[opendtv] Re: Lip sync problem resurfaces
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 20 Apr 2004 06:15:49 -0400
jeroen.stessen@xxxxxxxxxxx wrote:
<q
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.
/q>
For digital TV many displays these days have either digital inputs or
an integrated receiver. In either case it would seem you could have
both access to the signal and the needed knowledge of how much you
would like to delay the audio. So it would seem desirable to do so.
Even with DTCP or HDCP connections it would seem desirable to pass
through the audio to an AV Receiver, doing nothing but delaying that
audio in the digital domain. Video scalers should be able to do this
also, with each component adding the correct delay.
As long as it is some form of digital audio it would seem possible to
pass it through many components without degradation to a final AV
receiver that has to deal with the many user options and audio
processing choices that users might want these days.
With all the things it is possible to do to video signals these days I
don't think targeting zero delay video processing is the best choice.
Of course, once everyone starts being bothered by delay then even
low end AV receivers will probably start offering a variable audio
delay anyway.
- Tom
>
>
>
> 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.
> |------------------------------+-----------------------------------------|
> | From: Jeroen H. Stessen | E-mail: Jeroen.Stessen@xxxxxxxxxxx |
> |------------------------------+-----------------------------------------|
> | Building: SFJ-6.22 Eindhoven | Philips Digital Systems Laboratories |
> |------------------------------+-----------------------------------------|
> | Phone: ++31.40.27.32739 | P.O. Box 80002, (Street: Glaslaan 2) |
> |------------------------------+-----------------------------------------|
> | Fax: ++31.40.27.32572 | NL 5600 JB Eindhoven, the Netherlands |
> |------------------------------+-----------------------------------------|
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>
>
>
>
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