[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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