On 29.03.2016 20:59, Raymond Yau wrote:
>>
>> The USB driver will submit N silence URBs on startup in the prepare
and you will have to wait for those URBs to retire before the samples
are queued. There is very little 'USB processing'. If you want to
reduce this delay you have to use smaller periods, it'll decrease the
size of the URBs. I guess it could be possible to change the URB size
after the start but that's not implemented atm.
>>
For loopback, the source capture the same amount of data while you
wait for the retitement of those urbs
Yes, that is exactly the point.
> 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.
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