Subject: Re: [linux-audio-user] Re: Firewire, what's the story?
From: Steve Harris (S.W.Harris_AT_ecs.soton.ac.uk)
Date: Tue Dec 09 2003 - 13:22:54 EET
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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