Alc/uhjfilter.h

   1 #ifndef UHJFILTER_H
   2 #define UHJFILTER_H
   3
   4 #include "AL/al.h"
   5
   6 #include "alMain.h"
   7
   8 typedef struct AllPassState {
   9     ALfloat x[2]; /* Last two input samples */
  10     ALfloat y[2]; /* Last two output samples */
  11 } AllPassState;
  12
  13 /* Encoding 2-channel UHJ from B-Format is done as:
  14  *
  15  * S = 0.9396926*W + 0.1855740*X
  16  * D = j(-0.3420201*W + 0.5098604*X) + 0.6554516*Y
  17  *
  18  * Left = (S + D)/2.0
  19  * Right = (S - D)/2.0
  20  *
  21  * where j is a wide-band +90 degree phase shift.
  22  *
  23  * The phase shift is done using a Hilbert transform, described here:
  24  * https://web.archive.org/web/20060708031958/http://www.biochem.oulu.fi/~oniemita/dsp/hilbert/
  25  * It works using 2 sets of 4 chained filters. The first filter chain produces
  26  * a phase shift of varying magnitude over a wide range of frequencies, while
  27  * the second filter chain produces a phase shift 90 degrees ahead of the
  28  * first over the same range.
  29  *
  30  * Combining these two stages requires the use of three filter chains. S-
  31  * channel output uses a Filter1 chain on the W and X channel mix, while the D-
  32  * channel output uses a Filter1 chain on the Y channel plus a Filter2 chain on
  33  * the W and X channel mix. This results in the W and X input mix on the D-
  34  * channel output having the required +90 degree phase shift relative to the
  35  * other inputs.
  36  */
  37
  38 typedef struct Uhj2Encoder {
  39     AllPassState Filter1_WX[4];
  40     AllPassState Filter1_Y[4];
  41     AllPassState Filter2_WX[4];
  42 } Uhj2Encoder;
  43
  44 /* Encodes a 2-channel UHJ (stereo-compatible) signal from a B-Format input
  45  * signal. The input must use FuMa channel ordering and scaling.
  46  */
  47 void EncodeUhj2(Uhj2Encoder *enc, ALfloat *restrict LeftOut, ALfloat *restrict RightOut, ALfloat (*restrict InSamples)[BUFFERSIZE], ALsizei SamplesToDo);
  48
  49 #endif /* UHJFILTER_H */