[linux-audio-user] Re: Firewire, what's the story?
clemens at ladisch.de
Tue Dec 9 07:01:53 EST 2003
Steve Harris wrote:
> On Tue, Dec 09, 2003 at 09:25:42AM +0100, Clemens Ladisch wrote:
> > > Isoch - 1394 has a timer operating on the bus. This timer happens
> > > (roughly) every 125uS.
> > USB isochronous transfers happen once per millisecond.
> OK, so its the difference between 44 or 45 samples per packet at 44.1kHz
> (USB) and 5 or 6 (1394).
> So, a plausible jack period of 64 samples will be 11 or 12 1395 packets,
> but always 125uS of jitter of course, (10 or 11 packets at 48k).
> I dont really have any idea how bad that is, its 8% of the availble time
> slot for processing the 64 samples.
Both USB and 1394 transfer the data in packets (and the packet could
arrive near the end of a frame), so it must be buffered in the device
anyway. Consequently, this jitter does not appear in the data fed to
This additional buffering introduces another latency (of ca. one
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