Apply biquad and T60 filters using transposed direct form II

2 files changed, 17 insertions, 28 deletions

diff --git a/Alc/effects/echo.c b/Alc/effects/echo.c
index a3a4e120..282aa1d7 100644
--- a/Alc/effects/echo.c
+++ b/Alc/effects/echo.c
@@ -155,14 +155,12 @@ static ALvoid ALechoState_process(ALechoState *state, ALsizei SamplesToDo, const
     const ALsizei tap2 = state->Tap[1].delay;
     ALfloat *restrict delaybuf = state->SampleBuffer;
     ALsizei offset = state->Offset;
-    ALfloat x[2], y[2], in, out;
+    ALfloat z1, z2, in, out;
     ALsizei base;
     ALsizei c, i;
 
-    x[0] = state->Filter.x[0];
-    x[1] = state->Filter.x[1];
-    y[0] = state->Filter.y[0];
-    y[1] = state->Filter.y[1];
+    z1 = state->Filter.z1;
+    z2 = state->Filter.z2;
     for(base = 0;base < SamplesToDo;)
     {
         alignas(16) ALfloat temps[2][128];
@@ -182,11 +180,9 @@ static ALvoid ALechoState_process(ALechoState *state, ALsizei SamplesToDo, const
              * feedback attenuation.
              */
             in = temps[1][i];
-            out = in*state->Filter.b0 +
-                  x[0]*state->Filter.b1 + x[1]*state->Filter.b2 -
-                  y[0]*state->Filter.a1 - y[1]*state->Filter.a2;
-            x[1] = x[0]; x[0] = in;
-            y[1] = y[0]; y[0] = out;
+            out = in*state->Filter.b0 + z1;
+            z1 = in*state->Filter.b1 - out*state->Filter.a1 + z2;
+            z2 = in*state->Filter.b2 - out*state->Filter.a2;
 
             delaybuf[offset&mask] += out * state->FeedGain;
             offset++;
@@ -198,10 +194,8 @@ static ALvoid ALechoState_process(ALechoState *state, ALsizei SamplesToDo, const
 
         base += td;
     }
-    state->Filter.x[0] = x[0];
-    state->Filter.x[1] = x[1];
-    state->Filter.y[0] = y[0];
-    state->Filter.y[1] = y[1];
+    state->Filter.z1 = z1;
+    state->Filter.z2 = z2;
 
     state->Offset = offset;
 }
diff --git a/Alc/effects/reverb.c b/Alc/effects/reverb.c
index 0d486e63..831e4a3a 100644
--- a/Alc/effects/reverb.c
+++ b/Alc/effects/reverb.c
@@ -234,11 +234,9 @@ typedef struct T60Filter {
     ALfloat HFCoeffs[3];
     ALfloat LFCoeffs[3];
 
-    /* The HF and LF filters each keep a state of the last input and last
-     * output sample.
-     */
-    ALfloat HFState[2];
-    ALfloat LFState[2];
+    /* The HF and LF filters each keep a delay component. */
+    ALfloat HFState;
+    ALfloat LFState;
 } T60Filter;
 
 typedef struct EarlyReflections {
@@ -407,10 +405,8 @@ static void ALreverbState_Construct(ALreverbState *state)
             state->Late.T60[i].HFCoeffs[j] = 0.0f;
             state->Late.T60[i].LFCoeffs[j] = 0.0f;
         }
-        state->Late.T60[i].HFState[0] = 0.0f;
-        state->Late.T60[i].HFState[1] = 0.0f;
-        state->Late.T60[i].LFState[0] = 0.0f;
-        state->Late.T60[i].LFState[1] = 0.0f;
+        state->Late.T60[i].HFState = 0.0f;
+        state->Late.T60[i].LFState = 0.0f;
     }
 
     for(i = 0;i < NUM_LINES;i++)
@@ -1443,9 +1439,8 @@ DECL_TEMPLATE(Faded)
 static inline ALfloat FirstOrderFilter(const ALfloat in, const ALfloat *restrict coeffs,
                                        ALfloat *restrict state)
 {
-    ALfloat out = coeffs[0]*in + coeffs[1]*state[0] + coeffs[2]*state[1];
-    state[0] = in;
-    state[1] = out;
+    ALfloat out = coeffs[0]*in + *state;
+    *state = coeffs[1]*in + coeffs[2]*out;
     return out;
 }
 
@@ -1456,8 +1451,8 @@ static inline void LateT60Filter(ALfloat *restrict out, const ALfloat *restrict
     ALsizei i;
     for(i = 0;i < NUM_LINES;i++)
         out[i] = FirstOrderFilter(
-            FirstOrderFilter(in[i], filter[i].HFCoeffs, filter[i].HFState),
-            filter[i].LFCoeffs, filter[i].LFState
+            FirstOrderFilter(in[i], filter[i].HFCoeffs, &filter[i].HFState),
+            filter[i].LFCoeffs, &filter[i].LFState
         );
 }