1、 Rec. ITU-R BT.1675 1 RECOMMENDATION ITU-R BT.1675 System design and operational practices for minimizing disturbance from loop delay in broadcast systems (Question ITU-R 35/6) (2004) The ITU Radiocommunication Assembly, considering a) that sound or television broadcast programmes can include interv
2、iews or other interactive situations which involve inserts shot at different physical locations, linked together at a base location; b) that such inserts can be delayed due to propagation time or due to signal processing in codecs; c) that Question ITU-R 35/6 Tolerable round-trip time delay for soun
3、d programme and television broadcast programme inserts, has requested submissions on delay models for programme contribution loops and on tolerable limits for delay around such loops, recommends 1 that the signal path model in Annex 1 should be used as a basis for modelling delay in programme contri
4、bution loops. Annex 1 System design and operational practices for minimizing disturbance from loop delay in broadcast systems 1 Introduction This Annex outlines a model for calculating signal delay in programme contribution loops and outlines system design guidelines and operational practices which
5、can be used to minimize the loop delay, minimize the programme degradation arising from loop delay and echo as well as minimize the disturbance to the programme participants from loop delay and echo. 2 System modelling One half of the insert loop may be modelled as shown in Fig. 1. 2 Rec. ITU-R BT.1
6、675 1675-01BACDEFGHJFIGURE 1Model of audio/video production chain for calculating half-loop delayMicrophone,camera etc.Anti-shakeetc.MPEG, etc. ATM/GSM/CDMAsystemsRadiatedorcabledtransmissionMPEG, etc. Mixer, etc.Video monitor,headphone, etc.CapturePre-processingDatacompressionPacketswitchingdelayPr
7、opagationdelayDatadecompressionPost-processingDigitalanalogueconverterImage/soundsourceTransducerViewer/listenerAnaloguedigitalconverterTransducerTypical delays in the stages of this half-loop are shown in Table 1. Note that more than one row of Table 1 may apply to a given link e.g. calculation of
8、delay for digital video with MPEG-2 encoding transmitted via an asynchronous transfer mode (ATM) carrier needs information from two rows of the Table. These values are examples only. Measured or calculated values for the system under study should be used to determine the actual delay in that system.
9、 TABLE 1 Typical delays of loop components using the half-loop model of Fig. 1*Stage A/B C D E F G H J System Analogue audio 0 0 0 5 s/km cabled, 3.3 s/km radiated 0 Telephone audio, GSM 125 s 10-15 ms 5 s/km cabled, 3.3 s/km radiated 10-15 ms 125 s Telephone audio, code division multiple access (CD
10、MA) 125 s 20 ms 5 s/km cabled, 3.3 s/km radiated 3 ms 125 s Telephone audio, low-Earth orbit (LEO) satellite 20 ms 5-13 ms 20 ms Telephone audio, GSO satellite 20 ms 240-280 ms 20 ms Rec. ITU-R BT.1675 3 TABLE 1 (end) 3 System design and configuration factors Three general principles should be obser
11、ved in designing and configuring facilities for programme contribution loops: 3.1 Number of encode/decode stages The number of encode/decode processes within each half-loop should be minimized. This minimizes the encode/decode delay and has the added benefit of reducing other forms of signal degrada
12、tion such as group delay and non-linear distortion. 3.2 Differential audio-video delay The differential sound-to-picture delays should be kept within the limit of +25 ms to 100 ms as specified in Recommendation ITU-R BT.1359 Relative timing of sound and vision for broadcasting. This may generally be
13、 done by judicious selection of audio and video codecs and by compensation with audio delay units if necessary. Stage A/B C D E F G H J System Digital audio, 20 kHz BW, ISDN 21 s 10-200 ms 5 s/km cabled, 3.3 s/km radiated 10-200 ms 21 s Digital audio, ATM, AAL5 Typical up to 150 s per node, up to 40
14、 nodes 5 s/km cabled, 3.3 s/km radiated Analogue video Maximum 33 ms 5 s/km cabled, 3.3 s/km radiated Digital videophone To be developed 5 s/km cabled, 3.3 s/km radiated SD digital video MPEG 1 frame (33-40 ms) 1-4 frames (33-160 ms) 5 s/km cabled, 3.3 s/km radiated 1-4 frames (33-160 ms) 77 ns GSO
15、satellite link 240-280 ms *Additional network delays may be incurred, see text in 2. 4 Rec. ITU-R BT.1675 3.3 Foldback leakage limit Audio leakage between half-loops should be kept below the threshold indicated in Fig. 2. The 15 dB upper limit on each half-loop ensures that the loop has at least a s
16、ubstantial gain margin for stability and that the frequency response deviations from comb filtering are limited to less than 1.5 dB. The slope of the curve is derived from experimentally determined audibility thresholds. The 50 dB lower limit is set as the S/N for speech rarely exceeds 50 dB even un
17、der studio conditions. It may be necessary in some circumstances to reduce the leakage further than 50 dB, especially for long delays. The leakage limit should be applied to the worst-case sum of both the electric and the acoustic leakage. 1675-020 30 60 90 120 150 180 210 240 270 300 330 3606050403
18、02015100FIGURE 2Recommended limit for foldback leakage vs. delay in a full loopFull loop delay (ms)Relativeamplitude(dB)Folback leakage limit4 Operating practices Operation of facilities for programme contribution loops should conform with the following principles to minimize the leakage between hal
19、f-loops and to minimize the disturbance caused by loop delay. 4.1 Audio practices Audio leakage from foldback lines to programme lines should be controlled by: using a “mix-minus” or “clean feed” to foldback, and/or appropriate muting of foldback when microphones are open, and/or using closed headph
20、one monitoring or in-ear monitoring to minimize leakage into open microphones. Rec. ITU-R BT.1675 5 4.2 Video practices No picture transmitted along the return half-loop should show a live monitor fed by the forward half-loop. If an on-screen monitor is required, either use a blue screen and key the picture in downstream OR (preferably) use a separate camera to feed the return half loop, with no monitor in shot.
