[pulseaudio-discuss] alsa sink latency - how to account for startup delay

[Tanu Kaskinen]

On Tue, 2016-03-22 at 12:57 +0100, Georg Chini wrote:
> On 22.03.2016 12:51, Tanu Kaskinen wrote:
> > 
> > On Tue, 2016-03-22 at 12:33 +0100, Georg Chini wrote:
> > > 
> > > On 22.03.2016 12:20, Tanu Kaskinen wrote:
> > > > 
> > > > On Tue, 2016-03-22 at 10:11 +0100, Georg Chini wrote:
> > > > > 
> > > > > Hi,
> > > > > 
> > > > > when a sink is started, there is some delay before the first sample is
> > > > > really played.
> > > > > This delay is a constant part of the sink latency that will be always
> > > > > present, so the
> > > > > minimum sink latency cannot go below that start delay.
> > > > > Would it be acceptable to adjust the latency range for the device after
> > > > > each unsuspend
> > > > > to reflect that?
> > > > > USB devices (those I have access to) for example have a startup delay in
> > > > > the range of
> > > > > 10ms, but have a latency range that starts at 0.5ms which does not make
> > > > > a lot of sense
> > > > > in my opinion.
> > > > I don't understand why the startup delay would limit the minimum
> > > > latency once the stream is flowing. Imagine a sound card that is
> > > > powered by a nuclear power plant. I don't know how long it takes to
> > > > start a nuclear power plant, but let's say it takes a couple of days.
> > > > Now the sound card startup delay is a couple of days, but there's no
> > > > reason that the audio latency has to be a couple of days once the power
> > > > plant is running. Where would all that audio be buffered anyway?
> > > > 
> > > Hi Tanu,
> > > 
> > > you are wrong.
> > I don't believe you :)
> Look at the code of alsa-sink. It never drops samples. The only way to 
> compensate
> for the startup delay would be to drop audio as long as the sink is not 
> yet playing,
> but that is not done. I could try to implement that however and then you 
> would be
> right, but with the current code at least for the alsa-sink the startup 
> delay will persist.

The sink isn't responsible for dropping samples in any case. The
connection between the start delay and the runtime latency just doesn't
exist at the sink level. It's module-loopback that introduces that
connection, and if samples are to be dropped, module-loopback is where
that should happen. And indeed, module-loopback should drop samples
when waiting for the sink to start - I've seen a bluetooth situation
where starting the sink took something like 8 seconds to start, and
module-loopback kept buffering all that time, which did not result in a
pleasant phone call experience :)

By the way, your mails have bad word wrapping. See how it looks:
https://lists.freedesktop.org/archives/pulseaudio-discuss/2016-March/025830.html

-- 
Tanu