MPEG suspected in lip sync discrepancy
February 21, 2008
There is growing indication that MPEG-2 is the source of lip-sync problems in broadcast DTV audio.
“I can the same stream, play it back on two different receivers and get different lip sync problems,” said a broadcaster who has parsed the problem. “I can take the same stream and play it twice, and get yet different problems that before, leading to the belief that there’s a time stamp or a buffer problem within MPEG.”
Lip sync is notoriously bad in digital broadcast television, making a lot of content resemble the original English-dubbed Godzilla films. Washington, D.C. PBS affiliate WETA-DT, for example, often has a particularly pronounced gap between its audio and video, sometimes with the audio leading. Audio ahead of video is considered more objectionable to viewers than a converse latency.
The errors have been clocked in some cases, at full seconds.
The source said it’s unlikely that something in the ATSC DTV standard is causing lip-sync problems, because the same thing is happening in Europe. The same individual emphasized that while lip sync is often considered an audio issue, it could just as easily be a problem with dropped video frames.
Nailing down whether or not audio and video are losing track of each other in the MPEG stream will require comprehensive testing. If the problem is originating in the MPEG stream, fixing it could be formidable because of all the DTV sets and set-top boxes in the market that are programmed to decode MPEG-2 as it is.
The Society of Motion Picture and Television Engineers could shed some light on the subject in its joint effort with European Broadcasting Union to find a successor to the SMPTE time code and the creation of new reference signals. Digitized analog black bursts currently are used to synchronize digital signals. A digital synchronization method will be sought, and “whenever you start talking about synchronization, lip sync comes up,” the source said.
SMPTE and EBU are issuing a Request for Technology for the timecode a new color-sync standard that can handle the latest high resolutions, and a new black. Submissions are expected to be in review by June.