This code appears to implement a function to interpolate gain...

This code appears to implement a function to interpolate gain values for frequency bands based on their energy levels, aiming to generate a smooth transition of gains across the bands. Let's break it down step by step:

Function Inputs and Outputs:

• g (output): A pointer to an array (float *g) where the computed interpolation gains will be written.
• bandE (input): A pointer to an array (const float *bandE) containing energy values of the bands that will guide the interpolation process. Each value in bandE corresponds to a band energy.

Code Explanation:

1. Initialization:

   memset(g, 0, FREQ_SIZE);
   

   • The g array is zeroed out by setting its first FREQ_SIZE bytes to 0. This clears the array before interpolating the gain values.

2. Main Interpolation Loop:

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

   • Iterates over each frequency "band" starting from the second band (i=1) up to the NB_BANDS-1 band.

   • The variable eband20ms[i] and eband20ms[i+1] refer to indices that define the boundaries of each band in the frequency array. Hence, the size of the i-th band is calculated as:

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

   • The second loop iterates over all points within the current band:

     for (j=0; j<band_size; j++)
     

     • For each point in the band, a fractional value frac is calculated:

       float frac = (float)j / band_size;
       

       frac linearly scales from 0 to 1 across the band range.

     • The gain for this specific frequency bin is linearly interpolated between the energy of the previous band (bandE[i-1]) and the current band's energy (bandE[i]):

       g[eband20ms[i] + j] = (1 - frac) * bandE[i-1] + frac * bandE[i];
       

3. Handle First Band:

   for (j=0; j<eband20ms[1]; j++)
       g[j] = bandE[0];
   

   • Handles the frequency bins before the first band begins (band 0), assigning them the energy of the first band (bandE[0]).

4. Handle Last Band:

   for (j=eband20ms[NB_BANDS]; j<eband20ms[NB_BANDS+1]; j++)
       g[j] = bandE[NB_BANDS-1];
   

   • Handles the frequency bins after the last band ends, assigning them the energy of the last band (bandE[NB_BANDS-1]).

Purpose of the Function:

• This function creates a smoothly interpolated mapping of gain values (g) over a frequency range based on the given band energies (bandE).
• The eband20ms array determines the start and end indices for each band and is used to ensure the gains are interpolated within the proper ranges.
• The first and last segments of the g array are filled with constant energy values from the first and last bands, respectively.

Practical Applications:

• This type of interpolation is common in audio and signal processing, particularly in systems like equalizers or vocoders, where smooth transitions between spectral bands are needed to avoid artifacts such as abrupt changes in gain.