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R&D Report 1966-30 RESEARCH DEPARTMENT . Visibility of sound /chrominance pattern with the PAL system: dependence upon offset of intercarrier frequency and upon sound modulation RESEARCH REPORT No. G -102 UDC 621.391.837.41:621.397.132 1966/30 THE BRITISH BROADCASTING CORPORATION ENGINEERING DIVISION RESEARCH DEPARTMENT VISIBILITY OF SOUND/CHROMINANCE PATTERN WITH THE PAL SYSTEM: DEPENDENCE UPON OFFSET OF INTERCARRIER FREQUENCY AND UPON SOUND MODULATION Research Report No. 0-102 UDe 621.391.837.41: 1966/30 621.397.132 R.V. Harvey, B.Sc., A."1.I.E.E. for Head of Research Department This Report is the property of the British Broadcasting Corporation and may not be reproduced in any form without the written permission of the Corporation. TblB R~porl u~e8 SI units 1n ftctor­ daone with B .. 8. dO(lUII.8nt PD I'HJ86. Research Report No. 0-102 VISIBILITY OF SOUND/CHROMINANCE PATTERN WITH THE PAL SYSTEM: DEPENDENCE UPON OFFSET OF INTERCARRIER FREQUENCY AND UPON SOUND MODULATION Section Title Page SUMMARY .... 1 1. INTRODUCTION. 1 2. THE CHOICE OF A COLOUR SUBCARRIER FREQUENCY 1 3. THE CHOICE OF A SOUND CARRIER FREQUENCY. 2 4. ASSESSMENT OF PATTERN VISIBILITY WITH SOUND CARRIER OFFSET. 2 4.1. Experimental Procedure . 2 4.2. Dis~ussion of Experimental Results. 2 5. DETER\1INA TION OF THE OPTPvIU\1 INTER-CARRIER FREQUENCY AND ITS TOLERANCE ............................. 5 . 6. CONCLUSIONS . ....................... , ...... 5 7. REFERENCES. ............................... 5 June 1966 Research Report No. 0-102 UDe 621.391.837.41: 1966/30 621.397.132 VISIBILITY OF SOUND/CHROMINANCE PATTERN WITH THE PAL SYSTEM: DEPENDENCE UPON OFFSET OF INTERCARRIER FREQUENCY AND UPON SOUND MODULATION SUMMARY In order to minimize the visibility of the sound/chrominance beat pattern in compatible reception of PAL colour transmissions, an optimum sound/ vision intercarrier frequency may be specified, together with a frequency tolerance. For the U.K. Standard I the results of subjective assessments of the pattern show that an appreciable improvement, over the existing specifi­ cation of 6 MHz ± 1 kHz, would be obtained by using 6 MHz - 400 Hz ± 500 Hz. A more precisely controlled offset would have little advantage in prac­ tical conditions. 1. INTRODUCTION quency of about 4'4 \1Hz is chosen. This frequency is low enough to allow sufficient bandwidth for the In compatible reception of a colour television colour sidebands and high enough to reduce the transmission using Standard I, a coarse pattern visibility of the beat pattern (which may limit com­ may become visible (in some conditions of receiver patibility on a monochrome receiver) to a sufficient tuning) caused by interaction of the chrominance extent. s ubcarrier signal at about 4'4 'vIHz with the sound intercarrier signal at about 6'0 MHz. The visibility The subcarrier pattern will be most visible of the fine pattern caused by the chrominance signal when the subcarrier has a constant phase and a itself is minimized by a very careful and precise frequency, fse, which is an integral multiple of choice of subcarrier frequency in terms of the line the line-scan frequency (fL == 15625 Ilz) since the and picture frequencies. No such precision is phase of the brightness distribution will then be specified for the sound/vi sion inter carrier fre­ the same on each line of the picture, giving a quency; the usual choice is to make this an integral pattern of vertical lines. A gradual offset of the' multiple of the line frequency so that the coarse subcarrier frequency will cause the lines of the pattern and the fine pattern have similar structures. pattern to slope away from the vertical and there­ At the same time, a tolerance of 1 kHz is usually fore become less visible. At an offset of one half considered permissible for the intercarrier frequency of the line-frequency, the phase of the pattern will since the latter is frequency-modulated up to ± 50 0 be displaced by 180 on successive lines giving kHz deviation by the sound programme. minimum visibility. For example, the NTSC sub­ carrier frequency is chosen to be fse == 283h fL, The purpose of this report is to give a brief resulting in a subcarrier pattern having the appear­ explanation for the choice of the chrominance sub­ ance of a mass of upwards-moving dots rather than carrier frequency for the PAL system, to ques tion a series of sloping lines. the assumptions made about the optimum sound/ vision intercarrier frequency and to indicate the best compromise for its mean value and the tole­ The PAL system transmits saturation and hue rance, based on observations of the sound/chromi­ in a similar way to the NTSC system with one nance pattern on PAL colour bars wi th various important difference. The phase and amplitude of degrees of sound modulation. the subcarrier of either system may be resolved into two components in quadrature as shown by the following expressions for the total video modulating 2. THE CHOICE OF A COLOUR SUBCARRIER voltage in each system: FREQUENCY ~TSC EM == Fy + £6 sin(2TTfse t + cp) For Standard I (625 lines, 50 fields/second, 5' 5 \1Hz video bandwidth) a colour subcarrier fre- + El' cos(2TTfse t + cp) 2 PAL* EM = Ey + Et'; sin 2TTfsct lated and had a frequency equal to an exact multiple of the line frequency (e.g. 6 MHz or 384 fL, U.K. ± Ev cos 2TTfsc t Standard I) the sound/chrominance beat would have a minimum vis ibili ty automatically since the frac­ where Ey is the luminance voltage and EO., Er', tional offset of the beat frequency would be the Et';, Ev are the chrominance vol tages, all of which same as that of the subcarrier itself. However, are derived from the original red, green and blue the practical situation, in which the sound carrier voltages; cP is a phase constant for the NTSC is modulated and the two colour components may sys tem. The phase reference for the subcarrier, not be present to an equal extent, warrants experi­ frequency fsc, is supplied by a burst of subcarrier mental inves tigation. signal during the line-blanking interval. The important difference between the two 4. ASSESS'vIENT OF PATTERN VISIBILITY WITH systems is that the sign of Ev is reversed on alter­ SOUND CARRIER OFFSET nate line-scans in the PAL system. A half-line­ offset subcarrier frequency would therefore be 4.1. Experimental Procedure unsuitable for PAL since hues giving a predomi­ nance of the Ev signal will again show a vertical PAL Colour Bars were observed by viewing a beat pattern through cancellation of the phase commercial u.h.f. television receiver at a distance shift caused by the offset. of eight times the picture height. The observer (an experienced engineer) adjusted the sound carrier The best compromise for PAL is clearly to level at each frequency offset so that the sound/ choose some intermediate offset, say, plus or minus chrominance pattern was at the threshold of percep­ ~4 fL, in order to make the patterns slope equally tibility (grade lYz).* The receiver was detuned in opposite directions for the Eu and Ev signals, slightly towards the sound carrier-frequency so that even though they will still be closer to the vertical it was never necessary to use a sound carrier level 1 than in the NTSC pattern. It can be shown that, exceeding the normal value (-7 dB relative to peak for a 625-line system, the optimum subcarrier fre­ vision signal). For this latter reason the results quency for PAL is: may be taken to show only the variation in pattern visibility near to the threshold of perception, not fsc = (n - ~4)fL + fp the absolute susceptibility of a properly tuned recei ver. At the time of the experiment, the avail­ where n is usually chosen to be 284 and fp is the able PAL equipment produced alternation of the picture frequency, 25 Hz. The addi tional offset of Ey' component of the colour signal according to an 25 Hz results in the pattern most nearly interlacing earlier standard2 for the system, but it is not con­ on alternate field-scans. sidered that results for the new standards would be significantly differen t. The subjective appearance of the PAL sub­ carrier pattern, unlike that of the NTSC pattern, 4.2. Discussion of Experimental Results p. depends on the hue, being one or other set of diagonal lines for hues which have only one chro­ The visibility of the pattern depends upon its VI minance component and a combination of these two coarseness of structure, its rate of apparent motion Cl fr predominant patterns for hues with comparable and the degree to which it is made indistinct by amounts of both the Eu and Ev signals. the frequency-modulation of the sound-carrier. Fig. -~ 1 shows the dependence of visibility on the struc­ Cl ture of the pattern as a function of the sound carrier al 3. THE CHOICE OF SOUND CARRIER FRE­ offset over a line-frequency interval; Fig. 2 shows al QUENCY the dependence of visibility upon the movement of C( the pa ttern a s a function of the sound carrier offse t or The above discussion on the visibility of the over a picture-frequency interval. Because of the re PAL system subcarrier is equally applicable to the effect of small closely-controlled offsets shown in pe sound/ chrominance beat pattern and the general Fig. 2, Fig. 1 includes two curves for an unmodu­ Fi all requirement for minimum visibility still applies. lated carrier, one for the most visible condition th If, therefore, the sound carrier were unmodu- si, * This refers to the six-point impairment scale of the 011 EBU; grade 1 V2 corresponds to the condition, which an w, experienced engineer can assess, when about 50% of ea * Revised specification agreed by EBU Ad-Hoc Group ordinary observers would find it "imperceptible" (grade 1) on Colour Television, 1966.
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