Re: Distortion Measurement, use FFT or not ?

On Sat, 02 Feb 2008 22:01:32 +1100, Adam S <not.valid@nosuchaddress>
wrote:

Hi Bharat

I'm a bit confused as to how interpolation of FFT bin magnitudes can be
used to pin point the magnitude of tone laying at a frequency between
those bins.
As I understand, when a signal is not exactly a multiple of the sampling
frequency then the power of that signal is distributed across
surrounding FFT bins. So if one takes the average power (sum of squares
of magnitudes) over these bins then they will get almost the same power
as from a single bin when the input frequency is an exact multiple of
Fs. I just wrote a small test program to check this for myself.

Experimentally I found that when using a Hanning window the useful side
bins only extended about +-2 bin either side of the centre bin. The
spectrum power across these 5 bins showed to add up to about 0.9997 of
the total power.

I've heard FFT interpolation being used to estimating the frequency of a
single tone, but not its magnitude.

regards,

Adam

Magnitude is possible as well.

There are several techniques covered or linked here, I'd think there
might be one that'll suit your application. The only real caveat is
that the tones to be estimated need to be reasonably well isolated,
i.e., not a lot of nearby adjacent energy or the sidelobes from the
adjacent energy will affect the results.

http://www.ericjacobsen.org/fe.htm

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
.

Relevant Pages

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  • Re: Distortion Measurement, use FFT or not ?
    ... I'm a bit confused as to how interpolation of FFT bin magnitudes can be used to pin point the magnitude of tone laying at a frequency between those bins. ... when a signal is not exactly a multiple of the sampling frequency then the power of that signal is distributed across surrounding FFT bins. ...
    (comp.dsp)
  • Re: Distortion Measurement, use FFT or not ?
    ... surrounding FFT bins. ... of magnitudes) over these bins then they will get almost the same power ... single tone, but not its magnitude. ...
    (comp.dsp)