Alc/uhjfilter.c

   1
   2 #include "config.h"
   3
   4 #include "alu.h"
   5 #include "uhjfilter.h"
   6
   7 /* This is the maximum number of samples processed for each inner loop
   8  * iteration. */
   9 #define MAX_UPDATE_SAMPLES  128
  10
  11
  12 static const ALfloat Filter1Coeff[4] = {
  13     0.6923878f, 0.9360654322959f, 0.9882295226860f, 0.9987488452737f
  14 };
  15 static const ALfloat Filter2Coeff[4] = {
  16     0.4021921162426f, 0.8561710882420f, 0.9722909545651f, 0.9952884791278f
  17 };
  18
  19 static void allpass_process(AllPassState *state, ALfloat *restrict dst, const ALfloat *restrict src, const ALfloat aa, ALsizei todo)
  20 {
  21     ALsizei i;
  22
  23     if(todo > 1)
  24     {
  25         dst[0] = aa*(src[0] + state->y[1]) - state->x[1];
  26         dst[1] = aa*(src[1] + state->y[0]) - state->x[0];
  27         for(i = 2;i < todo;i++)
  28             dst[i] = aa*(src[i] + dst[i-2]) - src[i-2];
  29         state->x[1] = src[i-2];
  30         state->x[0] = src[i-1];
  31         state->y[1] = dst[i-2];
  32         state->y[0] = dst[i-1];
  33     }
  34     else if(todo == 1)
  35     {
  36         dst[0] = aa*(src[0] + state->y[1]) - state->x[1];
  37         state->x[1] = state->x[0];
  38         state->x[0] = src[0];
  39         state->y[1] = state->y[0];
  40         state->y[0] = dst[0];
  41     }
  42 }
  43
  44
  45 /* NOTE: There seems to be a bit of an inconsistency in how this encoding is
  46  * supposed to work. Some references, such as
  47  *
  48  * http://members.tripod.com/martin_leese/Ambisonic/UHJ_file_format.html
  49  *
  50  * specify a pre-scaling of sqrt(2) on the W channel input, while other
  51  * references, such as
  52  *
  53  * https://en.wikipedia.org/wiki/Ambisonic_UHJ_format#Encoding.5B1.5D
  54  * and
  55  * https://wiki.xiph.org/Ambisonics#UHJ_format
  56  *
  57  * do not. The sqrt(2) scaling is in line with B-Format decoder coefficients
  58  * which include such a scaling for the W channel input, however the original
  59  * source for this equation is a 1985 paper by Michael Gerzon, which does not
  60  * apparently include the scaling. Applying the extra scaling creates a louder
  61  * result with a narrower stereo image compared to not scaling, and I don't
  62  * know which is the intended result.
  63  */
  64
  65 void EncodeUhj2(Uhj2Encoder *enc, ALfloat *restrict LeftOut, ALfloat *restrict RightOut, ALfloat (*restrict InSamples)[BUFFERSIZE], ALsizei SamplesToDo)
  66 {
  67     ALfloat D[MAX_UPDATE_SAMPLES], S[MAX_UPDATE_SAMPLES];
  68     ALfloat temp[2][MAX_UPDATE_SAMPLES];
  69     ALsizei base, i;
  70
  71     for(base = 0;base < SamplesToDo;)
  72     {
  73         ALsizei todo = mini(SamplesToDo - base, MAX_UPDATE_SAMPLES);
  74
  75         /* D = 0.6554516*Y */
  76         for(i = 0;i < todo;i++)
  77             temp[0][i] = 0.6554516f*InSamples[2][base+i];
  78         allpass_process(&enc->Filter1_Y[0], temp[1], temp[0],
  79                         Filter1Coeff[0]*Filter1Coeff[0], todo);
  80         allpass_process(&enc->Filter1_Y[1], temp[0], temp[1],
  81                         Filter1Coeff[1]*Filter1Coeff[1], todo);
  82         allpass_process(&enc->Filter1_Y[2], temp[1], temp[0],
  83                         Filter1Coeff[2]*Filter1Coeff[2], todo);
  84         /* NOTE: Filter1 requires a 1 sample delay for the final output, so
  85          * take the last processed sample from the previous run as the first
  86          * output sample.
  87          */
  88         D[0] = enc->Filter1_Y[3].y[0];
  89         allpass_process(&enc->Filter1_Y[3], temp[0], temp[1],
  90                         Filter1Coeff[3]*Filter1Coeff[3], todo);
  91         for(i = 1;i < todo;i++)
  92             D[i] = temp[0][i-1];
  93
  94         /* D += j(-0.3420201*W + 0.5098604*X) */
  95         for(i = 0;i < todo;i++)
  96             temp[0][i] = -0.3420201f*InSamples[0][base+i] +
  97                           0.5098604f*InSamples[1][base+i];
  98         allpass_process(&enc->Filter2_WX[0], temp[1], temp[0],
  99                         Filter2Coeff[0]*Filter2Coeff[0], todo);
 100         allpass_process(&enc->Filter2_WX[1], temp[0], temp[1],
 101                         Filter2Coeff[1]*Filter2Coeff[1], todo);
 102         allpass_process(&enc->Filter2_WX[2], temp[1], temp[0],
 103                         Filter2Coeff[2]*Filter2Coeff[2], todo);
 104         allpass_process(&enc->Filter2_WX[3], temp[0], temp[1],
 105                         Filter2Coeff[3]*Filter2Coeff[3], todo);
 106         for(i = 0;i < todo;i++)
 107             D[i] += temp[0][i];
 108
 109         /* S = 0.9396926*W + 0.1855740*X */
 110         for(i = 0;i < todo;i++)
 111             temp[0][i] = 0.9396926f*InSamples[0][base+i] +
 112                          0.1855740f*InSamples[1][base+i];
 113         allpass_process(&enc->Filter1_WX[0], temp[1], temp[0],
 114                         Filter1Coeff[0]*Filter1Coeff[0], todo);
 115         allpass_process(&enc->Filter1_WX[1], temp[0], temp[1],
 116                         Filter1Coeff[1]*Filter1Coeff[1], todo);
 117         allpass_process(&enc->Filter1_WX[2], temp[1], temp[0],
 118                         Filter1Coeff[2]*Filter1Coeff[2], todo);
 119         S[0] = enc->Filter1_WX[3].y[0];
 120         allpass_process(&enc->Filter1_WX[3], temp[0], temp[1],
 121                         Filter1Coeff[3]*Filter1Coeff[3], todo);
 122         for(i = 1;i < todo;i++)
 123             S[i] = temp[0][i-1];
 124
 125         /* Left = (S + D)/2.0 */
 126         for(i = 0;i < todo;i++)
 127             *(LeftOut++) += (S[i] + D[i]) * 0.5f;
 128         /* Right = (S - D)/2.0 */
 129         for(i = 0;i < todo;i++)
 130             *(RightOut++) += (S[i] - D[i]) * 0.5f;
 131
 132         base += todo;
 133     }
 134 }