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