Re: usb-audio: variable periodsize / nframes per callback can shorten latency times

[Takashi Iwai <tiwai@suse.de>]

At Tue, 20 Aug 2002 13:34:03 +0200 (CEST),
Tim Goetze wrote:
> 
> Takashi Iwai wrote:
> 
> >> With the current setup and cycle code for mmap'd IO (like Jack for
> >> example does it), is it possible to use these devices at all?
> >
> >on 48kHz my usb speaker worked fine with 1ms or 2ms period size (not
> >under high loads, though), since the frames per urb becomes integer in
> >this case.  on 44.1kHz xrun could happen.
> 
> Yeah, a differing number of frames/cycle makes things hairy. But if
> one sets a period size of 45/90 frames/cycle and accepts 44/88/89 as
> well, things should work OK, shouldn't they?

well... for that, you have to know how much data to be written
_before_ actually you writing it.  i.e. you need an extra step, query
the (rest) period size -> then write/read, instead of always writing
the same amount of data.


> >> What kind of hw/sw params, especially period size, should one set?
> >
> >the frequency aligned to 1000Hz.
> 
> You mean divided by 1000, rounded up to the nearest integer, right?

yes.


Takashi


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