[opendtv] Re: ATSC and Lip Sync
- From: "Richard C. Ramsden" <ramsden@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 24 May 2009 00:27:54 -0400
The problem is worse.
A cheap set top might cause a frame or 2 of lack of sync. Yes, when the
source is really bad it might get off by a few tenths of a second.
Maybe I've never seen a bad cheap set top.
BUT!
have you noticed that the commercials are never out of lip sync?
The bottom line is the broadcasters don't care about anything that
doesn't cost them money.
Lip sync is not easy. There are no frame numbers in most source audio.
You have to keep track or get lost. There may or may not be any timing
info in the video, the fields are there, but they're not always filled in.
Mark Schubin wrote:
Cliff Benham wrote:
Almost ten years ago I wrote an email to this list [see attachment]
asking for discussion about the lip sync problems in ATSC.
Today it is much worse, not better than it was then.
Just watch any local or national news cast and you'll see/hear lip
sync disparities sometimes as much as two seconds [!!] worth in
various segments.
Somehow all audio must be embedded at 'the' source so it remains
synced with the video all the way into my eyes and ears.
It should just work.
There are three completely different issues here, and none of them is
the fault of the ATSC.
1. Programming distributors are doing more separate processing of
video and audio without compensating. As you point out, news is a
major culprit, and we started noticing the problem with the
near-concurrent arrival of digital video effects devices and
international news connections roughly 30 years ago (before there was
an ATSC looking into ghost cancellation, let alone DTT). DVEs, frame
syncs, and the like delay video; few delay audio to match.
Fortunately, there are many devices already on the market or coming
that can help this part of the problem, IF they are used (and, for
most of them, only if they are used in many stages of the production
process).
Even then, things are tricky. If you're doing a live interview
with a wireless camera, and you compensate for the encode-decode delay
of the wireless system, the frame syncs that might be used on the
wireless camera, the outbound path to the distant site, and the
inbound path from the distant site, the encode-decode delays on those
paths, and any DVEs that might be used, then, even without satellites,
you may be introducing a substantial conversational delay.
A solution is using things like the Boxx Meridian wireless system
(<1 ms delay) and the NTT ultra-low-latency transmission system
(likewise), both demonstrated at NAB 2009 and both requiring lots of
bandwidth. And even they don't solve the problem completely. Run
fiber between Europe and the U.S., and you're looking at delays
(ignoring all of the electronics) in the range of maybe three to ten
29.97-fps frames, depending on locations and routing -- not tragic for
a conversation but enough to inhibit intercontinental musician jams
and to require interesting echo cancellation even with "mix-minus"
production.
2. Non-CRT display technologies introduce video delays. At an HPA
Technology Retreat some years back, Bill Hogan showed a video sequence
shot of multiple monitors each getting the same video signal with
burned-in time code. Three different frame numbers were visible at
any time the sequence was frozen. Throw in multiviewers, and the
delay increases. The audio to the control room can be delayed to
match, but then there's a conflict with intercom and IFB. So it can
be hard even to determine what IS correct lip sync at the source.
3. There is no MPEG police force. Although the ATSC standard (like
most other audio-and-video bit-rate-reduction standards) includes a
perfect way to deal with any encode-decode slip (presentation time
stamps), receiver manufacturers rarely (if at all) check them after
acquiring a signal. When your lip sync goes out, try changing
channels briefly back and forth; it might lock in and then start
drifting again. Maybe this could come within FCC purview, but it is
not in any current requirements.
In summary, the bottom line is the bottom line. If you're willing to
spend $300 for your set-top box instead of $50, and you're willing to
pay $20 a month for your favorite news, these problems are resolvable.
If not, it's probably going to get a lot worse before it gets better.
TTFN,
Mark
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