DSP libraries - RFFT - strange results - c

Question

Recently I've been trying to do FFT calculations on my STM32F4-Discovery evaluation board then send it to PC. I have looked into my problem - I think that I'm doing something wrong with FFT functions provided by manufacturer.
I'm using CMSIS-DSP libraries.
For now I've have been generating samples with code (if that works correct I'll do sampling by microphone).
I'm using arm_rfft_fast_f32 as my data are going to be floats in the future, but results I get in my output array are insane (I think) - I'm getting frequencies below 0.
number_of_samples = 512; (l_probek in code)
dt = 1/freq/number_of_samples
Here is my code
float32_t buffer_input[l_probek];
uint16_t i;
uint8_t mode;
float32_t dt;
float32_t freq;
bool DoFlag = false;
bool UBFlag = false;
uint32_t rozmiar = 4*l_probek;
union
{
float32_t f[l_probek];
uint8_t b[4*l_probek];
}data_out;
union
{
float32_t f[l_probek];
uint8_t b[4*l_probek];
}data_mag;
union
{
float32_t f;
uint8_t b[4];
}czest_rozdz;
/* Pointers ------------------------------------------------------------------*/
arm_rfft_fast_instance_f32 S;
arm_cfft_radix4_instance_f32 S_CFFT;
uint16_t output;
/* ---------------------------------------------------------------------------*/
int main(void)
{
freq = 5000;
dt = 0.000000390625;
_GPIO();
_LED();
_NVIC();
_EXTI(0);
arm_rfft_fast_init_f32(&S, l_probek);
GPIO_SetBits(GPIOD, LED_Green);
mode = 2;
//----------------- Infinite loop
while (1)
{
if(true)//(UBFlag == true)
for(i=0; i<l_probek; ++i)
{
buffer_input[i] = (float32_t) 15*sin(2*PI*freq*i*dt);
}
//Obliczanie FFT
arm_rfft_fast_f32(&S, buffer_input, data_out.f, 0);
//Obliczanie modulow
arm_cmplx_mag_f32(data_out.f, data_mag.f, l_probek);
USART_putdata(USART1, data_out.b, data_mag.b, rozmiar);
//USART_putdata(USART1, czest_rozdz.b, data_mag.b, rozmiar);
GPIO_ToggleBits(GPIOD, LED_Orange);
//mode++;
//UBFlag = false;
}
}
}

Answer 1

I'm using arm_rfft_fast_f32 as my data are going to be floats in the future, but results I get in my output array are insane (I think) - I'm getting frequencies below 0.
The arm_rfft_fast_f32 function does not return frequencies, but rather complex-valued coefficients computed using the Fast Fourier Transform (FFT). It is thus perfectly reasonable for those coefficients to be negative. More specifically, the expected coefficients for your single-cycle sin test tone input with an amplitude of 15 would be:
0.0, 0.0; // special case packing real-valued X[0] and X[N/2]
0.0, -3840.0; // X[1]
0.0, 0.0; // X[2]
0.0, 0.0; // X[3]
...
0.0, 0.0; // X[255]
Note that as indicated in the documentation the first two outputs correspond to the purely real coefficients X[0] and X[N/2] (you should be particularly careful about this special case in your subsequent call to arm_cmplx_mag_f32; see last point below).
The frequency of each of those frequency components are given by k*fs/N, where N is the number of samples (in your case l_probek) and fs = 1/dt is the sampling rate (in your case freq*l_probek):
X[0] -> 0*freq*l_probek/l_probek = 0
X[1] -> 1*freq*l_probek/l_probek = freq = 5000
X[2] -> 2*freq*l_probek/l_probek = 2*freq = 10000
X[3] -> 3*freq*l_probek/l_probek = 2*freq = 15000
...
Finally, due to the special packing of the first two values, you need to be careful when computing the N/2+1 magnitudes:
// General case for the magnitudes
arm_cmplx_mag_f32(data_out.f+2, data_mag.f+1, l_probek/2 - 1);
// Handle special cases
data_mag.f[0] = data_out.f[0];
data_mag.f[l_probek/2] = data_out.f[1];

Answer 2

As a follow-up to the above answer, which is awesome, some further clarifications which took me an age to figure out.
The frequency bins are centered on the target frequency, so for instance in the example above X[0] represents -2500Hz to 2500Hz, centered on zero, X[1] is 2500Hz to 7500Hz centered on 5000Hz and so on
It's common to interpolate frequencies within the bin by looking at the energy of the adjacent bins (see https://dspguru.com/dsp/howtos/how-to-interpolate-fft-peak/) if you do this you will need to make sure that your magnitude array is large enough for the bins + Nyquist and that the bin above Nyquist is 0, but note many interpolation techniques require the complex values (e.q. Quinn, Jacobson) so make sure you interpolate before finding the magnitudes.
The special case code above works because there is no complex component of the DC and Nyquist values and thus the magnitude is simply the real part
There is a bug in the code above however - although the imaginary parts of the DC and Nyquist components is always zero, the real part could still be negative, so you need to take the absolute value to get the magnitude:
// Handle special cases
data_mag.f[0] = fabs(data_out.f[0]);
data_mag.f[l_probek/2] = fabs(data_out.f[1]);