# [PD] help with rfft~

Georg Holzmann grhPD at gmx.at
Tue Nov 29 09:39:07 CET 2005

```Hallo!

> The help for fft~ says (for that rfft~ at least) the left and right
> outputs can be considered as the coefficents of cosine and sine
> respectively. Does this mean that if frequency and phase of a sinusoid
> can br represented as A*sin(2*PI*f*t)+B*cos(2*PI*f*t), then the left
> output of fft~ is B and the right output is A?
> If so, how do I get frequency and phase? (My calculus book is back in
> Boulder)

well, the output of a fft (or rfft) are complex numbers - the left
output is the real part, right the imaginary part.
And as you can represent complex numbers with cos + sin - you have the
representation from above.
If you want the amplitude+phase you have to convert these numbers from
rectangular to polar coordinates.

> If I am going to convolve the two signals - multiply in the frequency
> domain - what happens to phase?
convolution is a simply multiplication, but it's a multiplication of
complex numbers ...
(and it's of course different, when you use rectangular or polar
representation)

LG
Georg

```