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 Filter1CoeffSqr[4] = {
  13     0.479400865589f, 0.876218493539f, 0.976597589508f, 0.997499255936f
  14 };
  15 static const ALfloat Filter2CoeffSqr[4] = {
  16     0.161758498368f, 0.733028932341f, 0.945349700329f, 0.990599156685f
  17 };
  18
  19 static void allpass_process(AllPassState *state, ALfloat *restrict dst, const ALfloat *restrict src, const ALfloat aa, ALsizei todo)
  20 {
  21     ALfloat z1 = state->z[0];
  22     ALfloat z2 = state->z[1];
  23     ALsizei i;
  24
  25     for(i = 0;i < todo;i++)
  26     {
  27         ALfloat input = src[i];
  28         ALfloat output = input*aa + z1;
  29         z1 = z2; z2 = output*aa - input;
  30         dst[i] = output;
  31     }
  32
  33     state->z[0] = z1;
  34     state->z[1] = z2;
  35 }
  36
  37
  38 /* NOTE: There seems to be a bit of an inconsistency in how this encoding is
  39  * supposed to work. Some references, such as
  40  *
  41  * http://members.tripod.com/martin_leese/Ambisonic/UHJ_file_format.html
  42  *
  43  * specify a pre-scaling of sqrt(2) on the W channel input, while other
  44  * references, such as
  45  *
  46  * https://en.wikipedia.org/wiki/Ambisonic_UHJ_format#Encoding.5B1.5D
  47  * and
  48  * https://wiki.xiph.org/Ambisonics#UHJ_format
  49  *
  50  * do not. The sqrt(2) scaling is in line with B-Format decoder coefficients
  51  * which include such a scaling for the W channel input, however the original
  52  * source for this equation is a 1985 paper by Michael Gerzon, which does not
  53  * apparently include the scaling. Applying the extra scaling creates a louder
  54  * result with a narrower stereo image compared to not scaling, and I don't
  55  * know which is the intended result.
  56  */
  57
  58 void EncodeUhj2(Uhj2Encoder *enc, ALfloat *restrict LeftOut, ALfloat *restrict RightOut, ALfloat (*restrict InSamples)[BUFFERSIZE], ALsizei SamplesToDo)
  59 {
  60     ALfloat D[MAX_UPDATE_SAMPLES], S[MAX_UPDATE_SAMPLES];
  61     ALfloat temp[2][MAX_UPDATE_SAMPLES];
  62     ALsizei base, i;
  63
  64     ASSUME(SamplesToDo > 0);
  65
  66     for(base = 0;base < SamplesToDo;)
  67     {
  68         ALsizei todo = mini(SamplesToDo - base, MAX_UPDATE_SAMPLES);
  69         ASSUME(todo > 0);
  70
  71         /* D = 0.6554516*Y */
  72         for(i = 0;i < todo;i++)
  73             temp[0][i] = 0.6554516f*InSamples[2][base+i];
  74         allpass_process(&enc->Filter1_Y[0], temp[1], temp[0], Filter1CoeffSqr[0], todo);
  75         allpass_process(&enc->Filter1_Y[1], temp[0], temp[1], Filter1CoeffSqr[1], todo);
  76         allpass_process(&enc->Filter1_Y[2], temp[1], temp[0], Filter1CoeffSqr[2], todo);
  77         allpass_process(&enc->Filter1_Y[3], temp[0], temp[1], Filter1CoeffSqr[3], todo);
  78         /* NOTE: Filter1 requires a 1 sample delay for the final output, so
  79          * take the last processed sample from the previous run as the first
  80          * output sample.
  81          */
  82         D[0] = enc->LastY;
  83         for(i = 1;i < todo;i++)
  84             D[i] = temp[0][i-1];
  85         enc->LastY = temp[0][i-1];
  86
  87         /* D += j(-0.3420201*W + 0.5098604*X) */
  88         for(i = 0;i < todo;i++)
  89             temp[0][i] = -0.3420201f*InSamples[0][base+i] +
  90                           0.5098604f*InSamples[1][base+i];
  91         allpass_process(&enc->Filter2_WX[0], temp[1], temp[0], Filter2CoeffSqr[0], todo);
  92         allpass_process(&enc->Filter2_WX[1], temp[0], temp[1], Filter2CoeffSqr[1], todo);
  93         allpass_process(&enc->Filter2_WX[2], temp[1], temp[0], Filter2CoeffSqr[2], todo);
  94         allpass_process(&enc->Filter2_WX[3], temp[0], temp[1], Filter2CoeffSqr[3], todo);
  95         for(i = 0;i < todo;i++)
  96             D[i] += temp[0][i];
  97
  98         /* S = 0.9396926*W + 0.1855740*X */
  99         for(i = 0;i < todo;i++)
 100             temp[0][i] = 0.9396926f*InSamples[0][base+i] +
 101                          0.1855740f*InSamples[1][base+i];
 102         allpass_process(&enc->Filter1_WX[0], temp[1], temp[0], Filter1CoeffSqr[0], todo);
 103         allpass_process(&enc->Filter1_WX[1], temp[0], temp[1], Filter1CoeffSqr[1], todo);
 104         allpass_process(&enc->Filter1_WX[2], temp[1], temp[0], Filter1CoeffSqr[2], todo);
 105         allpass_process(&enc->Filter1_WX[3], temp[0], temp[1], Filter1CoeffSqr[3], todo);
 106         S[0] = enc->LastWX;
 107         for(i = 1;i < todo;i++)
 108             S[i] = temp[0][i-1];
 109         enc->LastWX = temp[0][i-1];
 110
 111         /* Left = (S + D)/2.0 */
 112         for(i = 0;i < todo;i++)
 113             *(LeftOut++) += (S[i] + D[i]) * 0.5f;
 114         /* Right = (S - D)/2.0 */
 115         for(i = 0;i < todo;i++)
 116             *(RightOut++) += (S[i] - D[i]) * 0.5f;
 117
 118         base += todo;
 119     }
 120 }