[gst-devel] Gnonlin as a constant source of audio

Edward Hervey edward.hervey at collabora.co.uk
Tue Jun 10 15:43:10 CEST 2008


On Mon, 2008-06-09 at 22:46 -0400, Mike MacHenry wrote:
> I am writing a program using Gnonlin, which I feel might be a using
> the plugin in a way that it was not quite designed for. I am getting
> non-deterministic errors which are very difficult to pin down and I
> would like to just explain my design to the mailing list now. I am
> hoping to get some feedback on whether this design is too far from
> what gnonlin is supposed to do and if it's a wonder that it works. Or
> if maybe it's perfectly normal and the errors are well known.
> The design:
> Gnonlin seems to be designed to have several gnlfilesrc objects added
> to a gnlcomposition along with gnloperations and such and then played.
> There is a distinct separation between composition and playing. What I
> am doing is creating a composition with one file source in it and
> listening to the bus for a message telling me that that file has
> started playing. When it has, I add another file source to the
> composition after it. Once I get the signal that that next file is
> playing I queue up a third file and remove the first file source from
> the composition and stop it so that it can be cleaned up in memory.
> This goes on infinitely (in theory).
> The Problem:
> Occasionally, and eventually, it seems, for all streams I create, the
> composition gets confused. As soon as I queue up a file source for
> file number N with in the call back for file number N-1, I immediately
> receive a message that file N is playing, which causes my code to
> queue up a file source for N+1 and then N+1's message is also
> immediately sent and so forth until a ton of filesources are added. I
> immediately get an overload of file sources in the program and it
> crashes. It also seems to increase with load on the system. When 10
> clients play at once, they each run away at a faster rate.

  How exactly do you detect that a 'file is playing' ? GnlComposition
will preroll sources added to it, in order to have fast switching (from
one source to another). You might want to track the newsegment events
coming out of the composition (there will be one everytime the
composition switches sources).

> I can't figure out how the composition could get so out of sink. I
> suspect it has to do with its internal notion of the time line.
> Possibly I am confusing it with the way I add things. I welcome any
> ideas at all. Including questioning whether or not using gnonlin is
> right for this. Is there some other way of playing scheduled audio
> infinitely and dynamically? I have given the plugins a really good
> look over and I'm pretty sure there is no clear alternative but
> perhaps there is a clever use of other plugins I've not thought of.

  Try fixing the issue of when you add/remove the sources. Maybe the
rate issues is only related to the cpu overload. The composition doesn't
do any synchronization, it only modifies the values of the segment
events to provide proper synchronization downstream (in the sink

> Thanks for you help. I apologize for not having a reduced code example
> in this email but I tried very for a while to get a reduction that
> exhibits the issue but it's just so hard to reproduce consistantly. If
> you would like to read through the code that does this it is on source
> forge. Here is a link to the specific file of interest.
> http://roundware.svn.sourceforge.net/viewvc/roundware/server/roundwared/stream.py?revision=42&view=markup
> There is actually a function in there called has_runaway which is a
> hack to make the effects of this bug in my program more tolerable. You
> can find the rest of the code from their if you're interested. In case
> you are wondering what the big picture is, this code is a museum audio
> installation open now. Here is a link:
> http://aldrichart.org/exhibitions/burgund.php
> The link has an audio demo that may be illuminating as to what I'm
> doing with this code.
> -mike


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Edward Hervey <edward.hervey at collabora.co.uk>
Collabora Multimedia

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