Recommendation itu-r bt. 1675 System design and operational practices for minimizing disturbance from loop delay in broadcast systems




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Rec. ITU-R BT.1675

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 interviews 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 sound 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 contribution 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 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.


Typical 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 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.

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 (CDMA)

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







TABLE 1 (end)




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 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 satellite link













240-280 ms










* Additional network delays may be incurred, see text in § 2.


3 System design and configuration factors


Three general principles should be observed 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 degradation 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 done by judicious selection of audio and video codecs and by compensation with audio delay units if necessary.

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 substantial 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 under 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.


4 Operating practices


Operation of facilities for programme contribution loops should conform with the following principles to minimize the leakage between half-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 headphone monitoring or in-ear monitoring to minimize leakage into open microphones.


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.


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