Managing lip sync
Mar 1, 2009 12:00 PM, By Aldo Cugnini
Frame grab
A look at tomorrow’s technology
Mobile video downloads increasing
In Q4 2008, the average user downloaded six videos.
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Decoders vary in how often they recheck the PCR and PTS elements for synchronization, which can cause a problem if data is corrupted or missing. For instance, a simple decoder could be constructed that fills the buffer to some arbitrary point, and then proceeds to decode pictures without referring back to the PTS on an ongoing basis. Assuming all other data is correctly received, and the decoding frame rate is correct, the decoder could run indefinitely and appear to produce correct pictures and sound. But if there was an error in the timing algorithm, or if some data is lost in transmission, the playback timing could be sufficiently in error so as to produce an A/V sync error that persists indefinitely.
The problem with any such product is that there is no formal requirement that the decoding should work properly 100 percent of the time, other than that of product quality control. (And receiver manufacturers are loath to accept imposed requirements, as well.) In reality, any viewer encountering lip sync issues will almost certainly blame it on the program provider and not on the product. Any activity aimed at improving the situation would have to be from a cross-industry collaboration between broadcasters and consumer electronics manufacturers.
Few solutions at this time
In an earlier column, we took a look at some of the technologies that measure or control A/V sync at the broadcast plant. Part of the problem with their effectiveness is that the simplest test equipment requires the interruption of normal programming. Automatic online measurement and compensation could alternately provide a precise and self-correcting system. Technical committees are continuing to work on the problem, but the work is difficult.
IEC is working on standards relating to assessment, measurements and methods for A/V synchronization, but the results may not provide specifics for the broadcaster. The HDMI v1.3 and IEEE-1394 standards have features that help consumer equipment, but not in systems already installed.
CEA is working on a recommended practice, to be known as CEB-20, for DTV receiver implementers and developers, that relates to DTV receiver/decoder processing affecting a/v sync. Expected completion is mid-2009, after which ATSC will continue its own efforts.
More visibility needed
Unfortunately, A/V sync is the kind of problem that everyone knows about, but not all broadcasters and program distributors are willing or able to spend sufficient time or money in its solution, perhaps in part due to the difficulty of determining the actual effect on revenue. Perhaps therein lies an opportunity for manufacturers to develop solutions that are inexpensive and straightforward to implement.
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to: aldo.cugnini@penton.com
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