[pulseaudio-discuss] bluetooth latencies

[Tanu Kaskinen]

On Thu, 2017-02-02 at 11:29 +0100, Georg Chini wrote:
> On 31.01.2017 14:51, Tanu Kaskinen wrote:
> > On Mon, 2017-01-30 at 17:35 +0100, Georg Chini wrote:
> > > Hello,
> > > 
> > > in module-bluez5-device.c and module-bluez4-device.c, latencies for
> > > bluetooth are defined as follows:
> > > 
> > > #define FIXED_LATENCY_PLAYBACK_A2DP (25 * PA_USEC_PER_MSEC)
> > > #define FIXED_LATENCY_PLAYBACK_SCO (125 * PA_USEC_PER_MSEC)
> > > #define FIXED_LATENCY_RECORD_A2DP   (25 * PA_USEC_PER_MSEC)
> > > #define FIXED_LATENCY_RECORD_SCO    (25 * PA_USEC_PER_MSEC)
> > > 
> > > Is the fixed latency for SCO playback a mistake? Both headsets I own
> > > report around 28 ms actual latency for the SCO sink, so I cannot
> > > understand why the fixed latency is set to 125 ms. Should I send a
> > > patch to correct it?
> > 
> > I don't know how Lennart came up with these numbers. How did you get
> > the latency report from the headset? AFAIK, the reason why these
> > constants exist is that we don't have a way to query the latency with
> > bluetooth, and reporting some latency is likely more accurate than
> > reporting zero latency.
> > 
> 
> Hi Tanu,
> 
> the values (plus one write block size) are passed to pulse via
> pa_{sink,source}_set_fixed_latency_within_thread() calls.
> So pacmd list-sinks shows a fixed latency of 128 ms for the
> SCO sink.
> When I use module-loopback, the latency of the sink reported
> by pa_sink_get_latency() is 28 ms however, similar to what I
> get for A2DP.
> 
> Since all other fixed latencies are at 25 ms and this is also set
> for SCO recording, I think that the 125 is just a typo.
> If you agree, I would send a patch to correct it.

I thought that the latency constants would be included in the value
that get_latency() reports, but apparently that's only true for A2DP in
module-bluez4-device (which I believe you're still using). In module-
bluez5-device the fixed latency is added to all reports.

I'm fine with changing the constant from 125 ms to 25 ms, it may very
well be a typo.

-- 
Tanu

https://www.patreon.com/tanuk