[LAU] open hw soundcard (was Re: status usb2 for sound)

[Karl Hammar]

FA:
> On Wed, Nov 11, 2009 at 10:59:39PM +0000, Folderol wrote:
> 
> > An entire cycle of 48kHz is about 20 uS so jitter would have to be
> > significantly less than that to avoid 'cogging'.
> 
> It's not the amplitude of the jitter that matters but
> the spectrum. Even the best and most expensive audio
> cards have lots of very low frequency jitter. And if
> you lock them to an external reference the local VLF
> jitter is replaced by that of the reference (within
> the PLL bandwidth).
> 
> Jitter creates phase modulation, and the amplitude
> of the sidebands increases with signal frequency.
> To have a clean top end any non-VLF jitter should
> be below a nanosecond or so. It's not so difficult
> at all to achieve this with a PLL provided the
> reference is stable.
> 
> Syncing two or more cards (so they will produce
> time-aligned samples if given the same analog input)
> is another matter. But if the 'soundcard' is dedicated
> hardware there are relatively simple solutions for that.

So..., as a first try it is better to do a multi channel card, than to 
do multiple single channel ones and try to make them sync ?

Regards,
/Karl

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