Gate SSG pseudo-stereo on the same pan detection as FM

Sources/FMSynthesis/YM2608.swift

@@ -401,11 +401,22 @@ package final class YM2608 {
   package var pseudoStereoEnabled: Bool = false
 
   /// Set to true when any FM channel's pan register (0xB4) is written with non-center value.
-  /// Once set, pseudo-stereo is suppressed for FM until reset.
+  /// Once set, pseudo-stereo is suppressed until reset — for SSG as well as FM: a program
+  /// that places its FM channels itself authored a stereo image, and widening the SSG
+  /// underneath it would fight that image rather than complement it.
+  ///
+  /// Tracked unconditionally, not only while `pseudoStereoEnabled` is set, so that
+  /// enabling pseudo-stereo mid-tune still sees the pans the program wrote earlier.
   package private(set) var fmPanDetected: Bool = false
   package var chorusFM = ChorusEffect()                    // L=dry, R=delayed
   package var chorusSSG = ChorusEffect(delayLeft: true)    // L=delayed, R=dry
 
+  /// Rebuild `fmPanDetected` from the current channel pans. Used after a save
+  /// state load, where the latch itself is not stored but the pans are.
+  package func refreshPanDetection() {
+    fmPanDetected = (0..<6).contains { !(fmSynth.ch[$0].panLeft && fmSynth.ch[$0].panRight) }
+  }
+
   /// Audio sample rate for output (default 44100 Hz)
   package static let sampleRate = 44100
 
@@ -720,9 +731,10 @@ package final class YM2608 {
 
     if mask.contains(.ssg) {
       let ssgScaled = Int((ssgSample * 16384.0).rounded())
-      // Immersive takes priority: never apply pseudo-stereo chorus when
-      // spatial output is active (the two modes are mutually exclusive).
-      if pseudoStereoEnabled && !immersiveOutputEnabled {
+      // Same gating as the FM path in generateFMSamples(): suppressed once the
+      // program pans an FM channel itself, and immersive takes priority (the
+      // two modes are mutually exclusive).
+      if pseudoStereoEnabled && !fmPanDetected && !immersiveOutputEnabled {
         let (sl, sr) = chorusSSG.process(monoSample: ssgScaled)
         mixL = Self.storeSample16(mixL, sl)
         mixR = Self.storeSample16(mixR, sr)

Sources/FMSynthesis/YM2608Serialize.swift

@@ -235,11 +235,20 @@ extension YM2608 {
     adpcmReadPrefetch = pos < data.count ? data[pos] : 0
     if pos < data.count { pos += 1 }
 
-    // Clear transient audio buffer. The CD-mix delay lines are not part of the
-    // save state either, so drop their contents rather than carrying a tail
-    // from the pre-load audio into the restored scene.
+    // Clear transient audio buffer. The CD-mix and chorus delay lines are not
+    // part of the save state either, so drop their contents rather than
+    // carrying a tail from the pre-load audio into the restored scene.
     audioBuffer.removeAll()
     resetPostProcessors()
+    chorusFM.reset()
+    chorusSSG.reset()
+
+    // Rebuild the pan latch from the restored channels instead of storing it:
+    // FMCh.panLeft/panRight are part of the FMSynthesizer blob, so a load can
+    // recover it without a format change. A tune that pans a channel and later
+    // returns it to centre loses the latch across a load; the register writes
+    // that follow set it again.
+    refreshPanDetection()
 
     return true
   }

Tests/Bubilator88CoreTests/YM2608PseudoStereoTests.swift

@@ -0,0 +1,79 @@
+import Testing
+@testable import FMSynthesis
+
+@Suite("YM2608 pseudo-stereo gating")
+struct YM2608PseudoStereoTests {
+
+  /// SSG tone on channel A, loud enough that the Haas delay shows up as a
+  /// left/right difference in the mixed output.
+  private func makeSSGTone(pseudoStereo: Bool) -> YM2608 {
+    let sound = YM2608()
+    sound.reset()
+    sound.pseudoStereoEnabled = pseudoStereo
+    sound.writeAddr(0x00); sound.writeData(0x40)  // tone period low
+    sound.writeAddr(0x01); sound.writeData(0x01)  // tone period high
+    sound.writeAddr(0x07); sound.writeData(0x3E)  // mixer: tone A only
+    sound.writeAddr(0x08); sound.writeData(0x0F)  // channel A volume
+    return sound
+  }
+
+  private func hasStereoDifference(_ sound: YM2608) -> Bool {
+    stride(from: 0, to: sound.audioBuffer.count - 1, by: 2).contains {
+      sound.audioBuffer[$0] != sound.audioBuffer[$0 + 1]
+    }
+  }
+
+  /// Writes a non-centre pan to FM channel 1 (register 0xB4, right only).
+  private func panChannel1Right(_ sound: YM2608) {
+    sound.writeAddr(0xB4)
+    sound.writeData(0x40)
+  }
+
+  @Test("SSG output is widened while no FM channel is panned")
+  func ssgWidenedWhenCentred() {
+    let sound = makeSSGTone(pseudoStereo: true)
+    sound.tick(tStates: 80_000)
+    #expect(!sound.audioBuffer.isEmpty)
+    #expect(hasStereoDifference(sound))
+  }
+
+  @Test("SSG widening stops once the program pans an FM channel")
+  func ssgFollowsFMPanDetection() {
+    let sound = makeSSGTone(pseudoStereo: true)
+    panChannel1Right(sound)
+    #expect(sound.fmPanDetected)
+    sound.tick(tStates: 80_000)
+    #expect(!sound.audioBuffer.isEmpty)
+    #expect(!hasStereoDifference(sound))
+  }
+
+  @Test("Pan detection runs even while pseudo-stereo is off")
+  func panDetectedWithPseudoStereoDisabled() {
+    let sound = makeSSGTone(pseudoStereo: false)
+    panChannel1Right(sound)
+    #expect(sound.fmPanDetected)
+  }
+
+  @Test("A save state load rebuilds the pan latch from the restored channels")
+  func panLatchSurvivesSaveStateRoundTrip() {
+    let source = makeSSGTone(pseudoStereo: true)
+    panChannel1Right(source)
+    let state = source.serializeState()
+
+    let restored = makeSSGTone(pseudoStereo: true)
+    #expect(!restored.fmPanDetected)
+    #expect(restored.deserializeState(state))
+    #expect(restored.fmPanDetected)
+  }
+
+  @Test("A save state taken before any pan write leaves the latch clear")
+  func centredStateLoadsWithLatchClear() {
+    let source = makeSSGTone(pseudoStereo: true)
+    let state = source.serializeState()
+
+    let restored = makeSSGTone(pseudoStereo: true)
+    panChannel1Right(restored)
+    #expect(restored.deserializeState(state))
+    #expect(!restored.fmPanDetected)
+  }
+}