[LAU] Latency and USB interfaces

[Clemens Ladisch]

Tim Goetze wrote:
> [Robin Gareus]
>> On 01/08/2014 05:54 PM, James Stone wrote:
>>> Also, how meaningful is the reported jackd buffer size in terms of
>>> actual latency?
>>
>> jack buffer size is only meaningful within jack.
>
> I was under the impression that in an optimal setup (jackd -d alsa -d
> hw:$X), the jack buffer/period setup is identical to what the driver
> uses to communicate with the hardware.

The Jack buffer/period setup is identical to what Jack's ALSA driver
uses to communicate with the ALSA kernel driver.

> In fact, examining the ALSA driver parameters[*] while jackd is
> running confirms this for both the UCG-102 USB and the ice1712-based
> PCI interface here.

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.)


Regards,
Clemens