Late for the Future

Feb 1, 2008 12:00 PM, By Paul D. Lehrman

SYNC AND SWIM IN HDTV

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I guess I first noticed it a couple of years ago while I was watching an HD documentary on my local PBS channel. “Welcome to the future!” said the breathless hi-res promo just before the show. But then a talking head appeared, and it seemed that the future was going to be, well, a little delayed. I was seeing a very odd thing on my old 27-inch Panasonic CRT TV set: The person who was supposed to be an expert on whatever the program was about looked more like an actor playing a scientist in a badly dubbed, post-World War II, Japanese atomic-monster movie. The movements of his mouth and the sound I was hearing seemed to have very little to do with each other.

After a few minutes I figured it out: The program was out of sync, and the picture was later than the sound by something like half a second. A few minutes after that, something changed and the video and audio locked up.

In the months to come, I saw this happen a lot. Sometimes when I watched the news on a local commercial station that had switched to HD, the lip-sync would seem to shift whenever there was a remote pickup and some reporters were more out of sync than others. Sometimes even the anchors were out of sync for a little while, and then somehow it would get corrected.

When television programs relied on videotape and simple studio-to-transmitter coaxial or microwave links, the chances of the sound and picture getting away from each other were essentially nil. When the world of video was analog, if sound and picture left the plant in sync, then you could be sure that they would show up on the viewer's TV set that way. But as distribution systems became more numerous and elaborate, and digital technology entered the delivery path, opportunities were created for all sorts of gremlins to creep in. One memorable event was described by veteran TV mixer Ed Greene in his Hall of Fame acceptance speech at the recent TEC Awards banquet.

“I was watching an awards show — which I wasn't doing — that George Lucas, of all people, was speaking at, and he was seconds out of sync. The program was seconds out of sync for 20 minutes. So I called the mixer the next day, and I said, ‘What happened here?' And he said, ‘Well, when they took the program in back in New York, the primary [feed] was sent on fiber and the backup was sent on satellite. And they took picture from one and audio from the other.' [Cue giggles from the audience.] And then about 10 minutes into the program, they figured out it was wrong, and they switched — both of them. [Screams from the audience.] So I said, ‘How come at the end of it they didn't fix this for the West Coast [broadcast later]?' And he said, ‘Well, it was a Sunday, and that would have meant bringing somebody in on overtime.'” Loud groans. (You can watch the video at www.mixonline.com.)

But now that digital television signals are going right to the home, the chances of things screwing up have grown dramatically. And it's not just a problem with careless engineering. According to a lot of people whose business it is to make audio and video stay together, these problems are built into the system, and they're not going to go away soon.

In fact, this was the subject of a fascinating, if not particularly well-attended, session at the AES conference this pas fast fall entitled “Audio for HDTV: The Lip Sync Issue.” This seminar featured three presenters, none of whom were particularly optimistic.

Randy Conrod, digital products manager at Harris Corporation, discussed the nature of the problem. When it comes to viewer awareness of sync problems, he said, there are two timing thresholds: detectability, the point at which if the viewer tries to look for problems he will see them; and noticeability, the point at which the viewer notices them without trying. Both thresholds are much shorter if the audio leads the video, which is no surprise if you think about it for a moment. Sound arriving later than vision is part of the natural world, he explained, so when the sound is ahead, “Our brains find the experience particularly rattling.” He also pointed out that once a sync problem is noticed, the viewer is more likely to continue to be aware of it if the sound leads than if the picture leads.

Another member of the panel, Andrew Mason, who is an R&D engineer with the BBC, talked about some preliminary research he has done that suggests the problem is worse with HDTV — the acceptable delay window in either direction seems to be smaller with higher-resolution broadcasts.

Conrod pointed to an additional problem that is caused by delays, over and above their being annoying: When the audio is ahead, even if the timing differential drops below the detectability threshold, speech intelligibility and comprehension go way down. Maybe sportscasters don't care much whether their mouth movements match their words, but you can bet that the last thing an advertiser wants to hear is that because of some esoteric sync discrepancies, no one in the audience can remember their message.






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