# FFT result problem

Started by June 2, 2003
 Hello, I have Run 32 bit 1024 point Complex FFT on a Tiger Sharc. The DSP runs fft excalty as given in application note. Now i want to plot these results in real time using Streams in Visual DSP++ and using Visual DSP++ built in plot functions. Now Lets say i have Two tones 1200HZ and 700Hz. I want that plot should show Peak exaclty at 700Hz and 1200Hz. Is it possible by staying within Visual DSP++ envoirnment or do i have to export data to Matlab for Analysis. If this is so then What is use of plot function in Visual DSP++ Regards Maria Mountford
 Hi I did not try the plotting options in the VDSP. But i think that the plot result will depends on how we treat the data and how we select the axis. From ur explanation, the resolution of the FFT is Fs/1024, where Fs is the sampling frequency of the signal. So 700Hz should lies at the (700*1024/Fs)th point (array index, this may not be an integer. so the nearest point can approximate for the 700Hz). So a peak should be there at this point. Or u can scale the x-axis by Fs/N, now the x-axis corresponding to frequency. Note that the second half of the FFT spectrum is the mirror image of the first half it self. regards ajith --- In , Maria Mount wrote: > Hello, > I have Run 32 bit 1024 point Complex FFT on a Tiger Sharc. The DSP runs fft excalty as given in application note. Now i want to plot these results in real time using Streams in Visual DSP++ and using Visual DSP++ built in plot functions. Now Lets say i have Two tones 1200HZ and 700Hz. I want that plot should show Peak exaclty at 700Hz and 1200Hz. Is it possible by staying within Visual DSP++ envoirnment or do i have to export data to Matlab for Analysis. If this is so then What is use of plot function in Visual DSP++ > > > Regards > Maria Mountford