Date: Tue, 19 Mar 2002 10:31:08 -0500
From: "Steve Dove" <dsp@hifidelity.com>
Subject: Re: LF: Soundcard Mods
To: rsgb_lf_group@blacksheep.org
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Hi Andy,

As a couple of the other guys have pointed out, it depends on the soundcard.

Assuming it does have an onboard crystal or brick to determine sample-
rates, yes, of course one can take liberties with it.  Most audio-type sigma-
delta convertors have quite liberal specs and one can 'push' them
significantly;  50kHz for a nominally 44.1kHz part no problem.  (On a
digital stereo encoder I did for someone a while back, a 96kHz part cranked
to 128kHz satisfactorily . . . ).  One possible 'gotcha', although with ever-
faster machines much less likely to be a problem, if ever it was anway,
might be the service time of the interrupts starting to tread on their own
toes if the rate's cranked too hard.  Never had it as an issue, though.

We regularly mess with the clocks like that for better accuracy / weird
applications.

If you do feel the need to go 'after-market',  look at the external USB
interfaced codec units.  Gets the convertors out of THAT box, and they
definitely have their own rocks.  After-market units for the music industry
although still sadly prone to, um, hyperbolic specifications are far, far better 
than 'typical 'blasters.

With a touch of "living in a shoe-box beside the motorway", the microphone
input on my raptop is 3dB down at 80Hz.  Moral;  use the line in/outs. 

        Cheers,

                Steve        W3EEE

 

3/19/2002 9:59:54 AM, Talbot Andrew <ACTALBOT@dstl.gov.uk> wrote:

>Has anyone tried changing the clock source on a Soundcard for a different
>frequency ?  For a  job here at work we need to sample more accurately than
>the usual crystal oscillator (does this sound familiar ?) and at different
>sampling rates.  It would be ideal if we could push it to 50kHz, but 25kHz
>and 12.5kHz would do, as well as accurate 8 and 16kHz options.  One idea
>that immediately comes to mind is to remove the Soundcard clock crystal and
>feed in the input from a synthesizer.  Before diving inside a PC, I wonder
>if anyone has ever tried this ?  Modifying the integrated soundcard on the
>Dell machines here could prove a bit tricky just to try on spec.
>
>For LF use, how about an externally derived 1kHz sampling rate for input
>from receivers using a narrow CW filter and BFO pitch set below 400 Hz ?
>
>Andy  G4JNT - but with another hot on now.
>