Add an all-pass filter that replicates the band splitter's phase shift

2 files changed, 51 insertions, 0 deletions

diff --git a/Alc/bformatdec.c b/Alc/bformatdec.c
index 92a2aecf..ba6daac5 100644
--- a/Alc/bformatdec.c
+++ b/Alc/bformatdec.c
@@ -75,6 +75,45 @@ void bandsplit_process(BandSplitter *splitter, ALfloat *restrict hpout, ALfloat
 }
 
 
+void splitterap_init(SplitterAllpass *splitter, ALfloat freq_mult)
+{
+    ALfloat w = freq_mult * F_TAU;
+    ALfloat cw = cosf(w);
+    if(cw > FLT_EPSILON)
+        splitter->coeff = (sinf(w) - 1.0f) / cw;
+    else
+        splitter->coeff = cw * -0.5f;
+
+    splitter->z1 = 0.0f;
+}
+
+void splitterap_clear(SplitterAllpass *splitter)
+{
+    splitter->z1 = 0.0f;
+}
+
+void splitterap_process(SplitterAllpass *splitter, ALfloat *restrict samples, ALsizei count)
+{
+    ALfloat coeff, d, x;
+    ALfloat z1;
+    ALsizei i;
+
+    coeff = splitter->coeff;
+    z1 = splitter->z1;
+    for(i = 0;i < count;i++)
+    {
+        x = samples[i];
+
+        d = x - coeff*z1;
+        x = z1 + coeff*d;
+        z1 = d;
+
+        samples[i] = x;
+    }
+    splitter->z1 = z1;
+}
+
+
 static const ALfloat UnitScale[MAX_AMBI_COEFFS] = {
     1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
     1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f
diff --git a/Alc/bformatdec.h b/Alc/bformatdec.h
index 162f2652..8f44fc2a 100644
--- a/Alc/bformatdec.h
+++ b/Alc/bformatdec.h
@@ -60,4 +60,16 @@ void bandsplit_clear(BandSplitter *splitter);
 void bandsplit_process(BandSplitter *splitter, ALfloat *restrict hpout, ALfloat *restrict lpout,
                        const ALfloat *input, ALsizei count);
 
+/* The all-pass portion of the band splitter. Applies the same phase shift
+ * without splitting the signal.
+ */
+typedef struct SplitterAllpass {
+    ALfloat coeff;
+    ALfloat z1;
+} SplitterAllpass;
+
+void splitterap_init(SplitterAllpass *splitter, ALfloat freq_mult);
+void splitterap_clear(SplitterAllpass *splitter);
+void splitterap_process(SplitterAllpass *splitter, ALfloat *restrict samples, ALsizei count);
+
 #endif /* BFORMATDEC_H */