For those who want to calibrate the soundcard there is also a simple
hardware solution :
Take a CMOS4060 (14-stage divider with internal oscillator) and let it run
with a xtal in the 8-12MHz range. At the end of the divider chain (pin 3)
you will get the xtal frequency divided by 16384 (will be in the 500-700Hz
range).
If you can measure the oscillator frequency (available at pin 9) with a
100Hz accuracy you will know the audio frequency with 0.006Hz accuracy. You
get 100Hz accuracy with almost any frequency counter or by tuning into the
signal with a HF receiver.
With a better counter or a good RF receiver you can measure the oscillator
signal even up to 10Hz accurate (= 0.0006Hz for the audio signal).
Finaly, 'freaks' can take a 10MHz xtal and tune it zero beat against
Boulder or so. That will give a 1Hz accuracy on the xtal or 0.00006Hz on
the audio signal.
For those who want to try it but don't have acces to a CMOS databook I can
e-mail the circuit diagram.
73, Rik ON7YD
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