[alsa-devel] Enabling in-kernel synch for M-Audio boards
aldev at sound-man.co.uk
Tue Sep 18 12:40:33 CEST 2007
On Tue, Sep 18, 2007 at 10:07:05AM +0200, Ludovico Verducci wrote:
> John Rigg ha scritto:
> > On Mon, Sep 17, 2007 at 08:44:03PM +0100, John Rigg wrote:
> >> On Mon, Sep 17, 2007 at 12:22:19PM +0200, Ludovico Verducci wrote:
> >>> I'm developing a complex multichannel audio distribution system where
> >>> multiple linux boxes will stream audio data over ethernet and then
> >>> should play audio at sample level resolution synchronization. The boxes
> >>> clocks are synchronized over ethernet using PTP.
> >>> I need to keep in synch the audio board's clocks and I can't use an
> >>> external wordclock nor s/pdif.
> >> Won't this cause serious clock jitter problems? I don't see how the
> >> PCI bus can deliver precise enough timing, considering how much other
> >> data it has to handle.
> I didn't mean to directly drive the audio board's clock over the PCI
> bus. I think this is simply not feasible. But I think that using control
> signals periodically exchanged over PCI between the audio board and the
> kernel could be possible (if the hardware could support a similar
> feature, of course) to skew the board's clock to keep it in synch with a
> software reference.
Ah, I misunderstood what you meant.
> As far as I know the delta family boards drivers support the
> synchronization of up to 4 audio boards over PCI: at the moment I'm
> reverse engineering the hardware trying to understand how this can be
I'm aware that some Windows users are using several Delta 1010s
without external sync, but I'm not sure how it is done (or how
good it sounds). AFAIK it would require a VCXO so that the frequency
of the card's clock could be varied by enough to keep it in sync
(ie. making the clock oscillator part of a phase locked loop). Looking
at the PCI card on the 1010, I can only see standard fixed-frequency
crystals. The only PLLs appear to be the internal PLL in the S/PDIF
receiver and the 4046 PLL chip for the word clock input signal.
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