Subject: RE: [AVT] Sigalling for layered media -- do itgeneric or do itpayload specific

From: "Allison, Art" <[email-masked]>
Date: Mon, 5 Dec 2005 10:21:19 -0500

<lurker emerges>
Posit: There will continue to be new audio and video codecs developed.
If agree with that (which all should) then  - new transport means will
need to be developed for those with 'enough' business 'support'.
Therefore - serious consideration should be given to the parameterized
approach suggested here.
Logical conclusion  - better interoperability and fewer legacy issues
with a diverse set of supported codecs over time.
<lurk on>
______________
Art Allison
Director, Advanced Engineering
Science & Technology
National Association of Broadcasters
1771 N St. NW
Washington DC 20036
202 429 5418

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Andrea Lorenzo VITALI
Sent: Monday, December 05, 2005 9:09 AM
To: Stephan Wenger
Cc: 'Magnus Westerlund'; [email-masked]; 'Stephen Casner';
'AVT WG'; [email-masked]; 'David R Oran'; 'Colin Perkins';
[email-masked]
Subject: Re: [AVT] Sigalling for layered media -- do itgeneric or do
itpayload specific

I try to recap your mail and give my comments:

. Middleboxes cannot manipulate RTP streams without knowledge of the
signaling. This is ok to me, I would dare to say that it is obvious.

. Per-packet signaling is useful if different packets are muxed in the
same stream (as can be for H.264/RFC 3984) or if streams cannot be
prioritized (diffserv is not available). This is also ok to me.

In conclusion: I am not talking about media-unaware middleboxes. Instead
I am talking about a generic method to signal dependency and importance.

If you do not have such generic method, you are forced to implement
payload specific methods. This was done in RFC 3984 for H.264.

> three levels were sufficient for the then valid working draft.  That's

> where
:
> I could imagine technically valid arguments for more than two bits in
SVC.

As you say, defining a payload specific way to signal dependency and
importance can be a time consuming task. We have to reinvent the wheel
every time. Also, it may become soon obsoleted as it is tightly bounded
to the payload characteristics.

Generic signaling would have been better in this case.

Do you agree?

> I don't think the traditional MPEG-2 and H.263 Annex O style 
> enhancement layers (spatial and SNR scalability in the latter case) 
> need any "importance" or "priority" information.  In these cases, when

> you loose a packet you are basically done.  No chance of recovery but
intra.

Almost but not entirely true. Here is the scheme of MPEG-2 scalable
profile. IQ=inverse quantization, IT=inverse transform, P=prediction,
UP=upsampling, TS=temporal/spatial switch.

Harmonized hybrid decoder.

Enhance --> IQ1 --> IT --> + -----o-----> high res/quality
                            ^      |
                            |      |
                           TS<-P <-+
                            ^
                            |
                           UP
                         /    \
                        A      B
                        |      |
   Base --> IQ2 --> IT -o-> + -o---o-----> low res/quality
                            ^      |
                            |      |
                            +- P <-+

If the TS switch is in its "temporal" position, enhancement pictures are
predicted with motion-compensation in the enhancement layer. In this
case when you loose an enhancement packet, you you get only the base
quality until the next intra enhancement picture or until the TS switch
is in its "spatial" position.

If the TS switch is in its "spatial" position, enhancement pictures are
computed by upsampling pictures in the base layer. Upsampling can be
done in the error prediction domain (A) or in the pixel domain (B). In
this case when you loose an enhancement packet, only the current picture
is affected: you get only the base quality.

> To conclude: I don't think, the "priority", "importance' etc. is 
> something that should be specified in a media independent way.

I tried to explain that a media independent way can exist, and may be
worth implementing and using.

-- 
  ______________________________________________________________

   Andrea VITALI                            [email-masked]

   tel (+39) 039 603 7244             mobile (+39) 347 93 08 433
   fax (+39) 039 603 6129

   STMicroelectronics
   Centro Direzionale COLLEONI - Palazzo "DIALETTICA"
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  ______________________________________________________________

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt