[PD] fft beginner question

marius schebella marius.schebella at gmail.com
Thu Nov 22 16:38:25 CET 2007

in the rfft (as opposite to the fft) the missing dataset is supposed to 
be 0, that means if you think of a stereo channel as input, the second 
channel does not contain data/is muted. the missing data it is not 
related to time or index.
the information in both domains (time and frequency) is always exact the 

Frank Barknecht wrote:
> Hallo,
> hard off hat gesagt: // hard off wrote:
>>>> cos^2+sin^2=1
>> ah!
>> so, actually the FFT doesn't HAVE to use imaginary numbers....but they
>> are just used because "the complex-number system is
>> two-dimensional and that signals to be analysed are presumed to be
>> two-dimensional. They are decomposable into a sum of circular
>> trajectories"
>> every day a bit more of this falls into place.
> An interesting thing to note is that the rfft outputs two signals,
> where seemingly only one signal comes in. How can that be?  Actually
> what comes into the rfft object are two values as well: One is the
> usual signal amplitude visible as a patch cord, and the other is:
> time!
> Time is not visible in Pd, it's just there and normally always walks
> on. But inside of a FFT-subpatch, time is frozen for the duration of a
> block (or rather: the "current time" is assumed to be infinite or
> eternal) to allow calculating a different representation: the
> frequency domain representation. If you think about this freezing of
> time for too long this is actually quite mind-blowing as well.
> Ciao

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