[LAU] Sound card for recording guitar

[Ralf Mardorf]

On Fri, 2013-12-20 at 15:04 +0100, Clemens Ladisch wrote:
> Ralf Mardorf wrote:
> > I never tested it myself, however, I remember that it often is mentioned
> > not to use -n >2. Is there a reason to avoid -n >2 or is t juts a myth?
> 
> The buffer size is the product of the period size (-p) and the number of
> periods (-n).  A larger buffer size increases latency, but reduces the
> risk of underruns.  A smaller period size _slightly_ increases CPU usage
> because of the overhead needed for handling a period.
> 
> Therefore, when optimizing for low latency, one typcially uses two
> periods and makes -p as small as possible.
> 
> With USB devices, the period boundaries (where interrupts are supposed
> to happen) are not necessarily coincident with the USB frame boundaries
> (where interrupts actually happen).  This results in delays (jitter) of
> up to 1 ms in the timing of period interrupts; with very small buffer
> sizes, this increases the risk of underruns greatly.  So if, e.g., the
> machine is not able to handle "-p 64 -n 2" reliably, increasing the
> number of periods to 3 results in lower latency (3*64=192) than
> increasing the period size (2*128=256).  (Using "-p 96 -n 2" would have
> the same latency, but works only if that particular Jack version allows
> period sizes that are not a power of two.)

Thank you too. Jeremy already posted a link to a thread where _you_
explained it :) (less detailed than now). Résumé: As Fons pointed out,
the sound quality isn't affected by the -n size. I assume even sync
between audio and MIDI, between several devices isn't affected by the -n
size, but just some prosumer USB devices could cause less good sync
between audio and MIDI, irrelevant for this thread.

Regards,
Ralf