[time-nuts] Using cheap sound cards for measurements

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Aug 21 23:17:24 UTC 2009


Lux, Jim (337C) wrote:
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>> Behalf Of Mark Sims
>> Sent: Friday, August 21, 2009 3:06 PM
>> To: time-nuts at febo.com
>> Subject: [time-nuts] Using cheap sound cards for measurements
>>
>>
>> Just because the cards have the same ADC clock does not mean that they
>> will be sampling at the same time.  There will be differences in the
>> startup characteristics,  register programming, etc.  that can affect
>> just when the input is sampled.  They may be sampling at the same rate,
>> but the cards could be an indeterminate number of cycles off from each
>> other.
>>
>>     
>
>
> I think that's unimportant, as long as the relationship stays constant (or predictable) over the entire recording interval (many thousands of seconds, perhaps).  You're not really comparing the absolute phase of the two signals, you're comparing the relative frequency of the two signals, so it's only the change in phase that you're looking at.
>
> Which brings up yet another interesting approach.  Say you were to use a digital audio recorder (as a standalone box), sample for however many hours you want to run the test for, and then post process en-masse.  
>
> I suspect you might NOT want a recorder that does compression, so you're going to be accumulating some 600 MB/hr for two channels, but these days, that's no big deal on flash media. 
>
> There's a lot of pretty high quality recorders around (like the Edirol R-44 or R-4 and equivalent)
>
>   
If the 2 signals are sampled at different times then there is incomplete
cancellation of the phase noise of the offset oscillator.
This effect increases with the delay between the 2 samples.
In practice the interchannel sampling delay can be relatively long
(hundreds of microsec) with a low noise offset oscillator.

Bruce




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