[FFmpeg-user] RTSP stream fps 29.97 instead of 30

[Jonathan Viney]

On Tue, Oct 3, 2017 at 1:56 PM, Mark Thompson <sw at jkqxz.net> wrote:

> On 03/10/17 00:56, Jonathan Viney wrote:
> > Hi,
> >
> > I am pulling an RTSP stream from an Axis 4K IP camera. The stream from
> the
> > camera is 30fps, but the resulting stream from ffmpeg is 29.97fps. Here
> is
> > the command:
> >
> > ffmpeg -rtsp_transport tcp -i rtsp://10.9.9.1:554/axis-media/media.amp
> > -loglevel debug
> >
> > The rtsp log shows a framerate of 30:
> >
> > a=framerate:30.000000
> >
> > The full log output is below. Is there a way to force the framerate to 30
> > fps?
> >
> > This video undergoes a second pass where it gets re-encoded, so we could
> > adjust the frame rate there if necessary. But it would be preferable for
> it
> > to be right at this step.
> >
> > ...
> > [rtsp @ 0x40d0d80] SDP:
> > v=0
> > o=- 8374289283112756277 1 IN IP4 10.9.9.1
> > s=Session streamed with GStreamer
> > i=rtsp-server
> > t=0 0
> > a=tool:GStreamer
> > a=type:broadcast
> > a=range:npt=now-
> > a=control:rtsp://10.9.9.1:554/axis-media/media.amp
> > m=video 0 RTP/AVP 96
> > c=IN IP4 0.0.0.0
> > b=AS:240000
> > a=rtpmap:96 H264/90000
> > a=fmtp:96
> > packetization-mode=1;profile-level-id=640033;sprop-
> parameter-sets=Z2QAM6w0yAPABD/8BbgICAoAAAfSAAHUwdDAAGP/
> gAAMf+Nd5caGAAMf/AAAY/8a7y4b04A=,aO48MA==
>
> This SPS in the SDP decodes with:
>
> @126   VUI: timing_info_present_flag                                 1 (
> 1)
> @127   VUI: num_units_in_tick
> 00000000000000000000001111101001 (1001)
> @159   VUI: time_scale
>  00000000000000001110101001100000 (60000)
> @191   VUI: fixed_frame_rate_flag                                    1 (
> 1)
>
> That is, the H.264 stream indeed has a fixed framerate of 60000/1001/2 ~
> 29.97fps, and that is what ffmpeg is reading - the RTSP layer is likely
> lying about it.  (You may be able to count frames to check that.)
>
> If you want an exactly 30fps output then you should tell the camera to
> generate a 30fps stream rather than the 30000/1001fps it currently is.
>
> - Mark
>

Thanks Mark. That's interesting, I'll see if there is a way to adjust the
stream from the camera.

How did you decode the data from the sprop-parameter-sets value?

Thanks,
-Jonathan.