Re: sound card calibration?
Hi Jaye,
I tried a little bit with a USB soundcard. It has a 12 MHz crystal
reference. (I'm assuming it is using that for sampling and playback
and not the USB clock from the host. I'd have to look up the data
sheet on the audio codec device that was on the board.) I measured
the xtal at -2.something ppm using a nearby probe with shortwave
receiver and WWV's carrier. That seems pretty good, but I don't know
what's typical for sound cards.
You could try the same. Open up and see what the reference frequency
is. Common ones include 12 MHz, 48k*512 = 24.576 MHz, 48k*256 =
12.288 MHz, 44.1k*512, and 44.1k*256.
Or if you have a sig gen or a counter that's good enough, you could
use that with the sound card...
Take a closer look at fldigi. The webpage suggests that it has a
sound card sampling frequency error measurement feature. The version
in stable on my machine didn't match what was described in the fldigi
doc page I googled into exactly, so I need to dig into that more. It
seemed to do something, but not sure what the numbers meant. One
number that is there doesn't match the direct measurement very well.
Audacity does a spectral display of captures, but doesn't do a large
enough FFT to get the resolution I'd need to measure 2.something PPM.
WWV sends out 500 and 600 Hz during alternating minutes. So, I also
tried capturing a few minutes of WWV with arecord and an AM shortwave
receiver. I used just a bit of Python/numpy/scipy/matplotlib code to
get the spectrum. Promising, and in the ballpark of what I measured
directly. I want to repeat it and look at the numbers a bit more
closely. Also want to try it with a bit longer sample, so I can get
finer frequency resolution and perhaps a bit more SNR.
gnuradio & grc might be an interesting way to write an application to do this.
I take it you're looking for bit more of a canned solution like what
fldigi may offer?
Best regards,
Drew
kb9fko
Reply to: