Regarding pipeline latency and how to achieve a low-latency pipeline

[Nicolas Dufresne]

Le lundi 20 mars 2017 à 01:08 -0700, danny.smith a écrit :
> Hi!
> 
> I am working on a networked scenario in which one sender will transmit audio
> to multiple receivers whom will render the audio synchronized, following the
> description in this presentation:
> 
> https://gstreamer.freedesktop.org/data/events/gstreamer-conference/2015/Sebastian%20Dr%C3%B6ge%20-%20Synchronised%20multi-room%20media%20playback%20and%20distributed%20live%20media%20processing%20and%20mixing%20with%20GStreamer.pdf
> 
> and have some questions:
> 
> 1) Why is setting the total pipeline latency necessary if the receivers are
> identical?

You may need to add extra latency in case one devices takes a bit more
time to setup. Also, most of the time, the signalling is not done in
parallel, so there is a small delay.

> 
> 2) Which element will handle the additional buffering required by setting
> the total pipeline latency higher than the minimum latency?

The elements that have max_latency > min_latency in general. Most audio
src/sink, queues, etc.

> 
> 3) what is the explanation for min/max latency for a pipeline?

If you go lower then min latency, all frames will be late. Max latency,
is the duration that can be accumulated before data starts being
dropped.

> 
> 4) For the recievers; If I set a jitterbuffer latency of 100ms and alsasink
> buffer-time of 100ms, will this result in a total delay of 200ms?

Or more. The latency is a sum of each element latency.

> 
> 5) I am using the gstreamer netclock to achieve synchronized playback,
> sometimes during periods of high load on my network I get simultaneous skew
> on all my recievers (etime drifts roughly the same amount in the same
> direction). When the network load decreases this drift disappears. Is there
> a explanation for this and maybe a solution?

Reduce the network load or increase your bandwidth. The clock could
probably be improved for this case, but that would need research. I
don't myself know the details of this clock design.

> 
> 6) What would be the best approach to achieve a <100ms latency "best effort"
> pipeline?

Just setup your latencies so that they are below that level. Test to
make sure it does not stutter.

> 
> Would be grateful is someone could shed some light on the above topics :)
> 

Feel free to ask more question, and go in depth, we'll help as much as we can.

regards,
Nicolas
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