The provided code computes and distributes the energy from a...

The provided code computes and distributes the energy from a frequency spectrum (Fourier transform result) X into equally spaced frequency bands. It fills the array bandE (likely representing energy per frequency band) with the energy values normalized across bands using some smoothing.

Step-by-step explanation:

1. Inputs and Outputs:

   • Input:
     • X - Pointer to the frequency spectrum (kiss_fft_cpx array) obtained from a Fourier transform (typically complex-valued). Each element has a real (r) and imaginary (i) component.
     • eband20ms - Likely a global or external array representing the start and end indices of predefined frequency bands in X. This defines how the spectrum is divided into bands.
   • Output:
     • bandE - Output array into which the computed energy of each band is stored.

2. Temporary energy accumulator setup:

   • A temporary array sum is initialized with zeros for storing energy contributions. It has NB_BANDS + 2 elements to allow for smoothing across adjacent bands.

3. Energy computation for each band:

   • Iterates over all NB_BANDS+1 frequency bands:

     for (i=0; i<NB_BANDS+1; i++)
     

   • For each band:
     • Compute band_size: The width of the current band in the frequency spectrum is determined with:

       band_size = eband20ms[i+1] - eband20ms[i];
       

     • Accumulate energy across the band:
       • For each frequency within the band, energy is calculated as the sum of the squared real and imaginary components of the spectrum:

         tmp = SQUARE(X[eband20ms[i] + j].r) + SQUARE(X[eband20ms[i] + j].i);
         

       • Energy is distributed between the current band (sum[i]) and the next band (sum[i+1]), with a fractional weight (frac = j/band_size) ensuring smooth transitions:

         sum[i] += (1-frac)*tmp;
         sum[i+1] += frac*tmp;
         

4. Adjust boundary values:

   • The energy contributions at the boundaries (sum[0], sum[1] and sum[NB_BANDS], sum[NB_BANDS+1]) are smoothed by averaging neighboring bands:

     sum[1] = (sum[0]+sum[1])*2/3;
     sum[NB_BANDS] = (sum[NB_BANDS]+sum[NB_BANDS+1])*2/3;
     

5. Store computed band energies:

   • The energy of each band is copied from sum (starting from sum[1]) into the output array bandE:

     for (i=0; i<NB_BANDS; i++) {
       bandE[i] = sum[i+1];
     }
     

Purpose of the code:

The code computes the energy distribution across a set number of frequency bands (NB_BANDS) from a Fourier-transformed signal. It applies smoothing between adjacent bands to avoid abrupt transitions in energy. This kind of computation is typically used in audio signal processing, such as in codecs, noise suppression, or spectral analysis.

Key processes include:

1. Calculating the energy of the spectrum (X) for a given band.
2. Distributing energy using weights (frac) for smoother band boundaries.
3. Outputting the band-wise energy into bandE.