Lip sync errors are becoming a significant problem in the digital television industry because of the large amounts of video signal processing used in television production, television broadcasting, and fixed-pixel television displays such as LCD, DLP and plasma panels. Audio and video synchronization problems occur because video processing is more intensive than audio processing. Because of this, the audio is ready for playback before the video, and if audio is not delayed, what viewers hear will not match what they see on the screen.
Figure 1: Lip sync errors usually occur because video processing delays are larger than audio processing delays. Signals may arrive in-sync to a CE device and leave out-of-sync.
Anywhere video is processed, there will be a delay. Video processing filters, format conversion, compression -- all of these will add delay, perhaps as little as a few pixels or one line of video, or perhaps as much as many frames of video. Although faster processors and clever algorithms can minimize these delays, they can never completely eliminate them. Ignore the delays, and you have audio and video out of sync.
On the display side, video processing delays become significant for LCD and plasma display panels (PDPs), where memory-based video-processing algorithms, as well as panel response times, can cause a delay of more than 100ms.
Compressed and broadcast video brings yet another difficulty in the form of variable delays. Since the amount of compression varies with video material, the instantaneous compressed bit rate (bits per frame, for instance) will vary as well. In order to use the bandwidth efficiently, the rate needs to be smoothed to an overall constant bit rate, and that means that the delay will vary.
Yet another origin of synchronization failure is when different audio/video system components (or even STV/TV tuner channels for the matter) are used in the chain. In other words, the audio/video delay can actually jump to a different value when a new device is inserted within the stream.
Finally, audio synchronization issues can also arise from wireless multi-channel speaker applications. Because of the inherent processing delays of wireless transmission, it takes more time for wireless transmitted channels to output audio than non-transmitted audio channels. The non-transmitted channels, therefore, need an additional delay to synchronize them with transmitted channels. As television applications become more and more sophisticated, and more wireless components are being adopted, a real need has developed for Lip Synch correction solutions.
Next: Industry activities to address sync errors