Subject: RE: LIP synch and TMMBR; was Re: R: [AVT] CCMdraftreview-- should"bandwidth"includetheheaderoverhead

From: "Allison, Art" <[email-masked]>
Date: Mon, 5 Mar 2007 12:37:37 -0500

I think others more expert than I are addressing this more precisely
than I can. 

I jumped in because there was a discussion of codec only vs. codec plus
overhead alternatives,  I know that lip-synch is difficult to manage and
I thought it to be a factor that should be considered.
 
The signaling of each of the flows, at the codec level only for video
[and audio but assuming for simplicity it is the 'same method' - even
though audio is not in scope], would seem to make differential delay
assessment for the two flows through their paths more complex, and the
impact seemed relevant to contribute to the selection of which.  

The differences may be well handled by buffer models, but getting lip
synch right is a practical challenge for every digital system and the
offsets vary with flow rates for the components.  So from 100k meters
up, it seemed and still seems relevant to consider as a factor.

I leave to the advocates of  alternative flow definitions to assert
whether or not this consideration supports their case. 

See below for specific answers.

As background  <not directly applicable here>, see ATSC IS/191 at
http://www.atsc.org/standards/is_findings.html for determination of the
broadcast quality timing budgets.  I will note that the maintenance of
lip-synch within the 15ms lead and 45ms lag in outputs to fixed relative
delay delivery paths remains a challenge even with absolutely known (but
changing) differential path delays as the content changes. If it is hard
with complete knowledge, it will not be easier with less information....
:)

My concern is maintaining the quality of broadcasters' web streams to IP
devices.
Art
_____________
Art Allison
Director, Advanced Engineering
Science & Technology
National Association of Broadcasters
1771 N Street, NW
Washington, D.C. 20036
Phone: 202.429.5418
Fax: 202.777.4981
[email-masked]

The National Association of Broadcasters is a trade association that
advocates on behalf of more than 8,300 free, local radio and television
stations and also broadcast networks before Congress, the Federal
Communications Commission and the Courts.

-----Original Message-----
From: Magnus Westerlund [mailto:[email-masked]] 
Sent: Monday, February 26, 2007 5:40 AM
To: Allison, Art
Cc: Desineni, Harikishan; [email-masked]
Subject: LIP synch and TMMBR; was Re: R: [AVT] CCMdraftreview--
should"bandwidth"includetheheaderoverhead

Allison, Art skrev:
> How to control lip-sync or lack thereof is clearly out of scope here.
> 
> However, I respectively disagree with the AD that it is out of scope 
> as a factor in the decision between selection of a total bit rate 
> control vs. a partial bit rate control.

Can you please clarify what you mean with total vs partial bit-rate
control.
AA> Total includes overhead(s), partial is codec payload only. 
> 
> It is only one factor, but declaring it out of scope as a 
> consideration can be expected to impact the technical choice.
> 
> I request reconsideration and clarification, as surly the AD did not 
> intend to force the group to ignore a technical factor that is 
> material to the RFC content.
> 

To be clear, in AVT I am only another technical contributor. Cullen
Jennings is the responsible AD for this WG. I am not trying to force
something out. However I have so far failed to see how the issue you are
discussing do affect TMMBR. Looking at the fundamental synchronization
model RTP has, TMMBRs impact on lip-synch is purely implementation
specific. Sure there are things that in the implementation that controls
what is actually sent that could affect when packets get sent in such a
way that it has some impact on the lip-synch. Are there any special part
of the problem that you would like to have note about to help improve
implementations?
AA> Not really, it is just that receivers will have to deal with the
flow rate for both audio and video at the 'same' time, which impacts
complexity/processing power of the implementation. 
Thanks,
Art//

Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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S-164 80 Stockholm, Sweden | mailto: [email-masked]
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