1、 Rec. ITU-R BT.655-7 1 RECOMMENDATION ITU-R BT.655-7 Radio-frequency protection ratios for AM vestigial sideband terrestrial television systems interfered with by unwanted analogue vision signals and their associated sound signals (Question ITU-R 56/6) (1986-1990-1992-1994-1995-1998-2000-2004) The I
2、TU Radiocommunication Assembly, considering a) that accurate protection ratio values are required in order to permit the planning of terrestrial television services in an effective manner, recommends 1 that the protection ratios given in Annex 1 be used for planning terrestrial television services;
3、2 that studies should be undertaken to complete the information on protection ratios, in particular with reference to the items identified in 4 of Annex 1. NOTE Systems for the emission of digital terrestrial television services are being developed and the associated protection ratios are given in R
4、ecommendation ITU-R BT.1368. Annex 1 Radio-frequency protection ratios for terrestrial television systems 1 Introduction This Annex contains general information related to protection ratios for terrestrial television systems. It also contains a series of Appendices, each containing protection ratios
5、 required for the protection of an individual category of system or signal. Appendices 1 and 2 contain protection ratios for 525- and 625-line analogue television systems, respectively. Appendix 3 contains protection ratios for the sound signals of analogue television systems. 2 Rec. ITU-R BT.655-7
6、2 General The RF protection ratio is the minimum value of wanted-to-unwanted signal ratio, usually expressed in decibels at the receiver input, determined under specified conditions such that a specific reception quality is achieved at the receiver output. Measurements of protection ratio for the vi
7、sion signal of a wanted analogue television system should preferably be made with the subjective comparison method with a sine-wave reference interferer described in Recommendation ITU-R BT.1368, Annex 5. 2.1 The values of protection ratio quoted apply to interference produced by a single source. Ex
8、cept where otherwise stated, the ratios apply to tropospheric, T, interference and correspond closely to a slightly annoying impairment condition. They are considered to be acceptable only if the interference occurs for a small percentage of the time, not precisely defined but generally considered t
9、o be between 1% and 10%. For substantially non-fading unwanted signals, it is necessary to provide a higher degree of protection and ratios appropriate to continuous, C, inter-ference should be used (see Annex 2). If the latter are not known, then the tropospheric, T, values increased by 10 dB can b
10、e applied. Values applicable to limit of perceptibility, LP, are given for information only. 2.2 Significantly strong wanted input signals can require higher protection ratio values because of non-linear effects in the receiver. 2.3 For 625-line systems, the reference impairment levels are those whi
11、ch correspond to co-channel protection ratios of 30 dB and 40 dB with a frequency-offset between vision carriers close to two-thirds of the line frequency but adjusted for maximum impairment, the precise frequency difference being 10.416 kHz. These conditions approximate to impairment grades 3 (slig
12、htly annoying) and 4 (perceptible but not annoying) and apply to tropospheric, T, and continuous, C, interference, respectively. 2.4 It should be noted that the amplitude of a vision-modulated signal is defined as the r.m.s. value of the carrier at peaks of the modulation envelope (taking no account
13、 of the chrominance signal in positive-modulation systems), while that of a sound-modulated signal is the r.m.s. value of the unmodulated carrier, both for amplitude modulation and for frequency modulation. For planning purposes, it may be assumed that the power in the chrominance channel does not e
14、xceed a value which is 16 dB lower than the power in the vision carrier during peaks of the modulation envelope. 2.5 The protection ratio values are not affected if digital data are included in the field-blanking interval of the unwanted television signal. However, certain values are affected in the
15、 case of a full-field data unwanted signal; in particular, it is not possible to achieve the full advantages of precision offset operation. 2.6 The relationship between the vision carrier frequencies of the wanted and unwanted signal is as follows (see Annex 3): 2.6.1 Non-controlled condition No spe
16、cial control of the nominal frequency difference between the carriers of the wanted and unwanted signals. Rec. ITU-R BT.655-7 3 2.6.2 Non-precision offset The difference between the nominal frequencies of the wanted and unwanted carriers is suitably related to the line frequency, the tolerance of th
17、e carrier frequencies being 500 Hz. The line synchronization of television receivers must be sufficiently immune to periodic inter-ference if full advantage of carrier offset operation is to be achieved. 2.6.3 Precision offset (see Annex 4 for the case of 625-line systems) The difference between the
18、 nominal frequencies of the wanted and unwanted carriers is suitably related to the line and field frequencies, but with a tolerance of each of the nominal carrier frequencies of the order of 1 Hz and stability of the line frequencies equal to or better than 1 10-6. In order to take full advantage o
19、f precision offset when the interfering carrier falls in the upper video range (greater than 2 MHz) of the wanted signal, a line-frequency stability of at least 2 107is necessary. 3 Synchronized carrier operation Field and laboratory tests have demonstrated that synchronized carrier television syste
20、ms allow a similar reduction in co-channel interference to that achieved by use of precision offset techniques, when the same television programme is transmitted. Ratios of wanted-to-unwanted signals of 28 dB and 38 dB were found to correspond to impairment grades of 3.5 and 4.5, respectively. No de
21、gradation of picture quality was observed when the frequency difference between both vision carriers was less than 0.2 Hz and/or the phase fluctuations were less than 20. The use of synchronized carrier techniques simplifies the introduction of new television transmitters and transposers into existi
22、ng networks. Further studies in this field are required, especially for the case of different television programmes. 4 Further studies In a number of cases, the available protection ratio values are incomplete. In particular, this applies to: data signals, out-of-channel response, 525-line systems,
23、synchronized carrier operation, protection ratio values for digital television systems, protection ratio values for the protection of analogue television signals against interference from digital television signals. In addition, it is necessary to establish the relationships between picture quality
24、or impairment grade and protection ratio value. While the information is available for grades 3, 4 and 4.5, it is not yet available for lower grades. 4 Rec. ITU-R BT.655-7 Appendix 1 to Annex 1 Protection ratios for 525-line television systems 1 Protection from co-channel interference In this sectio
25、n, the protection ratio values between two television signals apply only for interference due to the modulated vision carrier of the unwanted signal. 1.1 Carriers separated by less than 1 000 Hz, non-controlled systems having the same or a different line-standard Protection ratio: 45 dB, tropospheri
26、c interference. 1.2 Carriers separated by parts of the line frequency ( fline), systems having the same line-standard, non-precision offset (see Table 1) TABLE 1 Protection ratio, tropospheric interference carrier separation up to about 36/12 fline(about 50 kHz) 2 Protection from adjacent-channel in
27、terference The given protection ratios apply to tropospheric interference and are defined in terms of wanted and unwanted vision carrier levels. For continuous interference, the values should be increased by 10 dB. Adjacent-channel protection ratios cannot be determined directly from the overlapping
28、 channel protection ratio curves shown in 4 because for certain systems the values may be affected by special measures in the receiver, e.g. sound traps. 2.1 Protection from lower adjacent-channel interference, VHF and UHF bands The worst interference to the picture signal from another signal using
29、the same standard results from the sound signal in the lower adjacent channel. However, an improvement of 2-3 dB can be achieved if the frequency difference between the wanted vision and unwanted sound carriers is an odd multiple of half the line frequency. During periods of no sound modulation, the
30、 improvement is as much as 10 dB. The figure below relates to the case where the separation between the wanted vision carrier frequency and the unwanted sound carrier frequency is 1.5 MHz and the ratio between the unwanted vision and unwanted sound powers is 10 dB. Offset of line frequency 1/2, 3/2,
31、 5/2, . 1/3, 2/3, 4/3, . 525-line system (dB) 25 28 Rec. ITU-R BT.655-7 5 A correction must be made for different vision to sound power ratios. Protection ratio: 13 dB. 2.2 Protection from upper adjacent-channel interference, VHF and UHF bands Protection ratio: 10 dB. 3 Protection from image-channel
32、 interference The protection ratio will depend on the intermediate frequency and image-channel rejection of the receiver, and on the type of unwanted signal falling in the image channel. It can be determined by subtracting the image rejection figure from the required protection ratio for overlapping
33、 channels. Table 2 shows image-channel rejection values. TABLE 2 Image-channel rejection 4 Protection from overlapping channel interference All Figures and Tables in this section give protection ratio values to be applied when a continuous wave (CW) signal lies within the vision channel of the wante
34、d transmission, the wanted vision signal being negatively modulated. Corrections to be made for other types of interfering signal are given in Table 3. TABLE 3 Correction values for different unwanted signals Image-channel rejection (dB) VHF UHF System M (Japan) 60 45 Other systems 40 Correction fac
35、tors (dB) Unwanted signal Wanted signal CW Vision signal negative modulated Vision signal positive modulated FM-sound AM-sound Vision signal negative modulated 0 2 0 0 +4 6 Rec. ITU-R BT.655-7 Figure 1 and Table 4 show protection ratios for tropospheric interference. For continuous inter-ference, th
36、e values should be increased by 10 dB. The unwanted signal is a CW carrier. For other types of unwanted signal, the correction factors given in Table 3 should be applied. 0655-016050403020100 3 2 101234567AABABA (Monochrome)(Monochrome)(PAL)Protection ratio (dB)FIGURE 1 and TABLE 4525-line systems (
37、M/NTSC and M/PAL) Tropospheric interferenceUnwanted signal: CW carrier(NTSC)Frequency difference, f = fu fw(MHz), between unwantedcarrier frequency fuand wanted carrier frequency fw4.1 Protection ratios for vision signals interfered with by terrestrial digital audio broadcasting (T-DAB) Figure 2 and
38、 Table 5 give protection ratios for negative modulated vision signals interfered with by a 1.5 MHz wide COFDM signal according to the T-DAB system (see Recommendation ITU-R BS.1114). (MHz) 1.5 1.0 0.75 0.3 1.0 2.5 3.0 3.5 3.7 4.1 4.5 A NTSC (dB) 50 50 45 A PAL (dB) 0 30 40 50 50 37 45 45 45 15 A Mon
39、ochrome (dB) B Monochrome (dB) 0 15 33 33 25 15 Protection ratio (dB) Curves A: non-controlled condition B: non-precision offset condition (1/3, 2/3, 4/3, 5/3, all of the line frequency) Rec. ITU-R BT.655-7 7 0655-02 30201001020304050 3 2 101234567FIGURE 2Protection ratios for vision signal interfer
40、ed with by T-DAB signalFrequency difference between centre frequency of unwantedT-DAB signal and wanted vision signal (MHz)Protection ratio (dB)(wanted visioncarrierpower/unwantedsignalpower)Tropospheric (ITU grade 3)Continuous (ITU grade 4)TABLE 5 Vision signal interfered with by T-DAB 5 Television
41、 signal affected by data signals The inclusion of digital data such as teletext in the field-blanking interval has no effect on the required protection ratios. However, the full improvement resulting from non-precision or precision offset operation is not achievable when the unwanted signal carries
42、a full-field data signal. No information is currently available regarding the protection ratios to be applied to a wanted 525-line system when the unwanted signal is full field teletext. Frequency difference between unwanted and wanted carriers (MHz) Luminance range Chrominance range Protection rati
43、o (dB) 3.0 2.0 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Tropospheric interference (T) 20 15 23 37 34 30 26 29 14 10 19 Continuous interference (C) 15 9 30 43 40 36 33 36 18 5 15 8 Rec. ITU-R BT.655-7 Appendix 2 to Annex 1 Protection ratios for 625-line television systems 1 Protection from co-channel inte
44、rference In this section, the protection ratios between two television signals apply only for interference due to the modulated vision carrier of the unwanted signal. Additional protection may be necessary if the wanted sound carrier is affected, or if the unwanted sound carrier lies within the want
45、ed vision channel (e.g. the unwanted sound carrier of system G lies within the vision channel of system K). For all protection ratio values in this section, the following corrections have to be made: when the wanted signal is modulated negatively and the unwanted signal is modulated positively (L/SE
46、CAM), the values should be increased by 2 dB; when the wanted signal is modulated positively and the unwanted signal is modulated negatively, the values should be reduced by 2 dB. 1.1 Carriers separated by less than 1 000 Hz, non-controlled systems having the same or a different line-standard Protec
47、tion ratio: 45 dB, tropospheric interference. 1.2 Carriers separated by multiples of a twelfth of the line frequency up to about 36/12 fline(about 50 kHz) These protection ratio values do not necessarily apply for greater carrier separations. TABLE 6 Protection ratio between 625-line systems* Offset
48、 (multiples of 1/12 line-frequency)(1)0 1 2 3 4 5 6 7 8 9 10 11 12 Non-precision Tropospheric interference 45 44 40 34 30 28 27 28 30 34 40 44 45 offset Transmitter Continuous interference 52 51 48 44 40 36 33 36 40 44 48 51 52 stability 500 Hz Limit of perceptibility (2)61 60 57 54 50 45 42 45 50 5
49、4 57 60 61 Precision Tropospheric interference 32 34 30 26 22 22 24 22 22 26 30 34 38 offset Transmitter Continuous interference 36 38 34 30 27 27 30 27 27 30 34 38 42 stability 1 Hz Limit of perceptibility(2)42 44 40 36 36 39 42 39 36 36 40 44 48 * The values shown in Table 6 may also be applied to PALplus signals (both as wanted and/or unwanted). (1)Value in the first column is only valid for the 0/12 case. All other values between 1/12 and 12/12 are the same by addition or subtraction of integral multiples of 12/
