[h323plus] Changing video codec transmit bit rate

Simon Horne s.horne at packetizer.com
Thu Nov 24 16:36:39 EST 2011



Yes you are correct  FlowControl for the InputDevice (change frame size and
rate) requires PTLIB 2.9 or above. Also FlowControl is not supported on
H.263 or H.261 which have fixed frame sizes. In H.264 you pass to the plugin
the required bitrate and it will return an appropriate frameWidth,
FrameHeight and FrameRate to match the proposed BitRate. In H.263 and H.261
the framesize is more rigid (CIF,QCIF) so really you can only adjust the


For H.261/H.263 If you set 

mediaFormat.SetBandwidth(yourrate) if SetFlowControl() fails in the
H323PluginVideoCodec::SetMaxBitRate(unsigned bitRate)  function then you'll
force the remote via the TCS to that bitrate. Then in your OpenVideoChannel
you set your inputdevice to a matching FrameRate (ie 128kbps/327kbps * 30 =
12)  then that should be a workaround to get the bitrate in both directions.


The limitation is you will not be able to change the bitrate during the






From: h323plus-bounces at lists.packetizer.com
[mailto:h323plus-bounces at lists.packetizer.com] On Behalf Of J.C Mercier
Sent: 25 November 2011 07:04
To: Simon Horne
Cc: h323plus at lists.packetizer.com
Subject: Re: [h323plus] Changing video codec transmit bit rate


Thanks Simon for your prompt response.

Having tested the code as suggested, it seems that the bit rate still
remains at 327kbps for the h.263 codec after i attempted to set it to

Is it possible that because this version of ptlib (2.8.5) does not support
flow control, changing the bit rate will not take affect?

I traced the SetMaxBitRate function, and it was called by the descendant
class in plugin, which is correct, which in turn looks as follows;

PBoolean H323PluginVideoCodec::SetMaxBitRate(unsigned bitRate)  
    if (SetFlowControl(codec,context,mediaFormat,bitRate/100)) {
         frameWidth =
         frameHeight =
         targetFrameTimeMs =
         return true;
    return false;

the SetFlowControl always returns false with the following message;

 - > "No Flow Control supported in codec"

Any ideas or other suggestions you may provide will be greatly appreciated.



On Thu, Nov 24, 2011 at 2:29 PM, Simon Horne <s.horne at packetizer.com> wrote:


Unfortunately the OpenVideoChannel is called after the H.245 TCS stage so
you cannot use it to negotiate a bandwidth limit for a call.

To set the Initial Bandwidth limit in the TCS there is a callback which is
called when building.
H323EndPoint::OnSetInitialBandwidth(H323VideoCodec * codec);

So you could do something like this in your code

void MyH323EndPoint::OnSetInitialBandwidth(H323VideoCodec * codec)
       unsigned bitrate = <your rate in kbps> * 1000;
       if ((bitrate != 0) && (codec->GetMediaFormat().GetBandwidth() >
bitrate)) {
               PTRACE(4,"My\tAdjusting maximum video bitrate to " <<

During a call the bitrate may change via FlowControl requests but will not
exceed the value you set in this callback.


-----Original Message-----
From: h323plus-bounces at lists.packetizer.com
[mailto:h323plus-bounces at lists.packetizer.com] On Behalf Of J C Mercier
Sent: 25 November 2011 05:08
To: h323plus at lists.packetizer.com
Subject: [h323plus] Changing video codec transmit bit rate

Hi all,

How would one go about adjusting the video codec max transmission bit rate?

I understand thus can be done in OpenVideoChannel by calling
codec.SetMaxBitRate but I I get  no flow control supported when I attempted
to do so.

I am using v 1.23 for H323plus downloaded from h323plus.org and pitlib v
2.8.5. I noticed that flow control is not supported in ptlib less than 2.9
but would there be another way of changing the bit rate?


Josh C.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.packetizer.com/pipermail/h323plus/attachments/20111125/929dfac9/attachment.htm>

More information about the h323plus mailing list