joeden

Hi, I'm new to the forum and I'm looking for help....I'm a beginner in electronics/microcontrollers and have always wanted to build my own Audio Graphic display, using lots of led's, probably 10-bands with 10 led's per band. I've found lots of info and source code on FFT's. I'm using a NEC uPD78F0034 8Mhz microcontroller and just wanted to know about the analogue sampling, how many samples, what resolution I should use etc. will 8Mhz be fast enough to do the sampling and FFT's. Is there any guideline projects already out there.

Thanks in advance,
Joe

kinjalgp

Don't know how much is the Machine Cycle of uPD78F0034 but standard 8051 (12 clock) running at 12MHz take 5 second to do 128 point DHFT (Discrete Fast Hartley Transform). FFT is still slower I think.

You need a real fast uC for spectrum analyzer. Using DSP will be better.

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bmcculla

I don't think the uPD78F0034 is an 8051. But it claims .12us instructions time which is one clock per instruction. The FFT is a very computation intensive algorithim. You might be better off with a filter bank approach. Instead of running the FFT you run a bunch of digital bandpass filters. If you only have a few graphic dispaly bars then you can cut down the computation required. With an FFT to get decent frequency resolution over the bandwidth required would probably require a window that would be longer than is computationaly possible in a short enough time. Your sample rate will have to be at least twice as fast as the highest frequency you want to see.

If you really want to do an FFT Silicon Labs has a 100MHz 8051 that is capable - they even have an App note for it. A DSP would be ideal though; they are designed for that sort of thing. You could check out Microchips DSPIC a PIC with DSP style instructions.

Brent

Nigel Goodwin

If you look on the PICList at http://www.piclist.com there's an FFT project using a high end PIC - they generate a VGA display in software as well to feed a PC monitor. Although it's not using your micro (which sounds a bit slow for FFT) it may give you a few ideas.

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crust

If you only have 10 bands, you dont need the full FFT. The FFT is designed to compute equal bins to the nyquist frequency. However, if you have already decided you only want 10 bins/lines, you dont need the full FFT. Just pick the frequencies you want and compute only that portion of the transform. You can find a pretty good basic description in my dissertation on my website, www.elkhashab.com

kinjalgp

I didn't say uPD78F0034 is an 8051. I was just comparing two controllers operating at 8MHz and 12MHz.

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bmcculla

kinjalgp: Sorry, I misunderstood what you were saying.

Brent