[time-nuts] Soundcard sampling Re: Picking a good HP 10811

Ulrich Bangert df6jb at ulrich-bangert.de
Wed Dec 3 10:45:48 UTC 2008


James and Stanley,

Bruce is already aware that I am doing sound card experiments since a
few years but you are not. 

That is why I would like to enter the discussion and encourage you to
perform some preliminary experiments on the topic that don't need the
real world existence of a suited sound card because you will generate
your "input" signals yourself the artificial way. This will enable you
to give your signals some known technical properties that are to be
found in real world signals too. 

I encourage you to test your algos for phase/frequency detection on
these artififial signals because if you discover problems with your algo
you will have an immediate understanding of the sources of your problems
(not necessarily a solution) because you made the signal in exactly this
way.

First generate yourself a sinusoidal signal at 1000 Hz and Amplitude 1
and a second at 1000.125 Hz and 80 dB down against the first signal. 

Const RunTime=1000;
      SampleRate=48000;
      f1=1000.0;
      f2=1000.125;
Var   I:Integer;
      Time:Double=0;
      Signal1,Signal2,Signal3:Double;

For I:=0 to (RunTime*SampleRate) do
begin;
  Time:=Time+1.0/SampleRate;
  Signal1:=Sin(2.0*Pi*f1*Time);
  Signal2:=1.0/10000.0*Sin(2.0*Pi*f2*Time);
  Signal3:=Signal1+Signal2;
end.

I write this down in a Pascalish style of language but it is easily
translated into any language of your choice. The thing modelled here are
two signals coming from a double mixer system with slightly different
frequencies. Typical cheap sound cards exhibit a crosstalk of -70 to -80
dB between the two channels. So Signal2 represents what is leaking from
channel 2 of the sound card into channel 1 of the soundcard and Signal3
repesents what you really find in channel 1 due to the crosstalk. 

Assumption is that the amplitude in channel 2 is close to "1" too.
Signal3 may be handled online or stored in a file or whatever you like.
Apply your algos to Signal3 and feed the results into an Tau-sigma
diagram, say on a second to second base (Tau0=1). The better your Algo
is the more you will see why I suggested this experiment.

Even -100 dB of crosstalk does not get you completely rid of this nasty
effect. You may even change the signals to integer entities and scale
not to "1" but to 32767 (for 16 bit resolution) to study the influence
of bit resolution of real world sound cards. After that experiment
decide yourself whether a cheap sound card with 80 dB channel separation
does the job.

After having understood that crosstalk is a limiting factor in using a
sound card for stability measurements I would like to draw your
attention to a case that has nothing to do with crosstalk at all but is
a fundamental property of ALL sampled systems with amplitude/time
quantization. Using something like:

Const RunTime=1000;
      SampleRate=48000;
      f1=1000.125;
Var   I:Integer;
      Time:Double=0;
      Signal1:Double;

For I:=0 to (RunTime*SampleRate) do
begin;
  Time:=Time+1.0/SampleRate;
  Signal1:=Round(32767.0*Sin(2.0*Pi*f1*Time)/32767.0;
end.

generate yourself a SINGLE signal of odd frequency and 16 bit
resolution. Apply your Algos to that and try to explain the small phase
modulation (similar to the crosstalk case) that you will find at a
repetition rate of 8 s. I know, the effect IS SMALL but after putting
your results into a tau-sigma diagram you notice how nasty it is
nevertheless.

I am writing you this because the real problems of a method are
sometimes to be found at places where one would not have expected them
at all.

Best regards
Ulrich Bangert
   

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Stanley Reynolds
> Gesendet: Mittwoch, 3. Dezember 2008 03:41
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Soundcard sampling Re: Picking a 
> good HP 10811
> 
> 
> Thanks Bruce,
> 
> Found it and reading ...
> 
> Stanley
> 
> 
> 
> 
> ________________________________
> From: Stanley Reynolds <stanley_reynolds at yahoo.com>
> To: time-nuts at febo.com
> Sent: Tuesday, December 2, 2008 8:32:27 PM
> Subject: Re: [time-nuts] Soundcard sampling Re: Picking a 
> good HP 10811
> 
> Found a basic common base amp would need to adjust values for 
> 10Mhz 
> http://i307.photobucket.com/albums/nn306/stanleyreynolds/commo
nbaseamp2.jpg
and this common emitter :
http://i307.photobucket.com/albums/nn306/stanleyreynolds/JPLCEtwostage.j
pg

Did not find the circuit for the NIST design just a article that
mentions it.

Stanley
_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.




More information about the time-nuts mailing list