Re: [LAU] Latency and USB interfaces

Tim Goetze wrote:
> [Clemens Ladisch]
>> In most cases, the ALSA buffer/period parameters directly reflect
>> hardware parameters, but in the case of protocols like USB where it is
>> not possible to transfer data directly out of a ring buffer, there is an
>> additional packet queue, which adds latency. (The length of that queue
>> is either constant or proportional to the period size, depending on the
>> driver.)
>
> Thanks you Clemens for clearing that up. How would one go about
> determining the size of this queue between ALSA and USB?

In the case of the current version of snd-usb-audio, the queue has the
same length as the buffer, but never more than 18 ms.

> And is it correct to assume that in a best-case scenario where the
> sample clock is synchronised to the USB clock

Pro devices typically don't do this.

> and the period size coincides with the interval at which USB frames
> are signalled,

This does not happen with Jack, where period sizes typically are a power
of two.

> the additional latency will be one period?

These two conditions have nothing to do with the queue length; they
ensure that USB interrupts happen exactly at period boundaries, i.e.,
that there is no jitter in the reported period interrupts.

Regards,
Clemens
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Received on Fri Jan 10 16:15:03 2014