[time-nuts] Question about SoundCard stability?

[jimlux]

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.