Real Time FFT - Is the Prop up to it?

Philldapill

We are getting into digital filter design(finally!) in my Digital Signal Processing class. I've been playing around with filters in Matlab, and have tons of ideas for things I want to build involving these filters. With the 8 cogs in a propeller, I'd love to get knee deep in turning the propeller into a voice distorter. I've got a pretty nice ADC that can pull in about 500ksps with one cog, so ADC sampling isn't a problem.

However, I'm wondering if the Propeller is up to doing real time Fast Fourier Transforms with this ADC input. I've got 7 cogs to work with... Surely a decent FFT can be made with the propeller... Has anyone done some work on this?

Mike Green

It's not a FFT, but look at what Chip did in his vocal tract synthesis program and the stereo synthesizer that goes with it. It's all in the Object Exchange. Look under "Speech and Sound".

ngl

Check the forum for the topic ·Propeller DSP Software where you will find a fft program based on the fast hartley transform.

nick

Ale

Look for the file fft1024.zip that I posted several times... yes the prop is up to it...

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Visit some of my articles at Propeller Wiki:
MATH on the propeller propeller.wikispaces.com/MATH
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pPropQL020: propeller.wikispaces.com/pPropQL020
OMU for the pPropQL/020 propeller.wikispaces.com/OMU
pPropellerSim - A propeller simulator for ASM development sourceforge.net/projects/ppropellersim

Cluso99

Chalk another one up to the prop. It's a great little (multiple) 32bit CPU even without 32bit hardware multiply.

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Drone

Hi Ale,

I looked for the file fft1024.zip using Google advanced search as well as the parallax general search. Can't find it (not surprised). Perhaps you can post a link or better yet add the .zip to ObEx and/or the Propeller Wiki. There is some FFT stuff on from you on the Propeller Wiki, is this the source for fft1024.zip?

propeller.wikispaces.com/FFT

Regards, David

hover1

Is it the one in this thread?

·http://forums.parallax.com/showthread.php?p=803389

Jim

Dave Hein

Ale,

Can you provide some details about your FFT?· It looks like it works with 16-bit data, is that correct?· Does it work with real data inputs only, or will it work on complex inputs also?· How many cycles does it take to compute a single 1024-point transform?

Dave

Ale

It is that one.
It uses 16 bit values (15 bit plus sign) and I should work with real/complex pairs. but be aware of the cleaning of the imaginary buffer and the lack of decimation of that data.

I really have to improve that code and put it into the obex... long due :-(

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Visit some of my articles at Propeller Wiki:
MATH on the propeller propeller.wikispaces.com/MATH
pPropQL: propeller.wikispaces.com/pPropQL
pPropQL020: propeller.wikispaces.com/pPropQL020
OMU for the pPropQL/020 propeller.wikispaces.com/OMU
pPropellerSim - A propeller simulator for ASM development sourceforge.net/projects/ppropellersim

Dave Hein

Ale said...
It is that one.
It uses 16 bit values (15 bit plus sign) and I should work with real/complex pairs. but be aware of the cleaning of the imaginary buffer and the lack of decimation of that data.

I really have to improve that code and put it into the obex... long due :-(

It's been a while since I worked with the FFT, but I believe there are faster versions of the algorithm that take advantage of the zero imaginary part of real data input.
Do you have any timing data for your FFT?· I'm just trying to understand how fast of a sampling rate it can handle in real time.

Ale

I don't really remember but it is something like 20 or 30 per second, 1024 points FFT

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Visit some of my articles at Propeller Wiki:
MATH on the propeller propeller.wikispaces.com/MATH
pPropQL: propeller.wikispaces.com/pPropQL
pPropQL020: propeller.wikispaces.com/pPropQL020
OMU for the pPropQL/020 propeller.wikispaces.com/OMU
pPropellerSim - A propeller simulator for ASM development sourceforge.net/projects/ppropellersim

BR

This is a topic I've been playing around with when I can find the time....I started this thread to catalog all the DSP-related stuff I've come across for the Prop.

Philldapill

I didn't doubt the Prop(or the lovely Propeller coding guru's out there) for a second.

One of the things I want to build, is an audio frequency shifter. What I'm thinking, is getting an FFT, then shifting all the data points up an octave or something, then reconstructing the signal with the shifted frequencies. Is THIS possible in real time?

ManAtWork

Hello Phil,

if the only thing you want to do is shifting the voice up one octave then it could be done without FFT. Find the fundamental frequency, cut the stream of samples at the zero crossing points and simply play each snippet twice at double the sampling rate

Cheers