amended Int32_To_Int24_Dither following feedback; first attempt at im…

…plementing PaUtil_Generate24BitTriangularDither

src/common/pa_converters.c

@@ -939,16 +939,26 @@ static void Int32_To_Int24_Dither(
 
     PaInt32 *src = (PaInt32*)sourceBuffer;
     signed char *dest = (signed char*)destinationBuffer;
+    signed char *scaledDitherResult = (signed char*)destinationBuffer;
     PaInt32 dither;
 
     while ( count-- )
     {
         /* REVIEW */
         dither = PaUtil_Generate16BitTriangularDither(ditherGenerator);
-        *dest = (signed char) ((((*src) >> 1) + dither) >> 7);
-
+        *scaledDitherResult = (signed char) ((((*src) >> 1) + (dither >> 8)) >> 7);
+
+#if defined(PA_LITTLE_ENDIAN)
+        dest[0] = (unsigned char)(*scaledDitherResult >> 8);
+        dest[1] = (unsigned char)(*scaledDitherResult >> 16);
+        dest[2] = (unsigned char)(*scaledDitherResult >> 24);
+#elif defined(PA_BIG_ENDIAN)
+        dest[0] = (unsigned char)(*scaledDitherResult >> 24);
+        dest[1] = (unsigned char)(*scaledDitherResult >> 16);
+        dest[2] = (unsigned char)(*scaledDitherResult >> 8);
+#endif
         src += sourceStride;
-        dest += destinationStride;
+        dest += destinationStride * 3;
     }
 }
 

src/common/pa_dither.c

@@ -86,6 +86,31 @@ PaInt32 PaUtil_Generate16BitTriangularDither( PaUtilTriangularDitherGenerator *s
 }
 
 
+PaInt32 PaUtil_Generate24BitTriangularDither(PaUtilTriangularDitherGenerator* state)
+{
+    PaInt32 current, highPass;
+
+    /* Generate two random numbers. */
+    state->randSeed1 = (state->randSeed1 * 196314165) + 907633515;
+    state->randSeed2 = (state->randSeed2 * 196314165) + 907633515;
+
+    /* Generate triangular distribution about 0.
+     * Shift before adding to prevent overflow which would skew the distribution.
+     * Also shift an extra bit for the high pass filter.
+     */
+#define PA_DITHER_BITS_24_BITS   (23)
+#define DITHER_SHIFT_24_BITS_  ((sizeof(PaInt32)*8 - PA_DITHER_BITS_24_BITS) + 1)
+
+    current = (((PaInt32)state->randSeed1) >> DITHER_SHIFT_24_BITS_) +
+        (((PaInt32)state->randSeed2) >> DITHER_SHIFT_24_BITS_);
+
+    /* High pass filter to reduce audibility. */
+    highPass = current - state->previous;
+    state->previous = current;
+    return highPass;
+}
+
+
 /* Multiply by PA_FLOAT_DITHER_SCALE_ to get a float between -2.0 and +1.99999 */
 #define PA_FLOAT_DITHER_SCALE_  (1.0f / ((1<<PA_DITHER_BITS_)-1))
 static const float const_float_dither_scale_ = PA_FLOAT_DITHER_SCALE_;

src/common/pa_dither.h

@@ -84,6 +84,17 @@ void PaUtil_InitializeTriangularDitherState( PaUtilTriangularDitherGenerator *di
 PaInt32 PaUtil_Generate16BitTriangularDither( PaUtilTriangularDitherGenerator *ditherState );
 
 
+/* REVEIW */
+/**
+ @brief Calculate 3 LSB dither signal with a triangular distribution.
+ Ranged for adding to a 1 bit right-shifted 32 bit integer
+ prior to >>7.
+ @return
+ A signed 32-bit integer with a range of +32767 to -32768
+*/
+PaInt32 PaUtil_Generate24BitTriangularDither(PaUtilTriangularDitherGenerator* ditherState);
+
+
 /**
  @brief Calculate 2 LSB dither signal with a triangular distribution.
  Ranged for adding to a pre-scaled float.