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Re: Fo frequency - Movie of Wendover Coi
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- Subject: Re: Fo frequency - Movie of Wendover Coi
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- Date: Mon, 18 Apr 2005 19:26:23 -0600
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Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 03:43 PM 4/18/2005, you wrote:
Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
At 03:38 PM 4/18/2005, you wrote:
Considering we have absolutely no idea how the original recording was
made or how the A/D conversion was made, all of this IMHO is a stretch,
at best. I believe any "data" you acquire this way is extremely suspect.
Yep!! I could almost see how a sliver of 35kHz might slip through all the
conversions but not 70kHz.
Fairly easy to get in. tens of kHz isn't all that far out of the passband,
and most systems have nothing like a brickwall filter (for one thing
they're expensive, for another, they have ringing and horrible phase
distortion). It was clearly recorded in "pre DSP FIR filter" days, so it
would have used analog filtering.
Once it's undersampled, it's down at baseband and will propagate all the
way through. What you wouldn't know is what all the intermediate sample
rates are, so relating a given frequency in the baseband back to a
pre-aliased frequency would be quite difficult.
One can sometimes infer a sampling rate, if the input signal is the result
of nonlinear mixing and has harmonically related spurs. Example.. say I
have a 30 kHz signal, and it has a strong second harmonic at 60 kHz. I
sample this at 11 kHz. The 30 aliases down to an apparent frequency of
(30-22 =) 8 kHz. The 60 aliases down to (60-55 =) 5 kHz. If the signal of
interest is moving, then the 5 kHz signal will move twice as fast as the 8
kHz signal (allowing you to identify it as a second harmonic).
Likewise, if you know that the 8 and 5 are the first and second harmonic,
you can solve for what sampling rates would cause that particular aliasing.
>>to look for fun and the 35kHz is "there"... The odds that it is the
Wendover coil's Fo frequency sneaking past all the electronics that is not
supposed to allow it though is pretty darn low.... Of course, the signal
is -60dB down just off the noise floor too...
And now you have practical experience at what TEMPEST engineers used to
do...(and may still do...for all I know)