frequency modulation (FM) code snippet

Question

I've written the following code for frequency modulation of an audio signal. The audio itself is 1 sec long, sampled at 8000 Hz. I want to apply FM to this audio signal by using a sine wave with a frequency of 50 Hz (expressed as a fraction of the sampling frequency). The modulating signal has a modulation index of 0.25 so as to create only one pair of sidebands.

for (i = 0; i < 7999; i++) {
    phi_delta = 8000 - 8000 * (1 + 0.25 * sin(2* pi * mf * i));
    f_phi_accum += phi_delta; //this can have a negative value
    /*keep only the integer part that'll be used as an index into the input array*/
    i_phi_accum = f_phi_accum;   
    /*keep only the fractional part that'll be used to interpolate between samples*/
    r_phi_accum = f_phi_accum - i_phi_accum; 
    //If I'm getting negative values should I convert them to positive
    //r_phi_accum = fabs(f_phi_accum - i_phi_accum);   
    i_phi_accum = abs(i_phi_accum);
    /*since i_phi_accum often exceeds 7999 I have to add this if statement so as to      prevent out of bounds errors   */  
    if (i_phi_accum < 7999)
        output[i] = ((input[i_phi_accum] + input[i_phi_accum + 1])/2) * r_phi_accum;            
}

Well, this code doesn't seem to work and I'm not even sure if it's supposed to work. I asked this question in another thread (stackoverflow.com/questions/8655121/frequency-modulation-fm) and simply tried to implement what I was told there. — dsp_user

Paul R Paul R · Accepted Answer · 2011-12-30T09:53:32

Your calculation of phi_delta is off by a factor of 8000 and an offset - it should be 1 +/- a small value, i.e.

phi_delta = 1.0 + 0.25 * sin(2.0 * pi * mf * i));

which will result in phi_delta having a range of 0.75 to 1.25.

frequency modulation (FM) code snippet

1 Answers