On Tue, 20 Feb 2007 23:02:18 -0500, "Arny Krueger"
wrote:
The designer of Spectra is a local Ham - I know his
capabilities, I don't know yours. All I have to go on is
the pedestrian choice of Blackman windowing that
demonstrates nothing in the way of "ideas on the effect
on actual results." As I offered, it is an ordinarily
suitable choice if you have no demanding requirements.
I got it now, I'm dealing with a FFT snob, not someone who just sees them as
a tool to get a job done.
Connoisseur is more appropriate, and as for getting the job done, I
did that on contract to HP for one of their many FFT audio analyzers
22 years ago. I've written 200,000 lines of fourier code for many
products that get jobs done.
I also have the seminal work by Blackman and Tukey that predates the
math of the Fourier transform: "The Measurement of Power Spectra."
An extract bears repeating:
"'... we were able to discover in the general wave
record a very weak low-frequency peak which would
surely have escaped our attention without spectral
analysis. This peak, it turns out, is almost certainly
due to a swell from the Indian Ocean, 10,000 miles
distant. Physical dimensions a 1mm high and
a kilometer long.'"
The Hann or Hamm windowing functions are preferable as even Blackman
would admit and even these two are hardly exemplars of outstanding
performance. My designs exhibited a noise floor of -200dB
(statistical noise from the transcendentals' math). A poor Blackman
window would throw away 120dB of that to offer only -80dB. -80dB is
certainly impressive to mundane applications, but most would agree
that very little more effort was needed to gain 12 more orders of
magnitude in performance.
Hi Hank,
If you've waded through my prior list of freely available Fourier
analyzers, more can be found at:
http://sourceforge.net/search/?type_...soft&words=fft
Not all are applications however.
However, you should probably try to get a copy of HP's Application
Note 243-1 "Dynamic Signal Analyzer Applications." Of particular note
for your studies into the nuances of investigating construction
materials in Guitars (I did it with Violins), you should study the
Fourier math relating to "Coherence" suited for a dual channel
analyzer.
73's
Richard Clark, KB7QHC