[PD] A "multiple problem" with video playing
Simon Wise
simonzwise at gmail.com
Thu Jan 28 10:16:39 CET 2010
Matteo Sisti Sette wrote:
>
> tep wrote:
>
> > I'm using a few computers to play some synced videos, but i can't get
> > satisfying results using pix_movie.
>
> What is that is unsatisfying?
>
> Is the playback on each computer unsatisfying by itself (in which case I
> cannot be of help, whatever unsatisfying means),
> or is the syncronization between computers that is not perfect?
>
> In this second case, "how much" do you lose syncronization, i.e., how
> many frames of difference between any two computers do you accumulate
> per minute?
>
> If you just send a "start" signal to all computer and have them play the
> videos, and expect them to keep synchronized just by the exactness of
> their clock, I think it is impossible (with whatever software or codec)
> to maintain an acceptable synchro (meaning no much more than a couple of
> frames of difference) for more than a few minutes.
>
> I once had to do a synced playback of 3 videos and, if left to drift,
> they could stay apparently synchronized for as long as about 5 minutes.
> Indeed I was surprised by GEM's "clock" stability, I didn't expect so
> much (well actually i did, but I had been surprised a few months earlier
> with a similar application).
>
> To guarantee synchro within a given tolerance "forever", I did a patch
> with one "master" and several (in my case 3) "slaves" where the master
> broadcast (via udp) its current time every N seconds (e.g. N=5) and each
> slave compared the received time with its own, and if the difference was
> greater than a threshold (e.g. 2 frames), it forced itself to skip or
> pause the needed amount of frames.
>
> It worked quite well. With a slightly more sofisticated patch you could
> correct the drift in a more gentle way than abruptly pausing or skipping
> a few frames.
>
>
> I used Windows (not sure whether xp or vista, I think xp) and DV-PAL codec.
>
> By the way, I never used pix_movie's "auto" mode: I turned it off and
> fed the right inlet with a counter incremented by bangs taken from the
> [gemhead]. But i dubt this makes any difference.
I have synced several computers on a LAN successfully using a master,
with a counter run by a [metro], sending frame numbers to the slaves via
[netsend] and [netreceive], sending the frame number to each [pix_film]
object.
This way the master can also play audio which remains in sync, and
pausing, scrubbing etc are all possible.
Simon
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