[opendtv] Re: ATSC and Lip Sync
- From: Cliff Benham <flyback1@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 30 May 2009 02:11:13 -0400
Mark Schubin wrote:
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?
If its a BBC production the thunder should come properly after the
lightning. If its a US production make the thunder simultaneous with the
lightning. #;^)
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?
I have a relatively inexpensive digital video converter that has 8
frames of storage in it. I can hold the thing in my hand.
Isn't there a broadcast quality synchronizer with enough storage
available to not reset in an hour long program or at least until during
a local break?
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?
You don't indicate whether this sequence is being produced as a portion
of a program to be edited later or if it is a 'live to air' event.
If it is being recorded for later editing, there are probably ways of
concealing the audio transition in post production.
If the entire sequence is live to air with music and singers or a
visible conductor, any shot that shows a performer 'out of rhythm' with
the music will be very disturbing to the viewer. The best alternative
here is to use discretion with the effects and minimize close-ups during
the sequence. It will be difficult to smoothly make unnoticeable
audio transitions live. Likewise, if the sequence is of dancers and lots
of long or moving shots are used it will be more difficult to perceive
an 'out of rhythm' condition.
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?
Find a wireless camera with latency ~equal~ to the other cameras or use
the wireless camera very judiciously.
To solve a problem created by just one camera by 'taxing' the rest and
dealing with the technical problems it causes is not worth what the
camera brings to the production.
5. A TV show is being presented in a large movie theater. Should the
lip sync be correct for someone in the front row?
NO.
In that case, it'll
be a frame out for someone roughly a dozen rows back.
Thats nearer to being correct.
A theater director I once worked for designed his shows for the middle
third of the audience which were the high priced seats. The sound
reinforcement was delayed to match that distance from the stage.
Also, for film in a theater I've read that the sound is acceptable even
if its as much as 3 frames behind the picture, but very detectable if
it's only two frames ahead.
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?
I don't see good lip sync for each person and conversational delay as
being mutually exclusive. The conversational delay is by far Einstein's
greatest curse to back haul TV over satellite.
I watch news shows face this problem every day. They have tried all
kinds of ploys like having the local host always end a question or
introduction with the distant guest's name as a cue, but it is a
workaround at best and a double hop 2 or 3 second delay, -but in perfect
lip sync- at best.
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).
One engineer has commented to me that the problem with their satellite
decoder audio is that management won't pay the licensing fee to use the
'better' audio coding available to them.
He also believes that embedding the audio at the source would help
eliminate lip sync problems down the line.
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.
My greatest complaint about consumer decoders is that when there is a
very slight video breakup, there is a corresponding 6 to 7 second period
of dead silence until the decoder 'catches up' again. Infuriating when
there are two or three in a row.
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.
I misused ATSC in the original post. I should have said 'digital video'
instead.
Cliff
TTFN,
Mark
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