restore q value to shelves

vendor/kotlinaudio/kotlin-audio/src/main/java/com/doublesymmetry/kotlinaudio/scope/EqCoefficients.kt

@@ -7,7 +7,7 @@ import kotlin.math.sin
 import kotlin.math.sqrt
 
 /**
- * Web Audio BiquadFilterNode coefficients, a0-normalized as b0,b1,b2,a1,a2.
+ * Audio EQ Cookbook coefficients, a0-normalized as b0,b1,b2,a1,a2.
  * Type ordinals match EQ_BAND_TYPE_ORDINAL in src/audio/eq.ts:
  * 0 lowshelf, 1 peaking, 2 highshelf, 3 highpass, 4 lowpass.
  */
@@ -34,7 +34,6 @@ internal object EqCoefficients {
     val a = 10.0.pow(gainDb / 40.0)
     val alphaQ = sinW0 / (2.0 * q.coerceAtLeast(MIN_FILTER_Q.toFloat()))
     val alphaQDb = sinW0 / (2.0 * 10.0.pow(q / 20.0))
-    val alphaShelf = (sinW0 / 2.0) * sqrt(2.0)
 
     var b0 = 1.0; var b1 = 0.0; var b2 = 0.0
     var a0 = 1.0; var a1 = 0.0; var a2 = 0.0
@@ -46,21 +45,21 @@ internal object EqCoefficients {
       }
       0 -> { // lowshelf
         val sqrtA = sqrt(a)
-        b0 = a * (a + 1 - (a - 1) * cosW0 + 2 * sqrtA * alphaShelf)
+        b0 = a * (a + 1 - (a - 1) * cosW0 + 2 * sqrtA * alphaQ)
         b1 = 2 * a * (a - 1 - (a + 1) * cosW0)
-        b2 = a * (a + 1 - (a - 1) * cosW0 - 2 * sqrtA * alphaShelf)
-        a0 = a + 1 + (a - 1) * cosW0 + 2 * sqrtA * alphaShelf
+        b2 = a * (a + 1 - (a - 1) * cosW0 - 2 * sqrtA * alphaQ)
+        a0 = a + 1 + (a - 1) * cosW0 + 2 * sqrtA * alphaQ
         a1 = -2 * (a - 1 + (a + 1) * cosW0)
-        a2 = a + 1 + (a - 1) * cosW0 - 2 * sqrtA * alphaShelf
+        a2 = a + 1 + (a - 1) * cosW0 - 2 * sqrtA * alphaQ
       }
       2 -> { // highshelf
         val sqrtA = sqrt(a)
-        b0 = a * (a + 1 + (a - 1) * cosW0 + 2 * sqrtA * alphaShelf)
+        b0 = a * (a + 1 + (a - 1) * cosW0 + 2 * sqrtA * alphaQ)
         b1 = -2 * a * (a - 1 + (a + 1) * cosW0)
-        b2 = a * (a + 1 + (a - 1) * cosW0 - 2 * sqrtA * alphaShelf)
-        a0 = a + 1 - (a - 1) * cosW0 + 2 * sqrtA * alphaShelf
+        b2 = a * (a + 1 + (a - 1) * cosW0 - 2 * sqrtA * alphaQ)
+        a0 = a + 1 - (a - 1) * cosW0 + 2 * sqrtA * alphaQ
         a1 = 2 * (a - 1 - (a + 1) * cosW0)
-        a2 = a + 1 - (a - 1) * cosW0 - 2 * sqrtA * alphaShelf
+        a2 = a + 1 - (a - 1) * cosW0 - 2 * sqrtA * alphaQ
       }
       4 -> { // lowpass
         b0 = (1 - cosW0) / 2; b1 = 1 - cosW0; b2 = (1 - cosW0) / 2

vendor/kotlinaudio/kotlin-audio/src/test/java/com/doublesymmetry/kotlinaudio/scope/EqCoefficientsTest.kt

@@ -1,6 +1,7 @@
 package com.doublesymmetry.kotlinaudio.scope
 
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
 import org.junit.Test
 
 class EqCoefficientsTest {
@@ -29,15 +30,34 @@ class EqCoefficientsTest {
   }
 
   @Test
-  fun shelfFiltersIgnoreQ() {
+  fun shelfFiltersUseQ() {
+    val lowWide = coeffs(type = 0, freq = 100f, gainDb = 6f, q = 0.5f)
+    val lowNarrow = coeffs(type = 0, freq = 100f, gainDb = 6f, q = 2f)
+    val highWide = coeffs(type = 2, freq = 8000f, gainDb = -4f, q = 0.5f)
+    val highNarrow = coeffs(type = 2, freq = 8000f, gainDb = -4f, q = 2f)
+
     assertCoefficients(
-      coeffs(type = 0, freq = 100f, gainDb = 6f, q = 0.1f),
-      coeffs(type = 0, freq = 100f, gainDb = 6f, q = 18f)
+      lowWide,
+      doubleArrayOf(
+        1.004523905875,
+        -1.978032948597,
+        0.973748440145,
+        -1.978092655842,
+        0.978212638774
+      )
     )
     assertCoefficients(
-      coeffs(type = 2, freq = 8000f, gainDb = -4f, q = 0.1f),
-      coeffs(type = 2, freq = 8000f, gainDb = -4f, q = 18f)
+      highWide,
+      doubleArrayOf(
+        0.746848511923,
+        -0.319264680328,
+        0.034120017138,
+        -0.641024137473,
+        0.102727986206
+      )
     )
+    assertCoefficientsDiffer(lowWide, lowNarrow)
+    assertCoefficientsDiffer(highWide, highNarrow)
   }
 
   private fun coeffs(type: Int, freq: Float, gainDb: Float, q: Float): FloatArray {
@@ -59,4 +79,12 @@ class EqCoefficientsTest {
       assertEquals("coefficient $i", expected[i].toDouble(), actual[i].toDouble(), 0.0)
     }
   }
+
+  private fun assertCoefficientsDiffer(first: FloatArray, second: FloatArray) {
+    assertEquals("coefficient count", first.size, second.size)
+    assertTrue(
+      "expected at least one coefficient to change",
+      first.indices.any { i -> kotlin.math.abs(first[i] - second[i]) > 1e-6f }
+    )
+  }
 }