ITU-R BT 1359-1-1998 Relative Timing of Sound and Vision for Broadcasting《声音和视频广播的相对定时》.pdf

1、Rec. ITU-R BT.1359-1 1RECOMMENDATION ITU-R BT.1359-1RELATIVE TIMING OF SOUND AND VISION FOR BROADCASTING(Question ITU-R 35/11)(1998)Rec. ITU-R BT.1359-1The ITU Radiocommunication Assembly,consideringa) that a perceptible time difference between the sound and vision components of a television signal

2、impairs theviewers reception of the programme;b) that separate picture and sound processing is becoming more and more widely used in broadcasting systems;c) that digital production and distribution equipment causes differential delay between the sound and visionsignals;d) that programme production m

3、ay involve tandem connected studios;e) that in studios the sound/vision relative timing should be the responsibility of the programme productiondirectors;f) that the transmitting equipment and the receiver may introduce an additional, variable timing difference;g) that subjective evaluations show th

4、at detectability thresholds are about +45 ms to 125 ms and acceptabilitythresholds are about +90 ms to 185 ms on the average, a positive value indicates that sound is advanced with respect tovision,recommends with reference to Figure 11 that the timing zero, as a reference for the subsequent measure

5、ment of the relative timing of sound and visionsignals is defined at the point of the final programme source selection element*;2 that the overall tolerance in sound/picture timing (between points 1 and 6) shall not exceed +90 ms or 185 ms;3 that the timing tolerance between image source (point 1) a

6、nd the zero reference point as defined inrecommends 1 above be taken as falling within the limits of +25 ms and -100 ms (Note that this is the zone within whichthe programme producer may exercise control over the relative timing of sound and picture. It is not possible to identifythe correct or inte

7、nded timing within this range, first because of the “plateau of undetectability” as noted in Appendix 1Figure 2 and secondly because the producer may have selected “for artistic effect” a non-zero relative timing.);4 that the timing difference in the path from the output of the final programme sourc

8、e selection element* to theinput to the transmitter for emission should be kept within the values +22.5 ms and 30 ms*;5 if correction of errors is not possible then each downstream segment that is not under the control of thebroadcaster shall not introduce any timing error in excess of 2 ms.Annex 1

9、outlines the user requirements for the corrections of relative timing difference of sound and vision signalswhich were used in preparing this recommendation.Appendix 1 is an explanation of the selection of the recommended timing difference values.Appendix 2 details currently used conditions for subj

10、ective assessment of sound/vision delay difference testing.NOTE 1 A positive value indicates that sound is advanced with respect to vision.NOTE 2 Studies should be made of time stamping in such a way as to facilitate the maintenance and correct timing atappropriate points in the broadcast chain._*Th

11、e definition of this point may vary depending on the particular broadcast organization and operating requirements. Typicalexamples are master control, network control, master switching or outside broadcast control.*Where the path from the output of the final programme source selection element to the

12、 input of the transmitter is comprised ofone or more digital codecs it should be noted that Recommendation ITU-R BT.1203 specifies that the delay error introduced byany single digital codec should be in the range 2 ms.2 Rec. ITU-RBT.1359-1 1359-01221 33 44 5112111665OutsideBroadcastOutsideBroadcastC

13、ompilationStation(1)LocalStation(1)CodecCodecContribution DistributionSTLLocaltransmitterFIGURE 1Simplified reference chain for television sound/vision timing(1)This includes synchronization equipment needed such as frame synchronizerRec. ITU-R BT.1359-1 3ANNEX 1User requirements for the correction

14、of relative timingdifference of sound and vision signalsWhen implementing the correction of relative timing difference or error of sound and vision signals, the following userrequirements should be satisfied.1 In the case of on-line correction of the timing error of sound and vision signals, the aud

15、io quality of the soundsignal when observed at the output of the sound signal timing corrector, should be maintained at the start of, during andat the end of the correction, with the quality of 4.5 or higher, when evaluated using the subjective evaluation methodbased on the ITU-R five-grade impairme

16、nt scale and the results are presented using the diffgrade.2 The correction of the relative timing error of the sound and vision signals should be carried out within aresponsibility boundary in the signal chain.3 The standard reference signals intended for the off-line use for the measurement and/or

17、 correction of the errorshould be able to be observed by the eye and ear, while they should be measured by at least using a piece of equipmentdisplaying the timing difference of the two signals.4 The cost of the equipment to produce the reference signals and/or to measure the timing difference shoul

18、d bewithin a reasonable range.APPENDIX 1Explanation for the selection of the recommended valuefor sound/vision timing difference1 It is known for many years from experience with film projection that the relative timing between picture andsound is very important and shows an identifiable point at whi

19、ch the timing error becomes objectionable to the viewer.Recommendation ITU-R BR.265 indicates that the precision of accuracy of location of sound and picture informationshould be within half a frame. For 24 fps film, this is an acceptable variation of about 22 ms.2 Differing imaging techniques gener

20、ating source television signals appear to introduce unavoidable uncertaintyof the actual sound/vision timing of about half a television field.3 Subjective evaluations undertaken in Japan, Switzerland and Australia show a high degree of similarity in thesensitivity of viewers to errors in sound/visio

21、n timing in television material for NTSC and PAL systems. Testsconducted have shown that the thresholds of detectability are about + 45 ms to 125 ms and thresholds of acceptabilityare about +90 ms to 185 ms on the average. Each set of test results indicates a broad area of acceptable timing covering

22、“sound leading” through zero timing difference to “sound delayed”. The range of timing between the “just detectable”limits of sound leading and sound delayed is about 170 ms. Each case also shows a clearly defined and rather consistentrange of values for the difference (1 grade) between detectable a

23、nd acceptable limits of about 45 ms for sound leadingand about 60 ms for sound delayed as shown in Figure 2.4 Rec. ITU-R BT.1359-1 1359-02 200 180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 2 10ABCCBA 1.5 0.5Subjectiveevaluationresults(Diffgrade)Sound delay wrtvisionSound advanced wrtvisionUndete

24、ctability plateauDetectability thresholdAcceptability thresholdFIGURE 2Detectability and acceptability thresholdsDelay time (ms)4 For the purpose of establishing a Recommendation concerning an agreed limit to sound/vision timing error in television, the range of values between the detectable limits

25、is not relevant. The actual timing value is the province of the programme producer in the studio. Because we do not necessarily know, and have no recommended way of determining, the precise timing difference, we accept as being correct the relative timing that occurs at the studio output. An unsatis

26、factory situation may now exist because the studio output timing may be set to be very close to one of the limits of perceptibility and thus there is a limited margin of additional error before the timing error is such as to become unacceptable. 5 Because of the undetectable plateau (C-C, see Figure

27、 2) the limit of allowable error should be constrained within 0.5 grade points (5 point impairment scale) above the subjectively evaluated detectable threshold (B-B). The subjective evaluations have shown that a one grade point impairment results in a 60 ms change in delay which is shown on Figure 2

28、 as the rising slope from A-B. The allowable delay should be constrained within a half grade point impairment which is 30 ms shown in Figure 2 on the rising slope from B-C. Likewise the advance limit is determined to be 22.5 ms from the rise on the slope from B-C. Rec. ITU-R BT.1359-1 5APPENDIX 2Cur

29、rently used conditions for subjective assessmentof sound/vision delay difference testing1 Distance between source andmicrophone50 cm2 Distance between loudspeakerand assessor200 cm Assessors should be able to easily see lip movementsof announcers.If a 22” diagonal monitor is used the display andloud

30、speaker will be in approximately the same positionand this equates to a 6H viewing distance.3 Parameter(s) for measurement Acceptable thresholds (i.e.rating 3.5 on DSIS)But it is also considered important to measure the“detectability” thresholds (i.e. rating 4.5 on DSIS).4 Camera type Tube camera5 E

31、valuation method Double stimulus impairmentscale method6 Viewing conditions Recs. ITU-R BT.500 and 11287 Range of sound/vision timingdifferences to be included ontape(s)Must include full range ofimpairments, grade 1through 5Based on Swiss PTT work the range 200 to +300 msshould be assessed.To obtain

32、 precision in assessment of “detectable”thresholds a second tape with a concentration of valuesaround the detectable threshold points may berequired.8 Test material Female newsreader To avoid viewer fatigue it is desirable that severaldifferent sequences and/or announcers be used.9 Duration of test

33、session Less than 30 minutes It may be necessary to hold two sessions if both“acceptable” and “detectable” thresholds are to beaccurately assessed.10 Type of assessors Expert and non-expert11 Number of assessors At least 1512 Age of assessors To be stated13 Visual acuity of assessors Normal (or corrected tonormal) acuity assessed usinga Snellen chart