[opendtv] Re: Lip sync problem resurfaces
- From: jeroen.stessen@xxxxxxxxxxx
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 20 Apr 2004 08:38:55 +0200
Hello all,
Frank Eory replied:
> There is no excuse for this. The system that decodes and processes
> the audio & video has all the tools it needs to maintain A/V sync.
It is not so simple !
Nowadays DVD players are embedded in "home theater sets", together
with (6) audio amplifiers that feed directly to the speakers. Only
the video signal goes to the display, where it may be delayed by
video signal processing and even the latency of the display itself.
There is no way that the audio can be actively delayed too, because
it simply doesn't pass through the processing circuits anymore.
And these affordable systems do not have the I/O connections for
applying an external audio delay. It is simple enough to delay the
audio IF you process it, but that is not always the case anymore.
I have once read a document in which it was stated that (IIRC) the
human tolerance for audio delay is between -30 and +80 ms relative
to the video. In other words: we tolerate better that the audio is
late than that the audio is early. Unfortunately, the typical
latency of even a minimal amount of video processing is already in
the order of 30 ms. We would be greatly helped if players and set-
top boxes would apply a nominal delay of e.g. 30 ms to the audio.
This would be well within the tolerable limits for a zero-delay
(i.e. plain CRT) display, and it would give other displays an extra
margin of 30 ms for high-latency video processing. Unfortunately,
everybody do their best to achieve zero delay difference, and then
it is the display processing that makes the delays unequal again.
> If that system is embedded in the display and there is an A/V
> sync issue, I call that a major design flaw.
It is a flaw, but you tell me how to avoid it ?!
> If that system is in an external STB, and the display does not
> provide a mechanism for bypassing its internal "value-added"
> video processing, I call that a major marketing flaw.
We have spent countless manyears in designing low-latency video
processing. Life would have been a lot easier if a higher latency
were allowed. But for the reasons stated above, it is not possible.
Also, some of this video processing can not be bypassed. For a
progressive matrix display, it is not an option to bypass the de-
interlacer. The minimum de-interlacer has one field memory, which
might be unused in case of fast motion in video mode, but it will
always be used in case of film mode ("segmented frames"). Frame
rate up-conversion algorithms (to get rid of 24 fps film judder)
need latency for calculating motion vectors and applying bidirec-
tional temporal interpolation. The plasma and micromirror displays
have essential frame memories for creating the sub-fields for their
digital pulse width modulation. The liquid crystal display has an
inevitable latency because the molecules need time to rotate to a
new state. And with a temporal aperture of a whole frame period,
even the fastest LCD would have an average latency of half that
period. This will also be true for continuously lit OLED displays.
(Some latency necessarily comes with reduction of field flicker.)
As I said: life would be a lot easier if audio came with a nominal
delay of e.g. 30 ms. Almost no modern display has zero latency
anymore. Only the classical (50-60 Hz) CRT can be "accused" of that.
Greetings,
-- Jeroen.
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| From: Jeroen H. Stessen | E-mail: Jeroen.Stessen@xxxxxxxxxxx |
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