Slashdot Mirror
Lip Sync Problems with New Digital Displays?
An anonymous reader writes "With all of the new digital TV displays flying out the door, its easy to to think that life is good on the road to high definition. But, as Audioholics reports today, cheaper displays are using inexpensive processors that result in video delays of up to 60 milliseconds (that's about 2 frames of video). This means that the video processing (deinterlacing, video scaling, etc) delays the picture so that the audio is out of sync. Add to this inherent delays in some LCD and plasma units and the problem can be more than a little noticeable. As of right now only a few manufacturers are building audio lip-sync delay into their products to compensate."
Actually, that's the only way to fix this is a work around. As not only the article states, but common sense states that to fix this, you have to make video processing faster. We cannot do this with our current technology. So we have to use a work around until the technology catches up.
Audio and Video processing happens asynchronously, so I don't know how you can avoid this. You can set a time limit, but then you will limit the amount of processing that can occur which sacrifices audio or video quality. I have a Panasonic 42" Plasma that does internal scaling. This is slower than doing Dolby Digital decoding. My Anthem AVM20 processor has an audio delay feature where now my audio and video are back in sync. Receivers are getting this feature so eventually it'll be commonplace.
Italian westerns? Ohh, you mean easterns...
No, Spaghetti Westerns. Typically made from the mid 60s and early 70s, they made Clint Eastwood into the star he is today. Fast cuts, trippy music, lots of gunplay, and they were heavily (and poorly) dubbed, as most supporting roles were cast with italian actors.
They are considered classics now, as are the likes of "Fistful of Dollars" and "The Good, the Bad, and the Ugly" Wonderfully loony and fun to watch.
The old way was to read in a frame into memory. An other HW block or processor would perform the next operation, by reading that frame, process it and store it into memory again. The whole chain could be quite long.
This was not really a problem, they thought, because the audio was processed at the same time, and the delay was under full control of the soft and hardware.
until someone tried to use an external audio path...
As far as I know, they solved the problem, and the delay is minimal. And non existant if you route the audio over the same processor.
You'll probably find that the "delay" on most receivers actually refers to the delay in putting audio through the rear channel, thus creating more of a "surround" effect if you have to sit in line with the rears.
See, to get a sweet spot in a home theatre set up (wish I had room for a sweet spot in mine) is to set the rears equidistant from your ears as the fronts are. Unfortunately, many room environments don't allow for this, so you can use the receiver to "delay" the rear signals by so many milliseconds to make the surround more convincing at close range.
Karnal
I used to develop code for digital set-top boxes, and I can tell you that this is not a trivial problem.
Because of the way MPEG-2 video works, there is an inherent delay in decoding (frame order in the bitstream isn't necesarily the display order because of the way P-frames and B-frames work.
Audio is slaved to the video through the use of timestamps, but the audio and video frame boundaries don't line up.
I'm not sure if the problem is really lip-sync delay, but building in enough buffering to account for video delay while not glitching audio.
Most people don't notice minor video problems, like repeating or dropping a frame, but they will hear lots of little audio glitches. Also, when a hardware audio decoder runs dry, you usually get a really bad artifact (it sound like stepping on a squealing mouse), and it takes 2 to 4 frames of audio to resync.
(S(SKK)(SKK))(S(SKK)(SKK))
Slow processors can significantly delay the generation of the output video. Not only that, but the amount of work the processor has to do, which depends on how many changes from frame to frame take place, will cause varying delays.
The way the problem usually manifests itself is that the delta between video and audio gets biggere and bigger, the two slowly drift apart. The video is, of course, being backed up in memory. At some point it will run out of buffer capacity. The olde way of dealing with this was to just flush the buffer, which brings thing back into sync (for a while), but usually causes a nasty glitch in the video (blank screen for a few frames) in most cases.
Newer techniques involve dropping frames, more of them as the buffer fills up.
A good indicator that you are getting buffer overflow is when you change channel, then change back again and all is back in sync (for a while). This will have flushed the video stream buffer, and life will be good, untill it backs up again.
Faster processors can deal with the overall data rate without having to resort to these extremes, but the inherent delay caused by having to buffer a frame (or more) to be able to decode the next (because we are dealing with frame deltas in MPEG) will still cause varying delays in the video.
The real answer is to use adquate processing power, and to modify MPEG to insert timing marks into the video and audio streams, and allow the system to automatically and incrementally adjust the audio delay to keep it in sync with the video.
Expect to see a squadron of flying pigs before this happens ...
An even better answer, of course, is to scrap this digital TV crap. The best digital TV signal doesn't hold a candle to the best analog TV signal. All that digital buys is the ability to squeeze another 150 shopping channels onto every satellite at the expense of video quality - but that doesn't matter, its marginally better than VHS, so what will the consumers ever know?