[LAD] Re : The future of videojack ?

Stéphane Letz letz at grame.fr
Wed May 7 11:06:19 UTC 2008

 >Well, this has been discussed to death on the jack-devel lists. I  
can see that from an audio developer's point of view,
 > it would be nice to have video within the same >server as audio.
  >However, there are fundamental differences between video and audio,  
which make this in my mind impractical.
 > Firstly, there is the problem of latency - for audio, generally the  
aim is to have a latency < 4ms. For video -
 > since we are dealing with much larger chunks of data, a >latency an  
order of magnitude greater than this is usually acceptible.
 >Second, the timing is different. For audio generally you have a 1024  
sample buffer and a rate of 44.1KHz or 48KHz. Video requires usually  
something like 25fps.
 > So you can either have two clocks, or you can split the video into  
chunks (ugh), both solutions have problems.
 > If you can solve these problems, then there is absolutely nothing  
stopping you running video and audio in the same server
 > (Video simply adds a new port type). Regards, Salsaman.

Video in jack1 won't happen because of several reasons that can be  
explained again: we want to fix and release jack1 soon and video in  
jack is a too big change to be integrated in the current state of the  
proposed patch.
The future of jack is now jack2, based on the jackdmp new  
implementation (http://www.grame.fr/~letz/jackdmp.html). A lot of work  
has already been done in this code base that is now API equivalent to  
jack2. New features are already worked on like the DBUS based control  
(developed in the "control" branch) and NetJack rework (developed in  
the "network" branch).
I think a combined "video + audio in a unique server" approach is  
perfectly possible: this would require having 2 separated graph for  
audio and video running at their own rate. Video and audio would be  
done in different callbacks and thus handled in different threads  
(probably running at 2 different priorities so that audio can  
"interrupt" video). Obviously doing that the right way would require a  
bit of work, but is probably much easier to design and implement in  
jackd2 codebase.
Thus I think a better overall approach to avoid "video jack fork" is  
to work in this direction, possibly by implementing video jack with  
the "separated server" idea first (since is is easier to implement).  
This could be started right away in a jack2 branch.
What do you think?

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