🛠️🐜 Antkeeper superbuild with dependencies included
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53 lines
1.7 KiB
53 lines
1.7 KiB
#ifndef UHJFILTER_H
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#define UHJFILTER_H
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#include "AL/al.h"
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#include "alMain.h"
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#include "almalloc.h"
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struct AllPassState {
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ALfloat z[2]{0.0f, 0.0f};
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};
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/* Encoding 2-channel UHJ from B-Format is done as:
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*
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* S = 0.9396926*W + 0.1855740*X
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* D = j(-0.3420201*W + 0.5098604*X) + 0.6554516*Y
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*
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* Left = (S + D)/2.0
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* Right = (S - D)/2.0
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*
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* where j is a wide-band +90 degree phase shift.
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*
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* The phase shift is done using a Hilbert transform, described here:
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* https://web.archive.org/web/20060708031958/http://www.biochem.oulu.fi/~oniemita/dsp/hilbert/
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* It works using 2 sets of 4 chained filters. The first filter chain produces
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* a phase shift of varying magnitude over a wide range of frequencies, while
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* the second filter chain produces a phase shift 90 degrees ahead of the
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* first over the same range.
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*
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* Combining these two stages requires the use of three filter chains. S-
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* channel output uses a Filter1 chain on the W and X channel mix, while the D-
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* channel output uses a Filter1 chain on the Y channel plus a Filter2 chain on
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* the W and X channel mix. This results in the W and X input mix on the D-
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* channel output having the required +90 degree phase shift relative to the
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* other inputs.
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*/
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struct Uhj2Encoder {
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AllPassState mFilter1_Y[4];
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AllPassState mFilter2_WX[4];
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AllPassState mFilter1_WX[4];
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ALfloat mLastY{0.0f}, mLastWX{0.0f};
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/* Encodes a 2-channel UHJ (stereo-compatible) signal from a B-Format input
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* signal. The input must use FuMa channel ordering and scaling.
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*/
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void encode(ALfloat *LeftOut, ALfloat *RightOut, ALfloat (*InSamples)[BUFFERSIZE], const ALsizei SamplesToDo);
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DEF_NEWDEL(Uhj2Encoder)
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};
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#endif /* UHJFILTER_H */
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