[vlc-devel] Synchronisation between players for network streams.

[Gildas Bazin]

Hi all,

I just added a new module called "netsync" which allows several players to 
be kept synchronised (relatively precisely) when playing the same network 
stream.

The original aim of this module is to avoid an echo effect when you have 
several VLC players which are playing the same stream in the same room.

I'm not actually sure that this module is that helpful because VLC already 
keeps a very good synchronisation with the server that is sending the 
stream (at least in case of MPEG TS). But theoretically this 
synchronisation is not precise enough because it can vary by a small fixed 
delay (because of some caching on the network stack of the hardware or OS 
or even induced by VLC itself). So basically I wanted to experiment a bit 
with this.

This module is still quite basic so right now you have to start a "master" 
VLC player (which will be used as the reference clock) like this:
"vlc --extraintf netsync --netsync-master udp://@blabla"
Then start the "slave" clients with:
"vlc --extraintf netsync --netsync-master-ip=yourmasterip udp://@blabla"

In the few tests I've done, I haven't really noticed any improvements in the 
synchronisation when doing udp multicast streaming (ie. the synchronisation 
was already really good without netsync). But admittedly, high bitrate 
streams (video+audio) shouldn't really be affected by caching issues in the 
OS/hardware. There seems to be a small gain however when running low 
bitrate audio only streams.

Netsync has the advantage though of harmonising the caching parameters of 
all the VLC players on the master's one. So for instance if all the players 
don't have the same caching value (--udp-caching/--http-caching) this won't 
matter.
The other big advantage is for http streaming (with VLC). HTTP streaming 
doesn't maintain a very good synchronisation between all the clients 
(because it is using PULL streaming). So here netsync is actually very 
useful.

Anyway, just wanted to share the few thoughts I had on this.

--
Gildas

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