Hi Olivier, On Wednesday 02 April 2014 00:31:59 Olivier Langlois wrote: > On Tue, 2014-04-01 at 15:49 +0200, Laurent Pinchart wrote: > > On Sunday 30 March 2014 00:23:01 Olivier Langlois wrote: > > > > > Yes. ffmpeg uses wall clock time to create timestamps for audio > > > > > packets from ALSA device. > > > > > > > > OK. I suppose I shouldn't drop support for the realtime clock like I > > > > wanted to then :-) > > > > > > > > > There is a bug in ffmpeg describing problems to synchronize audio > > > > > and the video from a v4l2 webcam. > > > > > > > > > > https://trac.ffmpeg.org/ticket/692 > > > > > > > > > > To workaround this issue, ffmpeg devs added a switch to convert back > > > > > > > > > > monotonic to realtime. From ffmpeg/libavdevice/v4l2.c: > > > > > -ts <int> .D.... set type of timestamps for > > > > > grabbed frames (from 0 to > > > > > 2) (default 0) > > > > > default .D.... use timestamps from the > > > > > kernel > > > > > abs .D.... use absolute timestamps > > > > > (wall clock) > > > > > mono2abs .D.... force conversion from > > > > > monotonic to absolute > > > > > timestamps > > > > > > > > > > If the v4l2 driver is able to send realtime ts, it is easier > > > > > synchronize in userspace if all inputs use the same clock. > > > > > > > > That might be a stupid question, but shouldn't ALSA use the monotonic > > > > clock instead ? > > > > > > I think that I have that answer why ffmpeg use realtime clock for ALSA > > > data. In fact ffmpeg uses realtime clock for every data coming from > > > capture devices and the purpose is to be able to seek into the recorded > > > stream by using the date where the recording occured. Same principle > > > than a camera recording dates when pictures are taken. > > > > > > now a tougher question is whether or not it is up to the driver to > > > provide these realtime ts. > > > > It makes sense to associate a wall time with recorded streams for that > > purpose, but synchronization should in my opinion be performed using the > > monotonic clock, as the wall time can jump around. I believe drivers > > should provide monotonic timestamps only. However, given that the uvcvideo > > driver has the option of providing wall clock timestamps, that option > > should work, so your patch makes sense. I'd still like to remove support > > for the wall clock though, but I don't want to break userspace. ffmpeg > > should be fixed, especially given that most V4L devices provide monotonic > > timestamps only. > > Please do not stop yourself to remove realtime ts support in your driver > because that would not break ffmpeg, IMHO. It is just me that have tried > to leverage options offered by your driver to remove the need to use > ffmpeg workaround for a sync issue. I apparently have been the first > ffmpeg user to try out! Then let's fix it first, and then I'll remove the option :-) > I am currently in the process to contribute the introduction of using > CLOCK_MONOTONIC inside ffmpeg. CCing their list because I think your > reply is relevant to the discussion we have on the topic there at the > moment. Very nice. Please keep me informed of the progress. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html