Phase-swappable allpass

VoidXH · Feb 25, 2024 · e4c21c0 · e4c21c0
1 parent ef18189
commit e4c21c0
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Showing 2 changed files with 43 additions and 4 deletions.
diff --git a/Cavern/Filters/Allpass.cs b/Cavern/Filters/Allpass.cs
@@ -5,7 +5,7 @@ namespace Cavern.Filters {
     /// <summary>
     /// Simple first-order allpass filter.
     /// </summary>
-    public class Allpass : BiquadFilter {
+    public class Allpass : PhaseSwappableBiquadFilter {
         /// <summary>
         /// Simple first-order allpass filter with maximum flatness and no additional gain.
         /// </summary>
@@ -41,10 +41,17 @@ public Allpass(int sampleRate, double centerFreq, double q, double gain) : base(
 
         /// <inheritdoc/>
         protected override void Reset(float cosW0, float alpha, float divisor) {
-            a2 = (1 - alpha) * divisor;
-            b0 = (float)Math.Pow(10, gain * .05f) * a2;
+            if (phaseSwapped) {
+                divisor = 1 / (1 - alpha);
+                a2 = (1 + alpha) * divisor;
+                b0 = 1;
+                b2 = (float)Math.Pow(10, gain * .05f) * a2;
+            } else {
+                a2 = (1 - alpha) * divisor;
+                b0 = (float)Math.Pow(10, gain * .05f) * a2;
+                b2 = 1;
+            }
             a1 = b1 = -2 * cosW0 * divisor;
-            b2 = 1;
         }
     }
 }
diff --git a/Cavern/Filters/PhaseSwappableBiquadFilter.cs b/Cavern/Filters/PhaseSwappableBiquadFilter.cs
@@ -0,0 +1,32 @@
+namespace Cavern.Filters {
+    /// <summary>
+    /// Simple first-order biquad filter with the option to invert the phase response.
+    /// </summary>
+    public abstract class PhaseSwappableBiquadFilter : BiquadFilter {
+        /// <summary>
+        /// Biquad filters usually achieve their effect by delaying lower frequencies. Phase swapping delays higher frequencies.
+        /// </summary>
+        public bool PhaseSwapped {
+            get => phaseSwapped;
+            set {
+                phaseSwapped = value;
+                Reset(CenterFreq, Q, Gain);
+            }
+        }
+
+        /// <summary>
+        /// State of phase swapping to be used when the filter is reset.
+        /// </summary>
+        protected bool phaseSwapped = false;
+
+        /// <inheritdoc/>
+        protected PhaseSwappableBiquadFilter(int sampleRate, double centerFreq) : base(sampleRate, centerFreq) { }
+
+        /// <inheritdoc/>
+        protected PhaseSwappableBiquadFilter(int sampleRate, double centerFreq, double q) : base(sampleRate, centerFreq, q) { }
+
+        /// <inheritdoc/>
+        protected PhaseSwappableBiquadFilter(int sampleRate, double centerFreq, double q, double gain) :
+            base(sampleRate, centerFreq, q, gain) { }
+    }
+}