Update some UHJ comments
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+8
-14
@@ -140,24 +140,18 @@ void allpass_process(al::span<float> dst, const float *RESTRICT src)
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} // namespace
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/* NOTE: There seems to be a bit of an inconsistency in how this encoding is
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* supposed to work. Some references, such as
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/* Encoding 2-channel UHJ from B-Format is done as:
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*
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* http://members.tripod.com/martin_leese/Ambisonic/UHJ_file_format.html
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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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* specify a pre-scaling of sqrt(2) on the W channel input, while other
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* references, such as
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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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* https://en.wikipedia.org/wiki/Ambisonic_UHJ_format#Encoding.5B1.5D
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* and
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* https://wiki.xiph.org/Ambisonics#UHJ_format
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* where j is a wide-band +90 degree phase shift.
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*
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* do not. The sqrt(2) scaling is in line with B-Format decoder coefficients
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* which include such a scaling for the W channel input, however the original
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* source for this equation is a 1985 paper by Michael Gerzon, which does not
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* apparently include the scaling. Applying the extra scaling creates a louder
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* result with a narrower stereo image compared to not scaling, and I don't
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* know which is the intended result.
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* The phase shift is done using a FIR filter derived from an FFT'd impulse
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* with the desired shift.
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*/
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void Uhj2Encoder::encode(FloatBufferLine &LeftOut, FloatBufferLine &RightOut,
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@@ -7,20 +7,6 @@
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#include "almalloc.h"
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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 FIR filter derived from an FFT'd impulse
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* with the desired shift.
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*/
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struct Uhj2Encoder {
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/* A particular property of the filter allows it to cover nearly twice its
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* length, so the filter size is also the effective delay (despite being
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