[time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop

Bob Camp kb8tq at n1k.org
Sun Oct 12 17:09:42 EDT 2014


A little more information:

If you are doing the ADC thing, you still need to estimate zero crossings. In all likelihood you would be doing bandpass filtering first (say 8 Hz to 12 Hz) on your 10 Hz note. Next you would do some sort of estimator to get the zero cross. A curve fit is one sort of estimator, there are others. A simple straight line fit over 4 or so points might do it. A higher order fit over a few more points is possible.  Why does that matter? The fit improves your accuracy quite a bit. It also reduces your vulnerability to odd single sample issues like popcorn noise. Since you are running at a very low frequency 1/f noise can be an issue.


On Oct 12, 2014, at 2:37 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> anders.e.e.wallin at gmail.com said:
>> Does it matter that the ADC in the sound-card is probably clocked by a
>> crystal clock that is 50ppm off and has bad ADEV? 
> You can calibrate the clock on the ADC.
> One way is to feed a known reference frequency in on the other channel.  
> (That's assuming you have a stereo setup and don't need the second channel 
> for something else.)
> Another way is to compare the sample rate with the PC clock.  That will 
> correct for any long term drift but may not track shorter transients.
> -- 
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