hp50.c

关于AMR-WB+语音压缩编码的实现代码

#include "../include/amr_plus.h"
/*
 * hp50_12k8
 *
 * Function:
 *    2nd order high pass filter with nominal cut off frequency at 50 Hz.
 *
 * Returns:
 *    void
 */
void hp50_12k8(Float32 signal[], Word32 lg, Float32 mem[], Word32 fscale)
{
   Word16 i;
   Float32 x0, x1, x2, y0, y1, y2;
   Float32 a1, a2, b1, b2, frac;
   y1 = mem[0];
   y2 = mem[1];
   x0 = mem[2];
   x1 = mem[3];
   if (fscale >= FSCALE_DENOM)
   {
      /* design in matlab (-3dB @ 20 Hz)
         [b,a] = butter(2, 20.0/6400.0, 'high');
         a = [1.00000000000000  -1.98611621154089   0.98621192916075];
         b = [0.99308203517541  -1.98616407035082   0.99308203517541];
         [b,a] = butter(2, 20.0/12800.0, 'high');
         a = [1.00000000000000  -1.99305802314321   0.99308203546221];
         b = [0.99653501465135  -1.99307002930271   0.99653501465135];
         difference:
         a = [               0  -0.00694181160232   0.00687010630146];
         b = [0.00345297947594  -0.00690595895189   0.00345297947594];
      */
      /* this approximation is enough (max overshoot = +0.05dB) */
      frac = ((Float32)(fscale-FSCALE_DENOM)) / ((Float32)FSCALE_DENOM);
      a1 =  1.98611621154089f + (frac*0.00694181160232f);
      a2 = -0.98621192916075f - (frac*0.00687010630146f);
      b1 = -1.98616407035082f - (frac*0.00690595895189f);
      b2 =  0.99308203517541f + (frac*0.00345297947594f);
      for(i = 0; i < lg; i++)
      {
         x2 = x1;
         x1 = x0;
         x0 = signal[i];
         y0 = (y1*a1) + (y2*a2) + (x0*b2) + (x1*b1) + (x2*b2);
         signal[i] = y0;
         y2 = y1;                                                              
         y1 = y0;
      }
   }
   else           /* -6dB 24Hz fs/2=6400Hz (3GPP AMR-WB coef) */
   {
      for(i = 0; i < lg; i++)
      {
         x2 = x1;
         x1 = x0;
         x0 = signal[i];
         y0 = y1 * 1.978881836F + y2 * -0.979125977F + x0 * 0.989501953F +
            x1 * -1.979003906F + x2 * 0.989501953F;
         signal[i] = y0;
         y2 = y1;                                                              
         y1 = y0;
      }
   }
   mem[0] = ((y1 > 1e-10) | (y1 < -1e-10)) ? y1 : 0;
   mem[1] = ((y2 > 1e-10) | (y2 < -1e-10)) ? y2 : 0;
   mem[2] = ((x0 > 1e-10) | (x0 < -1e-10)) ? x0 : 0;
   mem[3] = ((x1 > 1e-10) | (x1 < -1e-10)) ? x1 : 0;
   return;
}