[linux-audio-user] Re: Firewire, what's the story?

Steve Harris S.W.Harris at ecs.soton.ac.uk
Tue Dec 9 06:22:54 EST 2003

On Tue, Dec 09, 2003 at 09:25:42AM +0100, Clemens Ladisch wrote:
> > > I seem to recall that it does, but that the packet size is smaller,
> > > making the latency problem easier to deal with.
> >
> > 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.

You could lessen the effect of the jitter by having triple-buffering in
software, as in a 3x64 PCI soundcard - I dont know wether thats better
than just going to 128 samples or not.

FWIW, for my needs I rarely go below 256 samples/period anyway, even
though my setup is capable of it, but I understand there are people who
really need very low latency.

At 256 samples/period the jitter is only 2.1% of the time slice, which is
unlilkly to matter - cache temperature and so on has a far greater effect
than that.

- Steve

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