1 files changed, 29 insertions, 24 deletions

diff --git a/Alc/filters/filter.c b/Alc/filters/filter.c
index 4d757be7..fe59977b 100644
--- a/Alc/filters/filter.c
+++ b/Alc/filters/filter.c
@@ -9,7 +9,7 @@
 
 extern inline void BiquadState_clear(BiquadState *filter);
 extern inline void BiquadState_copyParams(BiquadState *restrict dst, const BiquadState *restrict src);
-extern inline void BiquadState_processPassthru(BiquadState *filter, const ALfloat *restrict src, ALsizei numsamples);
+extern inline void BiquadState_processPassthru(BiquadState *filter, ALsizei numsamples);
 extern inline ALfloat calc_rcpQ_from_slope(ALfloat gain, ALfloat slope);
 extern inline ALfloat calc_rcpQ_from_bandwidth(ALfloat f0norm, ALfloat bandwidth);
 
@@ -96,38 +96,43 @@ void BiquadState_setParams(BiquadState *filter, BiquadType type, ALfloat gain, A
 
 void BiquadState_processC(BiquadState *filter, ALfloat *restrict dst, const ALfloat *restrict src, ALsizei numsamples)
 {
-    ALsizei i;
     if(LIKELY(numsamples > 1))
     {
-        ALfloat x0 = filter->x[0];
-        ALfloat x1 = filter->x[1];
-        ALfloat y0 = filter->y[0];
-        ALfloat y1 = filter->y[1];
+        const ALfloat a1 = filter->a1;
+        const ALfloat a2 = filter->a2;
+        const ALfloat b0 = filter->b0;
+        const ALfloat b1 = filter->b1;
+        const ALfloat b2 = filter->b2;
+        ALfloat z1 = filter->z1;
+        ALfloat z2 = filter->z2;
+        ALsizei i;
 
+        /* Processing loop is transposed direct form II. This requires less
+         * storage versus direct form I (only two delay components, instead of
+         * a four-sample history; the last two inputs and outputs), and works
+         * better for floating-point which favors summing similarly-sized
+         * values while being less bothered by overflow.
+         *
+         * See: http://www.earlevel.com/main/2003/02/28/biquads/
+         */
         for(i = 0;i < numsamples;i++)
         {
-            dst[i] = filter->b0* src[i] +
-                     filter->b1*x0 + filter->b2*x1 -
-                     filter->a1*y0 - filter->a2*y1;
-            y1 = y0; y0 = dst[i];
-            x1 = x0; x0 = src[i];
+            ALfloat input = src[i];
+            ALfloat output = input*b0 + z1;
+            z1 = input*b1 - output*a1 + z2;
+            z2 = input*b2 - output*a2;
+            dst[i] = output;
         }
 
-        filter->x[0] = x0;
-        filter->x[1] = x1;
-        filter->y[0] = y0;
-        filter->y[1] = y1;
+        filter->z1 = z1;
+        filter->z2 = z2;
     }
     else if(numsamples == 1)
     {
-        dst[0] = filter->b0 * src[0] +
-                 filter->b1 * filter->x[0] +
-                 filter->b2 * filter->x[1] -
-                 filter->a1 * filter->y[0] -
-                 filter->a2 * filter->y[1];
-        filter->x[1] = filter->x[0];
-        filter->x[0] = src[0];
-        filter->y[1] = filter->y[0];
-        filter->y[0] = dst[0];
+        ALfloat input = *src;
+        ALfloat output = input*filter->b0 + filter->z1;
+        filter->z1 = input*filter->b1 - output*filter->a1 + filter->z2;
+        filter->z2 = input*filter->b2 - output*filter->a2;
+        *dst = output;
     }
 }