1 May
2003
1 May
'03
2:08 p.m.
Mark,
Thanks, your message did indeed suggest to me a way that using different
sound cards for input and output makes things more complicated for the
sound system. Only if the input analog-to-digital converter (ADC) and
the output digital-to-analog (DAC) use the same clock, will the number
of input samples available for reading match the number of output
samples that should be written, over a given span of time. If there are
two clocks, one will be slightly faster than the other. If some sound
processing application is in a loop of reading X number of samples,
processing them, and then writing X number of (different) samples,
eventually the faster interface has to wait for the slower one to catch
up. Waiting means lost samples, and lost samples sometimes cause audible
clicks or pops.
Paul,
You're completely on top of the problem now. The 'normal' way to get
around this problem. as Steve suggested earlier, is to use a common clock.
This is not necessarily impossible with your cards, but would take some
work. At the studio level there are fancy, expensive clocks, like 'Word
Clock', but that may not be necessary for your application.
The 9636 has a s/pdif output. s/pdif carries a clock. Many Sound Blaster
boards have s/pdif inputs. Since s/pdif carries a clock, if you can
configure the Sound Blaster to use s/pdif as it's clock source, and you hook
a cable from the s/pdif output on the RME to the s/pdif input on the Sound
Blaster, then conceptually they will both run locked together.
I have done this with my Digi 001, RME HDSP 9652 and my RME 9636, all
operating under Windows where I have drivers and configuration tools that
allow control over this sort of thing, but using the ADAT path instead of
s/pdif. It works well. Unfortunately, Linux configuration tools are not
always supporting all the features of different cards, so you would have to
figure out whether this stuff would work for you.
Thomas recently did a little app called hdspconf which allows you to set
the clock source on the RME card. This could work for you if you used the
Sound Blaster as the master and the RME as the slave. I noticed some funny
effects when doing this myself, so I think you'd just have to try it
yourself and see how it goes.
So, I think there may be solutions if you're willing to dig in a bit.
Good luck,
Mark