Question FFT spectrum rebuild and result interpretation
Posted: Mon Jan 20, 2020 1:20 pm
Hello!
I did a research again on some FFT things i did in the past and grabbed some older examples. But i always had an little error at iFFT that gave me a wron signal back (Not that wrong that you just notice it fast)
So i just to ask if someone can confirm this:
If you do a FFT with let's say 512 points you will get a magnitude spectrum that is "symetric" - But it is not that symetric as i thought:
Magnitude:
Point 0 is kind of Frequency 0 and defines the amp off-set from the whole wave
Point 1 to 255 is the spectrum
Point 256 is the amp of Frequency 1 - But it has added the symetric part too - so the real value is its half.
Point 257 to 511 is the amp-spectrum reversed without Point 256 and 0
So the point count of the real magnitude is 257 (N/2+1)
Phase:
Point 0 is the phase of the sin-offset
Point 1 to 255 phase for each frequenzy
Point 256 is the phase for frequenzy 1 - But it seems to have no symetric here
Point 257 to 511 is the phase of ech frequenzy inverted and reversed.
Am i missing something again?
Thx, C.Hackl
I did a research again on some FFT things i did in the past and grabbed some older examples. But i always had an little error at iFFT that gave me a wron signal back (Not that wrong that you just notice it fast)
So i just to ask if someone can confirm this:
If you do a FFT with let's say 512 points you will get a magnitude spectrum that is "symetric" - But it is not that symetric as i thought:
Magnitude:
Point 0 is kind of Frequency 0 and defines the amp off-set from the whole wave
Point 1 to 255 is the spectrum
Point 256 is the amp of Frequency 1 - But it has added the symetric part too - so the real value is its half.
Point 257 to 511 is the amp-spectrum reversed without Point 256 and 0
So the point count of the real magnitude is 257 (N/2+1)
Phase:
Point 0 is the phase of the sin-offset
Point 1 to 255 phase for each frequenzy
Point 256 is the phase for frequenzy 1 - But it seems to have no symetric here
Point 257 to 511 is the phase of ech frequenzy inverted and reversed.
Am i missing something again?
Thx, C.Hackl