CEA Publishes Recommended Practice on A/V Synchronization Implementation in Receivers

The Consumer Electronics Association recently published a Recommended Practice on A/V Synchronization Implementation in receivers, CEA-CEB20. This document was developed by CEA R4WG15, a working group of the R4 Video Systems Committee, under the chairmanship of Adam Goldberg. Adam has written the following contribution for TV TechCheck, covering the purpose of the RP and its main recommendations.

"Since the launch of over-the-air digital television (DTV) in the U.S., there has been some difficulty in maintaining accurate audio and video synchronization in television programming delivered to viewers. There have been (and still are) many reasons for this, including production, distribution and reception equipment each contributing to the varying delays of audio and video rendering. In mid-2008, there was growing concern that some DTV receivers were not using the MPEG-2 audio/video synchronization mechanisms properly. In response, the R4 standards committee in CEA began work on a document which sets out to explain to receiver designers who are familiar with but perhaps not expert on how synchronization provided for in the MPEG-2 Systems standard should be implemented.

There are some receivers/decoders which yield poor A/V synchronization, even in the presence of perfect streams. There are many reasons for this, among them poor or naive design choices, an imperfect understanding of how the synchronization mechanisms should work, or merely buggy implementations. Unfortunately, it is often the endpoint which gets blamed for problems which may be at the endpoint or may be upstream.

The document recommends specific hardware and software processing and explains how the MPEG timestamps (PCR and PTS) should be used. One of the specific industry concerns was that some receivers were only comparing PTS and system time once (at channel acquisition time), which yields A/V synchronization that drifts over time.

The hardware recommendations include specific functionality for the demultiplexer, and that receivers should include both a variable-rate oscillator driven hardware 27MHz clock and output delay hardware (buffers). The software recommendations include specific recommendation on how the encoder's 27MHz clock (the 'system time clock') should be recovered and maintained (using the PCRs in the stream) and that processing should continue over time (not just at acquisition time).

Finally, the document recommends against an overzealous reading of the MPEG-2 standard. Receiver designers should be aware of the theoretical operation of a receiver that is receiving perfect streams with idealized perfect hardware, but also should be aware of the practical considerations of a real-world system. Among other things, receivers should not attempt to detect every possible error and fail if any of them is received."

CEA-CEB20 is available for purchase on-line from the IHS Standards Store at this link.

Adam Goldberg has more than a 15 years experience in various facets of digital television, including the over-the-air digital transition, cable television, digital telecommunications activities, and home entertainment in general. He has been intimately involved with many aspects of DTV receivers, receiver silicon and broadcast equipment. He has held engineering, project management, strategic planning, standards and government relations roles for software vendors, decoder and silicon manufacturers, network and head-end vendors and consumer electronics companies. Now an independent consultant, he can be reached at adam@agp-llc.com or via www.agp-llc.com.

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Broadcast Equipment Auction Benefiting Bayliss Foundation Scholarship and
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