No. 306 (April 2006)
Out of sync?
Whilst watching TV programmes, you could easily conclude that many people appearing on TV have developed a special talent – that of being able to move their lips out of synchronization with their voices! Of course, the problem of lip-sync is not a recent phenomenon, but it has reached epidemic proportions to the point where being “in-sync” is now a rarity. What is the problem?
Broadcasters tend to blame consumer equipment, such as digital TV set-top boxes or displays. Some consumer devices, such as flat-panel displays, necessarily involve delays to the video signal – caused by, for example, the process of converting interlaced pictures to progressive pictures. It is true that such devices can cause lip-sync problems due to video processing, but they generally result in a fixed delay between audio and video signals. Some of these “high-end” devices now include a fixed delay in the audio chain – in the hope that this will ensure that the audio and video signals will be synchronized. However, this does not solve the problem in practice.
Careful examination of many broadcast programmes will soon convince you that the relative delay between audio and video is definitely not fixed: it often varies dramatically between different items within a single news programme. Production of TV programme material involves significant processing by standards converters, synchronizers, audio and video compression systems, noise reducers, digital video effects units, etc. All of these processes cause differential delays between the sound and video signals. Regrettably, most broadcasters do not bother to correct such problems as and when they occur. Although the relative delay caused by an individual piece of equipment may be relatively small, the overall delay through the entire production chain is often unacceptable.
Unless all of the delays are corrected by broadcasters, unsatisfactory results will be seen and heard by consumers. So what do most broadcasters do to correct these relative delays? It may be harsh of me to say “nothing!”
The EBU has recently re-issued its Recommendation R37 on “The relative timing of the sound and vision components of a television signal”. The EBU has long recommended that, for the broadcast signal, the relative delay between audio and video signals should be within the range –40ms to +60ms (i.e. the sound signal should not arrive more than 40ms before the picture or later than 60ms after the picture). Casual observation suggests that these limits are regularly exceeded by almost every broadcaster in Europe.
The new version of Recommendation R37 indicates that the accuracy of synchronization at each stage in the production chain should be in the range –5ms to +15ms. Note that these limits are much tighter than the overall limit – and below the threshold of perception. As noted above, the cumulative effect of numerous small delays can easily exceed the overall limit.
Engineers pay great attention to the quality of the audio and video signals – but, for some reason, they ignore the problem of delays between audio and video signals. It is true that some of the reported problems are caused by processing delays in consumer equipment – but, if these are corrected, it is immediately obvious that the broadcasters need to be blamed for the variable delays caused by the complex production processes.
When I ask broadcasters what they are doing to overcome these problems, the general reaction is one of embarrassment. Everybody recognizes that the current situation is unsatisfactory – and, indeed, unprofessional. What we now need is determined action by broadcasters to measure the delays and to correct them. To all of the broadcasting professionals around Europe (and, indeed, around the world), my simple message is “Do not wait for others to solve the problem, do something now”.
Philip Laven
Director
EBU Technical Department
10 April 2006
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