13 Nov
2009
13 Nov
'09
10:02 a.m.
FA:
On Wed, Nov 11, 2009 at 10:59:39PM +0000, Folderol
wrote:
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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Karl Hammar Aspö Data karl(a)aspodata.se
Lilla Aspö 148 Networks
S-742 94 Östhammar +46 173 140 57 Computers
Sweden +46 70 511 97 84 Consulting
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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.