Kevin Holland

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8 years, 9 months ago.

mbed-dsp FFT question

Can someone help me understand why when I do a FFT of a 60Hz test signal, my Maximum BIN is always 2X what it should be? See the code below, I have changed the SAMPLE_RATE to change the BIN resolution and the FFT always comes out as double. In the code below I set the SAMPLE_RATE to 2048 and the FFT size to 2048. That should give me a BIN resolution of 1Hz. So my max BIN location should be 60, but it comes out as 120. I have also tried a 10kHz SAMPLE_RATE that should have resulted in a Max bin of 12 or 13, but it comes out as 25. Am I missing something?

#include "mbed.h"
#include "dsp.h"
#include "arm_const_structs.h"

#define SAMPLE_RATE     2048
#define SAMPLE_SIZE     2048         
#define FFT_SIZE        SAMPLE_SIZE

#define INPUT_FREQ      60
#define INPUT1_PHASE    0

float32_t input1[SAMPLE_SIZE];

float32_t complex1[SAMPLE_SIZE * 2];

float32_t input1_mag[FFT_SIZE];
float32_t input1_maxValue;
uint32_t input1_maxIndex = 0;

const static arm_cfft_instance_f32 *S1;


Serial debug_pc(USBTX, USBRX);

int main() {
                     
    float32_t input1_RMS;
    int i;

    debug_pc.baud(115200);
                   
    /* --- Generate Test Signals --- */  
     
    Sine_f32 sine_input1(  INPUT_FREQ, SAMPLE_RATE, 1.0, INPUT1_PHASE, SAMPLE_SIZE);
    
    sine_input1.generate(input1);
    
    for(i=0;i<(SAMPLE_SIZE * 2) + 1;i=i+2) {
         /* --- Add IMAG value to complex sample --- */
         complex1[i] = input1[i];
         complex1[i+1] = 0.0;
    }   
    /* --- Calculate RMS of signal --- */
    
    arm_rms_f32(input1, SAMPLE_SIZE, &input1_RMS);
    
    printf("Input1 RMS: %1.4f\r\n", input1_RMS);
    
    /* --- Calculate Complex FFT --- */
    S1 = & arm_cfft_sR_f32_len2048;  
    arm_cfft_f32(S1, complex1, 0, 1);
    
    /* --- Get Max BIN --- */
    arm_cmplx_mag_f32(complex1, input1_mag, FFT_SIZE);
    /* Calculates maxValue and returns corresponding BIN value */
    arm_max_f32(input1_mag, FFT_SIZE, &input1_maxValue, &input1_maxIndex);
    printf("Input1 Max Bin: %d\r\n", input1_maxIndex);   
    printf("Input1 Max Value: %1.4f\r\n", input1_maxValue); 
    
    for(i=0;i<(SAMPLE_SIZE) + 1;i=i+2) {  //Don't display the top half of the FFT BINS
        printf("BIN %d, RE:%1.4f, IM: %1.4f\r\n", i / 2, complex1[i], complex1[i+1]);
    }

My problem is on line 40, I was skipping every other input sample. It should be like:

complex1[i] = input1[i / 2];
posted by Kevin Holland 07 Aug 2015
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