[vlc-devel] [PATCH] RFC: clock/aout: audio preroll discussion

Thomas Guillem thomas at gllm.fr
Wed Jun 20 16:23:20 CEST 2018

On Tue, Jun 19, 2018, at 19:37, Rémi Denis-Courmont wrote:
> Le tiistaina 19. kesäkuuta 2018, 18.09.32 EEST Thomas Guillem a écrit :
> > Currently, only the pulse audio module is able to start at a given time
> > (because the API allows to start a stream corked). Every other modules will
> > start right after the VLC Start callback.
> Err no. Start() does not have any audio data. Most audio outputs start on the 
> first Play(). And that first Play() actually conveys zero padding generated by 
> the core to time the playback correctly.

I didn't know that. Good to know.

> The problem is that this does not generally work well. It requires that the 
> latency is known correctly up-front. Not all audio drivers/hardware can 
> support this.
> > The problem is that the first play often need to be played in the future.
> That's not a problem: you can always defer audio, if you know how long.
> The opposite would be a problem.
> > There is often a 30/50ms pts delay (tested on current master). With the
> > future output clock, this delay should be arround the dejitter value of the
> > vlc_clock_t (200 or 500ms). No problem for pulse audio, but every other
> > modules will fail to start at this given time.
> If the audio output makes up a poor latency estimate, then this happens. 
> That's why PulseAudio does corking instead. If you remove the corking and 
> timer from PulseAudio, PulseAudio will simply exhibit the same problem.
> > We have some choices:
> > 
> >  1/ Simplify the date handling of aout modules: all modules behave the same
> >  way, and the core handle the delay (by playing silence when needed). So we
> >  move the delayed start functionally from PulseAudio to the core: this is
> > what this patch is doing.
> This is exactly what is already being done if TimeGet() succeeds before 
> playback starts.

So if I understand correctly: for pulseaudio, TimeGet() will fail before playback starts (because the uncork is delayed), but it's OK since this module handle the deferred start itself. 

For other modules, they should return a valid TimeGet() before playback is started in order to play some silence that will correspond to the audio device hw delay. Right ?

But how do you handle, in the future clock branch, a first play that is deferred by the clock dejitter time (200-500ms) ?
The core need to play some silence somehow, or should every module handle it themselves ?

> >  2/ Modify the aout API to handle 2 type of aout modules: one like
> > pulseaudio (that handle a play date), and all the others one.
> And this is the only way to get things working somewhat properly within the 
> current system, where the playback time is determined ahead.
> > I prefer the solution 1/ since it'll ease future debugging because all
> > modules will behave the same way.
> In other words, you want to keep the core as it is and rip out some code from 
> PulseAudio. End result: PulseAudio will work worse and the other outputs will 
> be exactly as broken as before.
> Well no thanks.
> > If you prefer the solution 2/, we need to think about a proper aout API.
> I prefer solution 2 and the proper API is what we have right now.
> -- 
> 雷米‧德尼-库尔蒙
> http://www.remlab.net/
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