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Discussion Groups | TMS320C54x | FFT questions!
Technical discussions about the TI C54x DSPs (including the c5401, c5402, c5402a, c5404, c5407, c5409, c5409a, c5410, c5410a, c5416, c5420, c5421, c5441, c549, c5470 and c5471).
FFT questions! - jjnews2000 - May 12 9:24:00 2004
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Hi, I need your help about FFT processing. Some questions will surely appear simple for many of you but please consider I'm not a specialist. I receive a signal which contains many frequencies, let's say from 0 to 30 kHz. What interests me is only the signal at about 9 kHz. In fact, I'd like to determine the magnitude of the incoming signal at about 9 kHz. First, i thought i should filter the incoming signal with a FIR filter for example and then process a FFT in order to get the magnitude at 9 kHz. But some questions appear to me : - Do I really need to filter the incoming signal? Tell me if I'm wrong, the FFT will process from the 0Hz until Fe/2 (I don't remember)? We can't make the FFT process from one given frequency to another one? So filtering isn't useful? - Of course I'd like the FFT to be the quickest as possible but giving sufficiently good results. So can anybody give me an advice as far as the number of points are concerned? - Tell me once again if I'm wrong : the result I'll get from the FFT is the square root of the sum of the squared real part and the squared imaginary part of the signal? In fact we get the "module" (in french) of the incoming signal for all the calculated frequencies? - As i work with the TMS320VC5402, can anybody suggest me which FFT code provided by TI in the dsplib i'd rather use for this application? - If you see another way to achieve this, please tell me! Just for information : the DSP works with a 100 MHz frequency. As far as the sample frequency is concerned i can't tell you now what it is its value. It will depend on the time i'll need to process the FFT and other things. If it's possible i'd like it to be 100 kHz minimum. Another thing which may help you to answer to my questions : the incoming signal simply comes from an ADC. So i guess it's real (not complex). I hope this is clear enough. If you need more data or if something isn't clear enough tell me about it. Anyway, thank you very much in advance. |
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Re: FFT questions! - sks_dsp - May 17 13:05:00 2004
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If you are interested in the frequency information of only one bin, use the sliding DFT or Goertzel algorithm, it will be more efficient than calculating an N point FFT with the resolution you are looking for. -Shawn Steenhagen www.appliedsignalprocessing.com --- In , "jjnews2000" <jerome_lapeyre_mirande@h...> wrote: > Hi, > > I need your help about FFT processing. Some questions will surely > appear simple for many of you but please consider I'm not a > specialist. > > I receive a signal which contains many frequencies, let's say from 0 > to 30 kHz. What interests me is only the signal at about 9 kHz. In > fact, I'd like to determine the magnitude of the incoming signal at > about 9 kHz. > First, i thought i should filter the incoming signal with a FIR > filter for example and then process a FFT in order to get the > magnitude at 9 kHz. > But some questions appear to me : > > - Do I really need to filter the incoming signal? Tell me if I'm > wrong, the FFT will process from the 0Hz until Fe/2 (I don't > remember)? We can't make the FFT process from one given frequency to > another one? So filtering isn't useful? > > - Of course I'd like the FFT to be the quickest as possible but > giving sufficiently good results. So can anybody give me an advice > as far as the number of points are concerned? > > - Tell me once again if I'm wrong : the result I'll get from the FFT > is the square root of the sum of the squared real part and the > squared imaginary part of the signal? In fact we get the "module" > (in french) of the incoming signal for all the calculated > frequencies? > > - As i work with the TMS320VC5402, can anybody suggest me which FFT > code provided by TI in the dsplib i'd rather use for this > application? > > - If you see another way to achieve this, please tell me! > > Just for information : the DSP works with a 100 MHz frequency. As > far as the sample frequency is concerned i can't tell you now what > it is its value. It will depend on the time i'll need to process the > FFT and other things. If it's possible i'd like it to be 100 kHz > minimum. > Another thing which may help you to answer to my questions : the > incoming signal simply comes from an ADC. So i guess it's real (not > complex). > > I hope this is clear enough. If you need more data or if something > isn't clear enough tell me about it. > > Anyway, thank you very much in advance. |
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