Remove the extra scaling on W for UHJ encoding

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.

2 files changed, 28 insertions, 10 deletions

diff --git a/Alc/uhjfilter.c b/Alc/uhjfilter.c
index cb1a4348..b410967a 100644
--- a/Alc/uhjfilter.c
+++ b/Alc/uhjfilter.c
@@ -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;
 
diff --git a/Alc/uhjfilter.h b/Alc/uhjfilter.h
index 5238e202..a5aa9275 100644
--- a/Alc/uhjfilter.h
+++ b/Alc/uhjfilter.h
@@ -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