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Remove the extra scaling on W for UHJ encoding

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There seems to be some inconsistent info about whether W should be scaled by
sqrt(2) for encoding. Not applying the scaling results in a wider stereo image,
which seems more appropriate.
This commit is contained in:
Chris Robinson
2016-03-01 13:37:12 -08:00
parent c89511b95e
commit 9fdca9e29f
2 changed files with 28 additions and 10 deletions
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Alc/uhjfilter.c
+26 -7
View File
@@ -16,6 +16,26 @@ static const ALfloat Filter2Coeff[4] = {
0.4021921162426f, 0.8561710882420f, 0.9722909545651f, 0.9952884791278f
};
/* NOTE: There seems to be a bit of an inconsistency in how this encoding is
* supposed to work. Some references, such as
*
* http://members.tripod.com/martin_leese/Ambisonic/UHJ_file_format.html
*
* specify a pre-scaling of sqrt(2) on the W channel input, while other
* references, such as
*
* https://en.wikipedia.org/wiki/Ambisonic_UHJ_format#Encoding.5B1.5D
* and
* https://wiki.xiph.org/Ambisonics#UHJ_format
*
* do not. The sqrt(2) scaling is in line with B-Format decoder coefficients
* which include such a scaling for the W channel input, however the original
* source for this equation is a 1985 paper by Michael Gerzon, which does not
* apparently include the scaling. Applying the extra scaling creates a louder
* result with a narrower stereo image compared to not scaling, and I don't
* know which is the intended result.
*/
void EncodeUhj2(Uhj2Encoder *enc, ALfloat (*restrict OutBuffer)[BUFFERSIZE], ALfloat (*restrict InSamples)[BUFFERSIZE], ALuint SamplesToDo)
{
ALuint base, i, c;
@@ -45,10 +65,10 @@ void EncodeUhj2(Uhj2Encoder *enc, ALfloat (*restrict OutBuffer)[BUFFERSIZE], ALf
D[i] = enc->Filter1_Y[3].y[1];
}
/* D += j(-0.3420201*W' + 0.5098604*X) */
/* D += j(-0.3420201*W + 0.5098604*X) */
for(i = 0;i < todo;i++)
{
ALfloat in = -0.3420201f*1.414213562f*InSamples[0][base+i] +
ALfloat in = -0.3420201f*InSamples[0][base+i] +
0.5098604f*InSamples[1][base+i];
for(c = 0;c < 4;c++)
{
@@ -63,10 +83,10 @@ void EncodeUhj2(Uhj2Encoder *enc, ALfloat (*restrict OutBuffer)[BUFFERSIZE], ALf
D[i] += enc->Filter2_WX[3].y[0];
}
/* S = 0.9396926*W' + 0.1855740*X */
/* S = 0.9396926*W + 0.1855740*X */
for(i = 0;i < todo;i++)
{
ALfloat in = 0.9396926f*1.414213562f*InSamples[0][base+i] +
ALfloat in = 0.9396926f*InSamples[0][base+i] +
0.1855740f*InSamples[1][base+i];
for(c = 0;c < 4;c++)
{
@@ -81,11 +101,10 @@ void EncodeUhj2(Uhj2Encoder *enc, ALfloat (*restrict OutBuffer)[BUFFERSIZE], ALf
S[i] = enc->Filter1_WX[3].y[1];
}
/* Left = (S + D)/2.0
* Right = (S - D)/2.0
*/
/* Left = (S + D)/2.0 */
for(i = 0;i < todo;i++)
OutBuffer[0][base + i] += (S[i] + D[i]) * 0.5f;
/* Right = (S - D)/2.0 */
for(i = 0;i < todo;i++)
OutBuffer[1][base + i] += (S[i] - D[i]) * 0.5f;
Alc/uhjfilter.h
+2 -3
View File
@@ -12,9 +12,8 @@ typedef struct AllPassState {
/* Encoding 2-channel UHJ from B-Format is done as:
*
* W' = W * sqrt(2)
* S = 0.9396926*W' + 0.1855740*X
* D = j(-0.3420201*W' + 0.5098604*X) + 0.6554516*Y
* 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