[opendtv] Re: ATSC and Lip Sync
- From: Mark Schubin <TVMark@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 29 May 2009 23:31:31 -0400
Cliff Benham wrote:
IT SHOULD JUST WORK. PERIOD.
Would that it were that easy. Here are a few scenarios:
1. A program on storms is showing lightning strikes. They are probably
being shot from pretty far away. Should they deliver true lip sync,
which would involve the crack occurring perhaps seconds after the flash,
or should they fake it?
2. A conscientious broadcaster is airing a remote feed via a frame
sync, with a matching audio delay. Eventually, the frame sync's buffer
will fill and need to be reset, should the audio follow, causing an
audible pop, or should it wait, out of sync, until a pause?
3. A TD/vision mixer is doing an interesting effect, involving pictures
passing through several effects systems, each adding a field of video
delay. Should the audio mixer delay the audio to match? If so, what
should happen when the TD/vision mixer cuts from the effect to the live
camera and suddenly loses the video delay? Should the audio pop, should
it be out of sync during the effect, or should there be some transition
period, again waiting for a pause?
4. A wireless camera is being used as one of many in a show, and its
encode-decode processing adds latency. Is it better to delay the other
dozen cameras and the audio to match, introducing multiple points of
failure (and cost) and adding to the director's reaction time? Is it
better to delay the audio only when the wireless camera is used? If so,
what about split screens or other effects? And what about getting in
and out of the audio delay?
5. A TV show is being presented in a large movie theater. Should the
lip sync be correct for someone in the front row? In that case, it'll
be a frame out for someone roughly a dozen rows back.
6. A news show involves a three-way conversation among people at great
distances. Is it better to have perfect lip sync or to avoid adding to
the conversational delay?
Those are just a few issues that don't lend themselves to a "IT SHOULD
JUST WORK. PERIOD" solution.
Then there are many situations that DO lend themselves to a simple
solution that is simply not being done: frame-rate conversion without
compensating delay, Dolby E or AC-3 coding without compensation for both
encode and decode delays, etc. I would love to see those wiped out
through training and, perhaps, the expenditure of a little money (audio
delay capability is built into many audio consoles these days, and in
post it can just be slewed).
And then there are consumer decoders -- not just ATSC, but also cable
and satellite (I don't have any telco-video home experience, so I can't
speak to theirs). They can lose lip sync even when the broadcaster does
everything right. I share your frustration, and I applaud your
complaint, but I'd like to direct it appropriately.
By all means, send a complaint to the FCC about not requiring frequent
PTS synching in receivers. I point out merely that it is not the ATSC's
fault.
TTFN,
Mark
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