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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 13:42:13 -0600
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Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
One might want to do some averaging over many frames.
In a former job, I spent much time looking at single shot power spectra,
and it's amazing how variable they are, even with a fairly good SNR.
If you wanted an extraction strategy, here's what I would do...
1) Figure out the break rate, so you can look only at the time spans with
the RF in them (i.e. no point in analyzing noise). There's typically some
amount of scatter in the precise timing of when the spark fires, but you
can probably pull out a 1 or 2 millisecond chunk that is "mostly" ringdown,
even if the actual start varies somewhat in the window.
2) Calculate the power spectrum (i.e throw away the phase information) for
each of those frames.
3) Calculate the mean and standard deviation in each frequency bin. (the
SD will provide a way to estimate SNR in that bin)
Start looking for aliases and intermods.
Since the original MPG was digitized, you really need to know what the
approximate sampling rate and compression method was (MP3?) and work at
those rates (after decompression).
Ideally, one could just digitize the audio track (I assume the original is
on videotape?) at a reasonably high rate, without compression (so the
compression doesn't introduce any artifacts.
> >By taking 4mS chunks of the sound track and letting the FFT
> >grind through
> >the numbers looking for higher frequencies, I "might" see the coils Fo
> >frequency too!!:
> >Way down in the noise, there is a consistent 35.8kHz signal
> >and a 69.8kHz
> >signal popping up during the streamers. That could be due to
> >many other
> >things and it is kind of a stretch to imagine that those
> >signal's would
> >survive through the years and media it was stored on.
> >But, we were thinking that the coil was running at about 40kHz and the
> >35kHz line does sort of agree with that...
> >Not sure I would bet a lot of money on this analysis, but it
> >is interesting!
> > Terry