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Re: How do YOU handle this disparity in fft?


From: Macy
Subject: Re: How do YOU handle this disparity in fft?
Date: Tue, 11 Feb 2014 06:06:43 -0800

Thank you for your response. True, I need to 'look' at exactly what the effect 
of sampling at the Nyquist rate means. sampling a cosine wave gets min/max 
numbers and sampling a sine wave gets ZERO! luckily the data point at p/2+1 is 
NOT used. ...thanks just got my answer for part of my reply to Mike.

I once had the problem of 'oversampling' causing that irksome numerical 
analysis limitation of having the sampling change so slowly [translate to 
change so little] for each sample that the information sought got lost in 
quantization noise.  ok, ok, introduce dithering. Dithering is your friend. 



--- address@hidden wrote:

>..snip....
I would like to explain that last paragraph a little more.
 Suppose you have a pure sine wave that is Fs/2. This means that it is
sampled 2 times per cycle.
Now if the first sample just happens to be at a zero crossing then all
samples will be at zero crossings, and the FFT will show 0 energy at that
frequency.
Likewise If the first sample is at 1/4 Vmax then all samples will be at +-
1/4 Vmax, and the FFT will show 1/4 of the power that it should.

I would likewise point out that the inverse FFT of these signals would be 0
for the first one, and a sawtooth  for the last one, not a sine wave.

Finally: If you really  need to capture information about a certain
frequency then you must sample at a much higher rate than 2 time that
frequency,
I use a minimum of 8 or 10 samples per sine wave.
IHTH

Doug Stewart
>..snip...


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