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

[Raymond Yau]

>>
>>> > I don't want to shorten the latency. I only want the latency reported
correctly. To me it still
>>> > looks like the real latency of the driver is not what it reports,
because the time that the
>>> > audio spends in the URB's is not taken into account. What I am seeing
is, that the real
>>> > latency is around 10ms longer than expected.
>>>
>>> The total number of URBs for the endpoint is not allowed to exceed
MAX_URBS (which the patch increases from 8 to 12).
>>>
>>> Do this match with your measurement
>>>
>>>
>> How much audio does one URB hold? The time I measure is between 8 and 9
ms and does not
>> depend much on the configured sink latency as far as I can tell. (I
tried latencies between
>> around 10ms and 2s). I did however not check the dependency in detail,
most observations
>> are with sink latencies in the range of 10 - 20ms.
>>
> OK, I did a few more measurements and the numbers I have given above are
not correct.
> The actual difference in overall latency is 12ms.
> When I run module-loopback with 40ms configured latency, I will see about
42ms with my
> code that accounts for the delay and 54ms with the old code.
> So if an URB holds 1ms of audio, this could match.
>
> I think the remaining 2ms are hardware delays, they are slightly
different for different
> combinations of source/sink and by setting small latency offsets (HDA
source: 0ms
> HDA sink: 2.8ms, USB source: 1.0ms and USB sink: 1.8ms) I am at 40ms
+/-0.5ms
> for all combinations.

Do module-loopback have higher latency than hda loopback mixing since most
hda codecs have analog mixer which support loopback mixing (e.g. mic
playback switch) ?
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