[time-nuts] Question about SoundCard stability?

[Alan Melia]

Hi I do not follow al the techniques in detail but a lot of work has been
done on soundcard sampling rates in the low frequency amateur radio groups
where GPS locking is used to extract very weak signals from the noise in
very narrow band widths. It has been found that some of the supposed
standard samping rates are not exact divisors of the clock crystal and are
achieved by a bodge in teh software but are regarded as "close enough" for
some audio work The 11kHz rate is a particularly odd one but many of the
8kHz rates are quite a way off. There are several ways of locking the
spectrogram software to a harmonic of the 1pps.

If there is interest I may be able to dig out some URLs  a quick check didnt
yield what I wanted to show.

Alan G3NYK

----- Original Message ----- 
From: "jimlux" <jimlux at earthlink.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Friday, October 15, 2010 2:26 PM
Subject: Re: [time-nuts] Question about SoundCard stability?


> Ulrich Bangert wrote:
> > Gents,
> >
> > I have already pointed to this paper
> > http://ipnpr.jpl.nasa.gov/progress_report/42-121/121G.pdf for a number
of
> > times but appearantly it is still too less known or too less understood.
Its
> > appendix explains completely the necessary signal processing for
frequency
> > and phase extraction from a sampled sine using ALL samples. While the
paper
> > itself addresses this algo to radio frequencies it naturally works as
well
> > at audio frequencies.
>
>
> And, in any case, the RSA described in the paper is sampling an audio
> frequency beat note, so it's exactly applicable to what is contemplated
> here.
>
> As Ulrich comments in the rest of his post, the math is straightforward,
> the performance is all in the hardware execution.  When measuring a
> gnat's eyelash, you need to worry about the bumps on the eyelash.
>
> Sound cards in PCs have all sorts of idiosyncracies.  Consider them as a
> 10 bit/ 60dB sort of device:  For instance, the sampling clock may be
> fairly stable, but it has interference from the processor clock on it,
> so you'll see spurs from that.  There's leakage between channels.  The
> low frequency response isn't very wonderful. etc.
>
> The folks doing ham software defined radios (in particular with the
> Flex-Radio boxes of the SDR1000 vintage a few years ago) spent a lot of
> time trying out different external sound interfaces: the performance of
> the interface directly affects the RF performance in the Flex direct
> conversion scheme.  Unfortunately, a lot of the mail reflector archives
> aren't on-line, but there was a lot of empirical data that some
> dedicated people collected.
>
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