The introduction of cascaded video and audio compression codecs, plus the typical frame delay of video processing units, means that without careful attention to propagation delays through the transmission chain in master control manes that the synchronism between video and audio can fall outside recommended guidelines. Audio arriving early is particularly confusing to the human brain, and means that hints to voice comprehension from lip movement are lost.
This was rarely an issue with analog processing, but now even the plasma screen in the home has a frame delay. To address this issue, SMPTE is requesting input on sources of lip-sync errors, measurement and correction. It has formed an Ad Hoc Group on lip-sync issues to review aspects of the problem and make recommendations for solutions.
The request for information includes the following areas:
- Sources of differential audio-video delay in television production, post-production, and distribution
- Audio-video delay issues through professional MPEG encoding and decoding systems
- Differential audio-video delay arising in consumer receiver, decoding and display devices
- Out-of-service methods of measuring differential audio-video delay
- In-service (during program) methods of measuring differential audio-video delay
- Devices for correcting differential audio-video delay at different points in the broadcast chain
For background purposes, SMPTE cites two documents: An ATSC Implementation Subcommittee Finding: "Relative Timing of Sound and Vision for Broadcast Operations," and an ITU recommendation labeled ITU-R BT.1359-1 - called Relative Timing of Sound and Vision for Broadcasting.
SMPTE wants to hear from manufacturers with practical solutions or proposals for measurement and correction of audio-video synchronization errors, and those willing to participate in development of related standards.
Responses should be sent to the ad hoc group chair, Graham Jones of NAB:
Telephone: 202-429 5345
1771 N Street, NW
Washington, D.C. 20036
For more information, visit www.nab.org.