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#ifndef UHJFILTER_H
#define UHJFILTER_H
#include "AL/al.h"
#include "alMain.h"
typedef struct AllPassState {
ALfloat x[2]; /* Last two input samples */
ALfloat y[2]; /* Last two output samples */
} AllPassState;
/* Encoding 2-channel UHJ from B-Format is done as:
*
* S = 0.9396926*W + 0.1855740*X
* D = j(-0.3420201*W + 0.5098604*X) + 0.6554516*Y
*
* Left = (S + D)/2.0
* Right = (S - D)/2.0
*
* where j is a wide-band +90 degree phase shift.
*
* The phase shift is done using a Hilbert transform, described here:
* https://web.archive.org/web/20060708031958/http://www.biochem.oulu.fi/~oniemita/dsp/hilbert/
* It works using 2 sets of 4 chained filters. The first filter chain produces
* a phase shift of varying magnitude over a wide range of frequencies, while
* the second filter chain produces a phase shift 90 degrees ahead of the
* first over the same range.
*
* Combining these two stages requires the use of three filter chains. S-
* channel output uses a Filter1 chain on the W and X channel mix, while the D-
* channel output uses a Filter1 chain on the Y channel plus a Filter2 chain on
* the W and X channel mix. This results in the W and X input mix on the D-
* channel output having the required +90 degree phase shift relative to the
* other inputs.
*/
typedef struct Uhj2Encoder {
AllPassState Filter1_WX[4];
AllPassState Filter1_Y[4];
AllPassState Filter2_WX[4];
} Uhj2Encoder;
/* Encodes a 2-channel UHJ (stereo-compatible) signal from a B-Format input
* signal. The input must use FuMa channel ordering and scaling.
*/
void EncodeUhj2(Uhj2Encoder *enc, ALfloat *restrict LeftOut, ALfloat *restrict RightOut, ALfloat (*restrict InSamples)[BUFFERSIZE], ALsizei SamplesToDo);
#endif /* UHJFILTER_H */
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