1、Rec. 654 1RECOMMENDATION 654SUBJECTIVE QUALITY OF TELEVISION PICTURESIN RELATION TO THE MAIN IMPAIRMENTS OF THEANALOGUE COMPOSITE TELEVISION SIGNAL(1986)Rec. 654The CCIR,CONSIDERING(a) that Recommendation 500 specifies the method to be used for the subjective assessment of the quality oftelevision p
2、ictures with a view to determining the relationship between that quality and the objective value of thespecific distortion;(b) that Recommendation 567 describes the objective parameters of typical transmission impairment and thecorresponding measurement methods and test signals;(c) that many experim
3、ental findings have been published concerning distortion to the composite signal in 525 and625-line colour-television systems in NTSC, PAL and SECAM coding and that, despite considerable dispersion, thoseresults can be used for determining an impairment characteristic which is representative of an a
4、verage situation;*(d) that it would be useful to have impairment characteristics which are accepted as reference values,UNANIMOUSLY RECOMMENDS that, with regard to the transmission characteristics applicable to the composite colour television signal, theimpairment characteristics referred to in this
5、 Recommendation should be regarded as a particular way of expressingthe relationship between the picture quality and the objective value of each of the distortions in question, assumingthat only one of them is present at any one time; that these impairment characteristics correspond to a viewing dis
6、tance equal to six times the picture height; that the use of these characteristics for I 2 (see Report 405, Annex III) (represented by a dotted line in Figs. 1 to 6of this Recommendation) requires great caution.1. Continuous random noiseThe signal-to-noise ratio is measured, unweighted, in accordanc
7、e with Recommendation 567, Part C, 3.2.1.This method is applicable to all 525 and 625-line systems.The reference impairment characteristic concerns only the impairment due to uniform-spectrum noise(white noise). The correction factors applicable to the different situations are known.The reference im
8、pairment characteristic is shown in Fig. 1. It conforms to the following relations: impairment factor: d = Nr.m.s.Lor D = LNr.m.s. dB= 20 log 1dmmmMMm(dB) mid opinion value (I = 1): dM= 0.0355 or DM= 29 dB slope: G = 2.22_*For the application of this Recommendation to countries using systems D and K
9、, further studies are needed.2 Rec. 654D01-scFIGURE 1 D01 = 11 cm2. Non-linear distortion2.1 Differential gainThe differential gain is measured in accordance with Recommendation 567, Part C, 3.4.1.3.Impairment of the picture depends on the distribution of distortion on the luminance scale. The most
10、criticalvariation corresponds to an increase in the amplitude of the superimposed colour sub-carrier as the luminance amplitudevaries from the black level (A0) to the white level (Amax), which corresponds to distortion measured in terms of value x.Impairment depends also on the colour coding system.
11、The reference impairment characteristic is illustrated in Fig. 2 and corresponds to a distortion situation offeringthe least favourable distribution. It conforms to the following relations:impairment factor: d = x = 100 Amax A0A0mid opinion value (I = 1):NTSC and PAL systems: dM= 43%SECAM system: dM
12、= 65% slope (deviation of d between I = 1 and I = 0.37):NTSC and PAL systems: S = 15%SECAM system: S = 13%Rec. 654 3FIGURE 2 Impairment characteristic for differential gain D02-scFIGURE 2 D02 = 11 cm2.2 Differential phaseThe differential phase is measured in accordance with Recommendation 567, Part
13、C, 3.4.1.3.The picture impairment depends on the distribution of distortion on the luminance scale. The most criticalvariation corresponds to a negative variation in the phase difference from the black level (0) to the white level (min),which corresponds to a distortion measured in terms of value y.
14、The impairment depends also on the colour coding system.The impairment characteristic is illustrated in Fig. 3 and corresponds to a distortion situation offering the leastfavourable distribution. It conforms to the following relations:impairment factor: d = y = | min 0|or D = log dmid opinion value
15、(I = 1):NTSC system: dM= 10PAL system: dM= 50SECAM system: dM= 65 slope:all systems: G = 2.5Note In the case of the PAL system, the fact that the impairment characteristic is a straight line has not been verified.4 Rec. 654D03-scFigure 3 = 11 cm3. Linear distortion3.1 Short-time waveform distortionS
16、hort-time linear distortion is measured in accordance with Recommendation 567, Part C, 3.5.1.4, byexpressing the difference, as a percentage, between the peak value of the pulse B1, i.e., B1, and the value of theluminance amplitude, i.e., L. The impairment characteristic value is:d = L B1L 100mmmMMm
17、(%)Impairment does not depend on the sign of L B1.Impairment does not depend on the colour coding system.The pulse-to-bar 2T ratio, i.e., B1L= 1 d100can be calculated from the value d.The impairment characteristic is illustrated in Fig. 4. It corresponds to the following conditions:impairment factor
18、: d = L B1L 100mmmMMm(%)Logarithmic scale in values of d;mid opinion value (I = 1): dM= 40% slope: G = 2.32Note Short-time linear distortion is often expressed by the K factor explained in Annex IV to Part C of Recommen-dation 567.This factor is such that K = d4.Rec. 654 5D04-scFigure 4 = 11 cm3.2 C
19、hrominance-luminance inequalities3.2.1 Gain inequalityChrominance-luminance gain inequality is measured in accordance with Recommendation 567,Part C, 3.5.3.1, the absence of distortion being expressed by equality between the luminance (EL) andchrominance (EC) amplitudes.Impairment depends on the cod
20、ing system and the curve is valid only for the NTSC and PALsystems. There is only an indirect effect on the SECAM system.The impairment characteristic is illustrated in Fig. 5. It conforms to the following relations: impairment factor: d = EC ELELmmmMMm(logarithmic scale) mid opinion value (I = 1):
21、dM= 63% slope: G = 2.336 Rec. 654D05-scFigure 5 = 11 cm3.2.2 Delay inequalityDelay inequality is measured in accordance with Recommendation 567, Part C, 3.5.3.2, using thesignal F (modulated 20T pulse for 625-line systems and 12.5T pulse for 525-line systems).The value thus measured corresponds clos
22、ely to that of the group delay of a band-limiting network inwhich gincreases with the frequency. In this case, the value of d corresponds to the value gmeasured atcolour sub-carrier frequency.This value is appreciably different from, and higher than, that obtained by direct measurement of thetime se
23、paration between a characteristic instant of a steep luminance front and the equivalent characteristicinstant of the associated chrominance transition front for the same impairment and therefore the same qualitylevel.The impairment characteristic meets the following conditions (Fig. 6):(a) for the 6
24、25-line systemsimpairment factor: d = gat 4.43 MHzg= delay differenceLogarithmic scale in expressed in ns;mid opinion value (I = 1): dM= 400 ns slope: G = 2.32(b) for the 525-line systemsimpairment factor: d = gat 3.58 MHzg= delay differenceLogarithmic scale in expressed in ns; mid opinion value (I
25、= 1): dM= 200 ns slope: G = 1.76Rec. 654 7D06-scFigure 6 = 11 cm4. EchoConsideration is given to the effect of superimposing a direct signal and a reflected signal producing apermanent “ghost” picture, an example of which is described in Report 478.Impairment depends on: the polarity of the echo, an
26、d it is noted that maximum impairment occurs with a positive echo; the time separation between the direct signal and the echo signal; the distortion of the echo signal.The impairment characteristic illustrated in Fig. 7 corresponds to the case of an undistorted positive echo 1 sbehind the direct sig
27、nal (E = echo amplitude, S = signal amplitude).When the echo delay time is less than 1 s, the following results apply to monochrome television or to theluminance signal in colour television. In the case of colour signals, additional effects occur which, in the PAL and NTSCsystems, can be interpreted
28、 by using the gain inequality characteristic.This characteristic meets the following conditions:impairment factor: d = ESor D = 20 log SEmmmMMm(dB) mid opinion value (I = 1): dM= 0.126 or DM= 18 dB slope: GM= 2.33For time separations other than 1s, the value D1should be corrected as shown in Fig. 8.Thus the value Dxfor t = X is corrected to D1= Dx D to obtain the corresponding impairment valuein Fig. 7.The slope of the characteristic is not substantially altered.8 Rec. 654D07-scFigure 7 = 11 cmD08-scFigure 8 = 12 cm_
