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Old 25th April 2005, 03:51 AM   #1
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Default DSP Recommendations for FFT Data

Hi-
I'm building a digitally controlled (for volume and preamp pots plus a remote control...also diy) amp based on LM3875... It's got a nice bitmap LCD screen (DIY font/graphic set/controller...)that displays information (ie, input). I'd like to be able to display FFT data of the audio that is being played... Any recommendations for cheap (less than $30) DSPs? How about DSP dev-kits? Doesn't have to be floating point, but I guess it would be a plus (for the educational value)... Thanks,

Doovieman
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Old 25th April 2005, 10:00 AM   #2
Giaime is offline Giaime  Italy
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Hi... I'm not an expert, but you can use some pass-band filters to select the frequency bands, and then feed them to some analog input in the microcontroller... and them write the code to sample the input signal and plot its value on the screen as a bar (or a line if you like)...
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Old 25th April 2005, 07:49 PM   #3
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Yeah, I could do that, but it's a lot of analog circuitry that I'd rather not deal with. Plus, I'd like to see the true FFT. Assuming I can do something like 20 frequency bins between 20Hz-20kHz, that would a lot of band pass filters... Anyway, if anyone has any suggestions, please let me know... Thanks,

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Old 25th April 2005, 08:29 PM   #4
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I'd like to alert you to the fact that the FFT does not provide octave bins (which for me would be a concern) to sample from, so you will have to create/calculate them yourself, which for low frequencies is a bit of a pain, you will have to decimate the signal a bunch and gather data for a much longer time in order to get much finer frequency bins out of your sampled signal. Also, your FFT size will be largely determined by the DSP chip, so you are going to have to decide on the size of the input you would like to gather at one time, which will lead to a very "jumpy" fft output unless you plan on working on an averaging or some sort of algorithm to record peaks.

I'm not saying it can't be done, but make sure your DSP purchase takes this into account. I don't have extensive knowledge in the field but just looking for a cheap chip might get you into trouble. Someone else who knows more feel free to interject.

The analogue method is easy.
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Old 25th April 2005, 08:30 PM   #5
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Sorry I forgot to mention, if you'd like to really get into the math, read up on the chirp-z transform which is similar to the FFT but will give you octave bins.
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Old 25th April 2005, 08:35 PM   #6
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Yeah, I know all that math stuff (had to take the class last fall...ugh), most DSPs are fast enough that it won't be that jumpy with, as you mentioned, some smoothing trickery. I'm looking for something like a SHARC that's cheaper (I want a dev kit and SHARC dev kits are expensive...)... Any thoughts?
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Old 25th April 2005, 08:45 PM   #7
ble0t is offline ble0t  United States
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You might be able to find some FFT code floating around, although all the bits I've seen have been written up in ASM (and for use in a microcontroller, not a DSP). You definitely are going to want to have a good understanding of the math behind it to properly implement it....
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Old 25th April 2005, 09:20 PM   #8
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Thanks for the replies so far...YES I do understand teh math very well...I have already implemented FFT on a DSP for a class... It's not very hard...DSPs come with FFT libraries. The question I'm asking is what DSP to use... I'm looking for something fast enough to do FFT of an audio signal and something cheap (less than $30)...if you have a recommendation, please let me know. Thanks,

Doovieman
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Old 25th April 2005, 09:33 PM   #9
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Default freescale dsp563xx

The freescale dsp563xx might be interesting. The have esai (enhanced serial audio interface) which is easily programmebly for I2S. The Altium Tasking DSP563xx IDE has a FFT sample. There is a working demo version of de IDE available. The DSPs are available as free samples. Creating the circuit around the dsp is probably the bigest concern. The dsp is tricky to solder and requires a external program rom.
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Old 26th April 2005, 09:06 AM   #10
Giaime is offline Giaime  Italy
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Maybe there is a dedicated chip just behind the corner!

Really, I bet you can salvage it from an old boombox. Do you remember those with the funny "bar-graphs" that show the average input on 5/6 fixed bands?
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