Re: [Linphone-developers] Video receive\send buffers\birate control, h264.

[Jonathan Rosser]

On 12/11/10 10:31, Станислав wrote:
> Hello!
> I have found recently a thread about h264 new features - 
> (http://x264dev.multimedia.cx/archives/249)
> I'm trying to use these new features. With them its possible to get 
> <100 ms delay in video calls.
> But there is some problems, and main is: buffers and packet sizes.
> Is it possible to reduce(or remove) receive\send buffers in linphone 
> and where we set video packet size(its set to 1440 by default ?)  ?
> As you can see from thread above buffers are not needed if 
> "single-frame VBV" and "Periodic Intra Refresh" features of h264 are 
> used.
> With "slice-max-size" we can set slice size to packet size and so 
> buffers are not needed because every packet received = frame.
> Also i can see there is code in linphone that can reduce bitrate of 
> video if we get 10% frame loss...but there is no code to increase 
> bitrate to its original value...?

I have rewritten a large part of the msx264 plugin to use the 
"zerodelay" mode of recent x264 releases. This works very well and the 
latency is very low. I have concentrated on applications with high 
bandwidth WAN links - so testing is required in other more normal scenarios.

For high bitrates you need to ensure that the rfc3984 packing mode is 
set to 1 to allow NAL units to span multiple RTP packets (i think - 
someone may have more correct detail on what that actually does)

The VBV buffer size calculation is unique for zerodelay mode, and should 
be constrained to one picture worth of bits. Follow the method in the 
x264dev blog post and it is fairly straightforward.

I have not payed attention to the buffer size within linphone but can 
see that optimising this would reduce the latency even more.

The x264_picture_t initialisation code in msx264 is not correct for 
recent x264 releases and needs fixing to avoid a segfault.

> last - is it possible to make linphone to go fullscreen mode(windows 
> with video) when video call is established ? 

I would really like this to happen as well. I would like the video to 
full-screen, and remember which monitor of a multi-head configuration to 
open on. My setup uses auto-answer and I want to have the minimum of 
input from users.

Cheers,
Jonathan.

Senior Research Engineer
BBC Research & Development