# [PD] help with rfft~

Georg Holzmann grhPD at gmx.at
Tue Nov 29 23:50:14 CET 2005

```Hallo!

> Wait, (a+b*i) = frequency? If so that makes more sense as to why even
hm, frequency ... the bins are the frequencies. If you have e.g. a FFT
size of 1024 you will get 1024 different frequencies (well, 512, because
the other half is the negative spectrum ...)
e.g. 44100/1024 = 43, so the first sample of the 1024-block out of the
fft will give you a and b (or r and phi = Magnitude and Phase) of the
frequencies from 0 to 43 Hz (this is the first bin), the next bin is 43
to 86 Hz and so on ...

> that to mean more then just frequency. For example in the xy plane if
> you add two vectors you don't neccessarily just get a y value back.
how do you add 2 vectors and don't get a y value ?

> Multiplication in the frequency domain involves frequency magnitude, but
> also frequency phase and frequency value. It seems to me that frequency
> space is 3D where as time space is 2D also.
I don't understand what you mean with frequency value, both time and
frequency space can be real or complex ...

> I think AM is only time domain...Having looked at
> www.dspguide.com the correct equations are, unlike the
> last thing I posted (sorry!):
>
> ReY[f] = ReX[f]ReH[f] - ImX[f]ImH[f]
> ImY[f] = ImX[f]ReH[f] + ReX[f]ImH[f]

yes, that's what I wrote before:
(a1 + b1*i)*(a2 + b2*i) = a1*a2 - b1*b2 + (a1*b2 + a2*b1)*i
real            imaginary

LG
Georg

```