>I need to record at least 16 channels with no phase error between channels.
>
>Is it possible with alsa to start (trigger) two cards or two ADAT ports on
>same card such way, that first samples from both devices are exactly from
>the same moment? Devices are of course word clock connected. Time/phase
>error of two devices must be within one sample.
>
>I'm thinking of using two M-Audio Delta1010's or one RME Hammerfall cards.
certainly in theory. just use snd_pcm_sync() on the handles, and all
is done.
however, this relies on the assumption that the code path between
issuing the start instruction (in whatever form that is; probably a
register write) to the first card and doing the same to the second
occurs in significantly less than 1 frame of audio time.
this is not a bad assumption: my measurements with my trident and a
hammerfall suggested on the order of several hundred to a few thousand
instructions, which even on a 500MHz machine is on the order of 2
microseconds (versus a 48kHz sample time of 20usec).
but its still possible that if an interrupt occured at the wrong time,
the delay between issuing the two start instructions could be
sufficient to mess them up. its hard to work around this, though there
are approaches (using the devices as free-running abstractions is one
approach). fundamentally, without a multiprocessor, 2 threads and a
h/w interrupt that is handled by both processors, there is no way to
avoid a delay of some sort between issuing both start
instructions. you can just hope that the delay is insignificant.
--p
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