utilization is affected by the carrier spacing and also by the necessary bandwidth of em
5、ission; (g) that adjacent-channel interference decreases in areas relatively close to the wanted transmitter, where, for a transmitter of medium to low power, a larger concentration of audience can usually be presumed: (h) that where adjacent-channel interference is minimized by appropriate geograph
6、ical spacing of stations, some advantage can be taken of bandwidths of emission significantly greater than the channel spacing, thus increasing system bandwidth, particularly where receivers of wider bandwidth are employed, UNANIMOUSLY RECOMMENDS that where required, either for optimizing spectrum u
7、tilization or for providing an improved overall system AF response, the overall system can be optimized and planning problems can be reduced by taking advantage of the existing knowledge of the interrelation between system bandwidth, channel spacing and adjacent-channel protection ratio, as given in
8、 Annex I. ANNEX I NECESSARY BANDWIDTH OF EMISSION IN LF, MF, AND HF SOUND BROADCASTING 1. Introduction In an amplitude-modulation double-sideband sound broadcasting system the bandwidth of emission is approximately twice the audio-frequency bandwidth of the programme and, therefore, greatly influenc
9、es the quality of reception. On the other hand, for a given frequency separation between adjacent channels, a limitation of the bandwidth of emission is desirable to avoid mutual interference. The difference between the transmitted bandwidth for amplitude-modulation sound broadcasting and the receiv
10、er bandwidth has led to research CCIR, 1966-69a and b; Netzband and Sverkriibbe, 1968; Sverkriibbe, 1969: Petke, 19731 aimed at improving the whole transmission system. It appears that it would be useful to fix values for the audio-frequency bandwidth of the programme to be radiated as well as for t
11、he overall response of the receivers and to obtain these values by the use of band-limiting filters. If both these bandwidths are nearly equal and are suitably related to the channel spacing the transmission system provides for the full utilization of the transmitted bandwidth as well as for the mos
12、t favourable protection against adjacent channel interference Eden, 19671. * * This Recommendation should be brought to the attention of Study Group 1. The essential contents of Report 457 having been transferred into Annex I of this Recommendation, Report 457 is hereby cancelled. CCIR VOLUME*X-3 90
13、 m 4855232 0503948 7 m 70 Rec. 639 2. Necessary bandwidth of emission 2.1 Bands 5 (LF) and 6 (MU Obviously, the bandwidth of emission, as well as the passband of the receivers, should be chosen in such a way that there is no unnecessary impairment of reception quality or any increase in adjacent-cha
14、nnel interference Netzband and Sverkrbbe, 19681. In areas where adjacent-channel interference is expected not to be negligible, the use of equal values for channel spacing, bandwidth of emission and receiver passband would be a good solution, In areas where less adjacent-channel interference is to b
15、e expected, different values may be suitable, e.g., the bandwidth of emission and the receiver passband may be equal and considerably exceed the channel spacing. This is especially true if the same transmitter network is operated during day and night. In such circumstances receivers, equipped with f
16、ilters of switchable bandwidths, may be used successfully to improve the reception quality under different propagation conditions. 2.2 Band 7(HF) In shortwave broadcasting, the necessary bandwidth of emission for AM-DSB should in no case exceed the value of 9 kHz. Recommendation 640 specifies a maxi
17、mum of 4.5 kHz necessary bandwidth for AM-SSB broadcasting. 3. General considerations 3.1 There exists a well-known interrelation between system bandwidth, carrier spacing and adjacent-channel radio-frequency protection ratio Sverkrbbe, 1969; Petke, 19731. 3.2 The theoretically obtainable optimum va
18、lue of protection against adjacent channel interference can be assessed by using an ideal receiver with rectangular passband characteristics. In this case the radio-frequency protection ratio is mainly determined by non-linear distortion in the transmitter. 3.3 A theoretical study of the energy spec
19、trum including, out-of-band radiation caused by transmitter non-linearities is contained in Kettel, 19681. Experimental investigations of the energy spectrum of a high-power transmitter operating in band 6 (MF) CCIR, 1966-69c show that the term occupied bandwidth as defined in Article 1, No, 147 of
20、the Radio Regulations does not give an adequate indication of the effects of bandwidth limitation on adjacent channel interference. 4. 4.1 Measurement results Relationship between AF bandwidth, RF protection ratio and channel spacing Measurements of the radio-frequency protection ratios for the case
21、 of various values of audio-frequency bandwidths, which are equal at both transmitter and receiver, and at different channel spacings have been carried out in the Federal Republic of Germany Sverkrbbe, 19691 using the objective two-signal measuring method given in Recommendations 559 and 560. For th
22、e measurements a high quality commercial receiver with an almost ideal passband characteristic was used. The interrelation between?the parameters involved is shown in Fig. 1. For a given channel spacing there are many pairs of values of audio-frequency bandwidths and adjacent channel protection rati
23、os. If, however, two of the parameters have been chosen, the third is definitely fixed. 4.2 Computation results The relationship between system bandwidth, adjacent channel protection ratio and channel spacing can be determined by means of the numerical method (Recommendation 559). Studies carried ou
24、t were based on the assumption that both the carrier spacing and the adjacent channel protection ratio are predetermined values. Using Recommendation 560, a relative value of the radio-frequency protection ratio of -26 dB corresponding to a channel spacing of 9 kHz hac been assumed. Thereby due acco
25、unt has been taken of the characteristics of current types of receivers. Any amplitude-modulation sound-broadcasting system has, in principle, the same effect on the reception quality as a low-pass filter. Amplitude-modulation systems designed in conformity with the channel spacing and protection ra
26、tio requirements mentioned above may, therefore, differ to some extent in bandwidth and rate of cut-off of the overall amplitude/frequency response. The investigations carried out were, therefore, extended to cover this aspect of the problem of the quality of reception. CCIR VOLUME*X-L 90 4855232 05
27、03949 9 = Rec. 639 71 Radio-frequency protection ratio relative to coihannel value (dB) It was assumed that the influence on the overall amplitude/frequency response of the entire system was equally distributed between the transmitting and receiving ends. This approach should, however, be considered
28、 as a first attempt only and additional studies will have to be carried out for different conditions. As a result of the calculations made it was found that any one of the three overall amplitude/frequency response curves shown in Fig. 3 would provide satisfactory adjacent-channel protection in an 9
29、 kHz channelling system. The curves of Fig. 2 present pairs of values for the bandwidth, b, and rate of cut-off, Q, required at either end of the AM sound-broadcasting system. The solid.curve is only valid if use is being made of a notch filter in the receiver to eliminate the beat-note between adja
30、cent channel carriers, whereas the broken line applies to the case where there is no notch filter. The particular points in Fig. 2 numbered 0, 0 or 0 correspond to terminal equipment characteristics that would provide the overall amplitude/frequency response curves, A, B or C, respectively, in Fig.
31、3. The results obtained are in close agreement with Fig. i which should be considered to provide limiting values, since it applies to the ideal case of rectangular passband characteristics. The system bandwidth thus decreases rapidly with decreasing rate of cut-off. 4.3 Listening tests The influence
32、 of reproduction quality of an amplitude-modulation sound-broadcasting system with 9 kHz channel spacing and a relative protection-ratio value of -26 dB for adjacent channel interference can be simulated by using three specified low-pass filters. The passband characteristics of these filters are tho
33、se of curves A, B and C in Fig. 3. Subjective listening tests then show quite clearly that a better subjective quality impression can. be obtained with frequency response curves A and B than with curve C. However, the difference in quality obtained with curves A and B is very small, a fact which may
34、 be of considerable economic interest, since the rate of cut-off of the receiver is 40 dB/octave less with curve B than with curve A. 72 60 c O O CCIR VOLUME*X-L 90 H 4855212 0503950 5 = Rec. 639 System bandwidth, b (kHz) FIGURE 2 - Characteristics of an amplitude-modulation sound-broadcasting syste
35、m for optitnum quality of reproduction Basic assumptions : Channel spacing: 9 kHz Relative adjacent-channel protection ratio : - 26 dB : characteristics including the effect of ij notch filter for elimination of the carrier beat -_- : characteristics without notch filter 5. Radio-frequency and inter
36、mediate-frequency passband characteristics of current types of receiver Receiver characteristics have been collated in various countries and are partly reproduced in. Report 333 (New Delhi, 1970). Radio-frequency and intermediate-frequency passban values between the 6 dB points are quoted ranging be
37、tween 5 and 10 kHz. It should be noted that the reproduced audio-frequency bands are about half these values, The highest values mentioned are those of “first category” receivers in the USSR CCIR 1966-69d with variable selectivity. It is known that there are many receivers with even smaller passband
38、s than those mentioned in the above references. It has, however, been indicated that in some areas there exist receivers with larger passbands. O -10 -20 CCIR VOLUME*X-L qo m 4455232 0503,3 7 m 73 Rec. 639 -40 -50 -60 O 1 2 3 4 5 6 7 a Audio-frequency bandwidth (kHz) FIGURE 3 - Oyera11 amplitudelfre
39、quency response for an amplitude-modulation sound-broadcasting system for optimum quality of reproduction Curve A: overall rate of cut-off for system - 120 dB per octave B : overall rate of cut-off for system - 80 dB per octave C : overall rate of cutoff for system - 40 dB per octave 6. Use of bandw
40、idth limitation in operational practice Even though the use of bandwidth limitation has been common practice for many years, the public reaction to the effect on programme quality has been negligible. On the other hand, improved reception has been reported in many cases where adjacent channel interf
41、erence had previously been severe. According to the Geneva Plan, a large number of transmitters are not operating in bands 5 (LF) and 6 (MF) with a limited bandwidth. In the LF band 50.6% of the total number of transmitters has a bandwidth of emission equal to or less than 10 kHz, whereas in the MF
42、band this value is only 31.3%. 7. A bandwidth-saving overtone transmission and reception system A new method has been described Gassman, 1972 and 19731, applicable in bands 5 (LF), 6 (MF) and 7 (HF), which allows improved sound quality at the receiver while the audio-frequency modulating signal is r
43、estricted in bandwidth. The system is based on the fact that the human ear is unable to identify overtone frequencies above about 4 kHz in relation to the fundamental tone. CCIR VOLUME*X-L 90 m 4855232 0503952 9 m 74 Rec. 639 The improvement of the sound quality is effected by the addition of artifi
44、cial overtones generated in the receiver. The amplitudes of the overtones are controlled by a pilot tone at the upper end of the audio-frequency passband. The pilot tone carries the information on the amplitude of the overtones and the necessary synchron- izing signal in the form of a single-sideban
45、d modulation. . 8. Out-of-band spectrum of double-sideband sound-broadcasting emissions Recommendation 328, 0 3.5.1, gives the limit curves for the level of the out-of-band radiation of amplitude modulated double-sideband broadcast emissions. The curves have no fixed relation to the level of the car
46、rier since this relation depends on: - - - which depends only on the power distribution within the sidebands. the modulation factor of the transmitter (ramas. value); the necessary bandwidth of the emission; the bandwidth of the spectrum analyser. However, the limit curves have a fixed relationship
47、to the maximum level of the sideband components Detailed information on the corresponding values is contained in Report 325. 9. The effect of audio signai processing on bandwidth Some transmitters operating in band 7 (HF) employ an audio shaping filter and limiter to achieve higher average modulatio
48、n. The use of the filter is commonly referred to as the trapezoidal modulation technique. Measurements in India CCIR, 1982-86aI have indicated that in the case of trapezoidal modulation, the occupied bandwidth of emission is greater than in the case of conventional modulation. Table I summarizes the
49、 results of the measurements. TABLE I - Occicpied bandwidth for trapezoidal arid coiivetitional modiilation Occupied bandwidth (kHz) Trapezoidal modulation Conventional modulation Upper limit of modulating Percentage modulation (%I Upper limit of modulating frequency frequency (kHz) (kHz1 4.5 5.0 4.5 30 50 70 11.0 11.7 13.7 12.0 12.9 14.6 10.3 11.1 12.1 5.0 11.5 12.0 13.0 However, when the highest frequency of a coloured-noise modulating signal (see Recommendation 559) is not restricted to a value of 10 kHz by a low-pass filter, the occupied bandwidth in the case o
