[Libav-user] latency of mpegts handling in libavformat

כרמל מגן carmel.work at gmail.com
Wed Jul 27 16:23:13 CEST 2011

I am sending and receiving h.264 video on mpeg2-ts over udp.
After noticing some lag, I added code that prints a timestamp and
framenumber after sending and after av_read_frame returns. when comparing
timestamps between frames with the same frame number, there is a ~60 ms
delay (when sending 30 fps). Furthermore, if I stop at a breakpoint and
delay the sending of frame number X, the timestamp of receiving frame number
X-2 corresponds to the time of sending of  X (i.e., the time between
receiving frame number X-3 and  frame number X-2 is longer if I delay the
sending of X longer). The unavoidable conclusion is that av_read_frame *does
not return* with frame X-2 until frame X starts arriving.


2011/7/27 Alex Cohn <alexcohn at netvision.net.il>

> On Wed, Jul 27, 2011 at 10:27, כרמל מגן <carmel.work at gmail.com> wrote:
>> av_read_frame does not return until the third frame arrives. This
>> introduces 2 frames delay *before* the decoding.
> What do you mean "av_read_frame does not return"?
> from avformat.h:
>  * This function returns what is stored in the file, and does not validate
>  * that what is there are valid frames for the decoder. It will split what
> is
> ...
>  * @return 0 if OK, < 0 on error or end of file
> So, does it return negative number? Or the packet is not a video packet?
> Maybe, you have the first video packet spliced into three network packets?
> Note that the first video frame is large (it is a key frame, naturally). The
> subsequent packet are much much smaller, becasue they encode only delta.
> Cheers,
> Alex
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