On Sun, Dec 09, 2012 at 12:12:15PM +0100, Jörn Nettingsmeier wrote:
> i could imagine how a situation like you describe could come about
> when wiring the cards this way, but it feels a bit weird. iiuc,
> connecting each card to an always-active wc source directly should
> have avoided the issue, right? when i get the chance, i'll check
> that.
(this may be related)
The way the (Linux) hdspm driver behaves has changed over time.
Originally, when the card was configured for external clock but
no clock signal was present, it would silently fall back on
internal, *and remain in that mode* even if later the external
clock became available. I complained about that, and apparently
things have changed. The behaviour I see now is that when the
card is configured for external clock but the clock is not
present, opening the device fails. Which is to be preferred as
it provides a clear indication that something is wrong.
Apart from that, syncing cards requires more than just a common
clock, you also need a single 'start' signal distributed in HW
('start' meaning that the next sample will be at position 0 in
the first buffer). If the start signal is just distributed in
software, then no matter how fast this is done there will always
be corner cases when one card sees the start in the current clock
period and another sees it in the next, resulting in a one sample
delay difference. I've even seen this happen between the capture
and playback sides of a single card (which ALSA sees as separate
devices).
Ciao,
--
FA
A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)
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