Foolou

I know .. 512 Instructions per cicle isn't much, but i think it should be possible to do a 4-point FFT.

Dunno if it's worth trying to build one, as the possibilities of such a filter would prolly be very limited.


Don't wanna bore you with my thoughts about the topic so stop reading if you ain't into those discussions:

With such an FFT-filter the second or third sample of the re-fft'd data would go to the output, the signal would be 2-3 samples delayed.
It could be possible to store values of the calculation on the tram to avoid recalculating multiplications, maybe it's even possible to go up to 8 sample-ffts that way. Really difficult to tell without taking a very close look on ffts.

Anyone got experience in progging an FFT-filter in dane? All i could see in this forum is that it is generally considered as not possible because of limited memory on the dsp.

Tiger M

no one has implemented FFT yet, so keep us in touch with the results if you achieve something

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Foolou

Took a look at the fourier stuff, and encountered a little problem:

The registers range from -1 to 1, but the first 'real' value of the fouriertransformed block of 4 samples is them all added together :
(f(x) is the samples, F the fouriertransformed, the entire thing looks pretty short because the input data is all real only.)

F(real)(0) = f(0)+f(1)+f(2)+f(3)
F(real)(1) = f(0)-f(2)
F(real)(2) = f(0)-f(1)+f(2)-f(3)
F(real)(3) = f(0)-f(2)
F(imag)(0) = 0
F(imag)(1) = f(3)-f(1)
F(imag)(2) = 0
F(imag)(3) = f(1)-f(3)

Actually it's not that much math needed to do a 4point-ft of real data.

But it would be necesarry to store values of up to 4 times the max of a normal register.
Would that work or cause problems due to saturated variables?

kokoon

how far is it from 4 point filter to a freq shifter? do you think it can be done? I WANT IT!

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[kokoon]

Foolou

Well without knowing details about frequency-shifting-FT-filters i assume it's done by shifting the FTdata.
With a 4-point-FT you could do samplerate/4 shifts, that'd be 12KHz shifts.

So unless you wanna talk and understand some being that communicates at frequencies up to 24KHz i fear a 4-point-FT wouldn't be of much help.

kokoon

hmm i kinda forgot all i learnt in the 1year of uni, but ... FT-ing a function (audio signal) gives you 2 arrays (real and imaginary) and shifting one (hmm maybe both) of them left/right gives you a very nice effect, kinda like pitch shift.

samplerate/4 should be more than enough, the question is can you shift contents of the entire arrays and can you shift them for less than 12KHz.

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[kokoon]

samplehunter

It definetly can be done, because there is a frequency shifter effect in the original CL drivers. But it's surely based on a Hilbert transformer and not on shifting FFT bins and iFFT. This would be a bit too heavy for this little Emu-thing.