Propeller Head Plaza

RE: Q: long....

130.199.3.130

In Reply to: RE: Q: long.... posted by gymwear5@hotmail.com on March 25, 2008 at 11:14:12

gymwear: ""
Usually FFT analyzers (but not ours) only let you look at the magnitude spectrum(no phase information). In this case there are an infinite number of time signals that can produce a given magnitude spectra. ""

Umm, that's been my point.
gymwear: ""
Whoa... but you have been lead on the wrong path, grasshopper.

Um, not exactly.. As you concur, ""there are an infinite number of time signals that can produce a given magnitude spectra"".. In other words, there are very distinct possibilities of signal manipulations that will be transparent to standard FFT analysis (single channel) magnitude spectra. In other words, a single channel analysis with identical results does not guarantee identical time varying signals were present..
In my specific construct, identical results using single channel could indeed be a false negative, eh?
gymwear: ""
Two issues, the single channel FFT is still a great tool for analyzing a signal that passes through a system - that is, if you know what goes in (say from an oscillator) if you get something different out then there is either linear (frequency response errors) or non-linear (new frequencies created).""

Agreed. I use the single channel FFT when appropriate.
gymwear: ""
Well, what if you had two or more channels of analysis? (B&K can offer well over 200 time synchronized channels.) Then you can measure the input to the system and the output of the system. And, here's the great part; you retain the phase information of the transfer function. The transfer function, in the signal analysis world, is called a frequency response function, and just like the input signal has as a real, imaginary, maginitude and phase representations. Of course, our analyzers can display all this stuff. With multichannel analysis you can easily document the differences - or more precisely contributions - each measured section offers in the signal path. The other thing that comes ot of the frequency response function is the ability to distinguish between linear and nonlinear "distortions" of the signal. Very cool.""

Um, which part of "collaboration" did you not understand?
While I can easily put together a good set of high bandwidth IA's and summing blocks to crunch the difference with analog circuits, I have approached you because I cannot afford a 30K instrument.
Positive results for my hypothesis could indeed result in increased interest in at least two channel phase sensitive FFT instruments such as yours.
Imagine being able to spot localization errors in the design/prototype stage for a piece of equipment...one of those high end audio holy grails, it is...

gymwear: ""
We don't sell much of this stuff to audio or electronics developers though because (1) it's pretty expensive with a 5 channel system going for about $30K and(2) a simpler single or dual channel analyzer can get them the information they need to develop the product. ""

I'm not interested in purchasing a big ($) system such as that...A collaborative effort is more to my liking.
Cheers, John

Edits: 03/25/08 03/25/08

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