[time-nuts] Question about SoundCard stability?

Fri Oct 15 10:14:51 UTC 2010

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. 

Those who are thinking of using a soundcard for serious time nuts
applications, say as a phase detector in a double mixer system, may be
warned: Not the math is the problem, even the soundcard's clock is easily
locked to a stable reference and this even if the soundcard is not prepared
for that. The real enemies are there where you won't expect them. If you
have never seen the worse impact that even a > 100 dB damped channel to
channel crosstalk (a very good value for semi-prof soundcards, bad ones may
give you 60 dB or less) has on a tau sigma diagram then you won't believe.
Been there, done that. 

A tau sigma diagram merciless reveals everything that is periodic in time
and has a period > Tau0. The combined phase/amplitude modulation that
results from sitting of a damped version of one channel's signal on top of
the other channel's signal due to crosstalk may be small but the tau sigma
diagram will reveal it with umpteen dBs up and down bumps in the graph where
you otherwise would have expected a straight line. The position of the first
bump is directly related to the beat frequency's period length. When I
noticed these artefacts in my real-world measurements it took me quite a
time to understand that it was due to crosstalk. In order to find out if
crosstalk in such a small amounts could give this big impact I wrote me a
piece of software where instead of sampling real world sines two sines were
computed and where I could add noise and crosstalk to the signals just as I
liked to do. When I set the noise level according to the value that the
manufacturer of the soundcard would claim for his product and did the same
for the crosstalk then I received EXACTLY the bad artefacts that I had seen
in my real world measurements.  

I have even tried to improve the crosstalk by mathematics. In principle that
is easy: If Crosstalk is merely ADDING one signal to another then remove the
crosstalk by SUBTRACTING a damped copy of the other channel's signal. But as
it is in life: Things that are easy in principle may be a problem in
reality. As it turned out the level of the subtracted signal was very
difficult to adjust to give a satisfying cancellation of added and
subtracted signal. In addition it turned out that the signal due to the
crosstalk had a phase delay against the signal in the producing channel. So
I needed to construct me not only a damped version but also a phase delayed
version of the sampled signal with damping AND phase delay freely setable.
And it seemed as if these parameters were slightly changing in time, making
necessary a permanent variation of the cancellation parameters. That
increased the necessary processing power to a point where the software would
not more run stable. Note that Greenhall's paper applies the algo offline to
signals which you have been sampled into files while I was going to compute
everything online to chunks of data worth one second of samples signals.   

Best regards

Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von shalimr9 at gmail.com
> Gesendet: Donnerstag, 14. Oktober 2010 22:36
> An: Time-Nuts
> Betreff: Re: [time-nuts] Question about SoundCard stability?
> 
> 
> I think that's what Jim is saying. If you try to fit to the 
> signal using only the zero crossing, it will be hard unless 
> you have a lot of zero crossing, because you will have only 
> one point per period to fit to. If you fit 10 or 100 points 
> per period, you improve your fitting considerably. That 
> assumes the signal waveform is stable of course.
> 
> Didier
> 
> Sent from my Verizon Wireless BlackBerry
> 
> -----Original Message-----
> From: David McClain <dbm at refined-audiometrics.com>
> Sender: time-nuts-bounces at febo.com
> Date: Thu, 14 Oct 2010 00:08:58 
> To: Discussion of precise time and frequency 
> measurement<time-nuts at febo.com>
> Reply-To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] Question about SoundCard stability?
> 
> > Or, now that I think about it, it's similar to what we do when
> > measuring ADEV.. you can do a crude "how many zero 
> crossings in the  
> > time window" or you can do a "fit a sinusoid to a series of ADC  
> > samples".  One has an uncertainty of "one count/epoch", the other  
> > can be substantially better.
> 
> 
> How could it be substantially better for the same analysis period?  
> Unless the frequency under test is an integral number of periods  
> during the analysis period, you will have a variation in the sine  
> fitting due to starting phase.
> 
> OTOH, as admonished in Horowitz & Hill, if the frequency to be  
> counted is substantially below your counter timebase, then 
> you should  
> count zero crossings of the higher timebase frequency in the period  
> of the lower frequency under test.
> 
> Dr. David McClain
> Chief Technical Officer
> Refined Audiometrics Laboratory
> 4391 N. Camino Ferreo
> Tucson, AZ  85750
> 
> email: dbm at refined-audiometrics.com
> phone: 1.520.390.3995
> web: http://refined-audiometrics.com
> 
> 
> 
> On Oct 13, 2010, at 22:30, jimlux wrote:
> 
> > Jim Lux wrote:
> >> That's not precisely true.  You can get a frequency estimate that  
> >> is substantially more precise than 1/T if the snr is high.   
> >> Consider super-resolution in an interferometer which is
> >> mathematically similar.  What you give up is ambiguity.  Probably  
> >> one of the oldest techniques is that of Prony, but there are lots  
> >> of others
> >
> > Or, now that I think about it, it's similar to what we do when
> > measuring ADEV.. you can do a crude "how many zero 
> crossings in the  
> > time window" or you can do a "fit a sinusoid to a series of ADC  
> > samples".  One has an uncertainty of "one count/epoch", the other  
> > can be substantially better.
> >
> >
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