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

fons at kokkinizita.net fons at kokkinizita.net
Wed Nov 11 18:47:49 EST 2009

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.



Io lo dico sempre: l'Italia è troppo stretta e lunga.

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