[Asterisk-Dev] iax2 video frame timestamps

[Derek Smithies]

Hi,
 Sending the I frames as full frames does not make much sense.

Consider H.261/H.263, which sends all video frames in the same UDP 
stream. The codec is required to "cope" with missing I frames.

If the I frame was sent reliably (as the proposal below) is dropped in
transmission, and then resent, imagine how it would look. The entire image
would then "jerk back" to what it was at the time of the I frame. This
effect will be less noticiable on a fast link. It will be noticeable on a
link where ping times are over 300 (or so) ms.


What video codecs often require is additional information sent in a 
reliable fashion. H323 sends this additional info with TCP channels.
My view is that we can reliably send this additional info with a video 
class of IEs.
 I am not keen to try and bodge in a reliable sending mechanism for 
mini-video frames that contain additional control info.



Derek.




On Sun, 1 May 2005, Steve Kann wrote:

> 
> On Apr 30, 2005, at 11:25 PM, Ben Lear wrote:
> 
> > Kenny Shumard wrote:
> >> On 4/29/05, Jeff Grollo <jgrollo at horizonlive.com> wrote:
> >>> The enlightening iax(2) spec released yesterday describes timestamps
> >>> for meta video frames as 16 bits. However, the diagram for meta video
> >>> frames displays a '?' as bit 0 in the timestamp. chan_iax2.c (line
> >>> 7324) only uses the lower 15 bits of the timestamp to construct a
> >>> full 32-bit timestamp:
> >>>
> >>>     fr.ts = (iaxs[fr.callno]->last & 0xFFFF8000L) | (ntohs(mh->ts) &
> >>> 0x7fff);
> >>>
> >>> Is that bit of the timestamp reserved or should the full 16 bits be
> >>> used in timestamp calculations?
> >>>
> >>> Jeff Grollo
> >>>
> >>
> >> This was exactly my question, and why I had that '?' there.
> >> Any insight from anyone on that bit?
> >
> > It would be nice if it is/could be used to indicate a complete image 
> > frame,
> > basically the equivilent of an RTP Headers "M" bit.
> 
> That's interesting;  I was thinking of sending I frames via full 
> frames, as opposed to "mini-video" frames,  Which would implicitly do 
> the same thing (and also get a retransmit/acknowledgment for them).    
> My feeling was that it made sense to send them reliably, since 
> otherwise the following P (or preceeding B) frames don't make much 
> sense without the I frame.
> 
> -SteveK
> 
> 
> 
> 
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-- 
Derek Smithies Ph.D.                         
IndraNet Technologies Ltd.                
Email: derek at indranet.co.nz         
ph +64 3 365 6485                          
Web: http://www.indranet-technologies.com/