feat(audio): non-linear APU mixing and mapper expansion audio (VRC6, FME-7, Namco163)

src/runtime/audio.rs

@@ -56,11 +56,27 @@ impl AudioMixer {
         let samples = self.sample_accumulator.floor() as usize;
         self.sample_accumulator -= samples as f64;
 
-        let pulse_out = 0.00752 * (f32::from(channels.pulse1) + f32::from(channels.pulse2));
-        let tnd_out = 0.00851 * f32::from(channels.triangle)
-            + 0.00494 * f32::from(channels.noise)
-            + 0.00335 * f32::from(channels.dmc);
-        let sample = pulse_out + tnd_out;
+        // NES non-linear APU mixing (Blargg's reference formulas).
+        // Pulse channels use a shared lookup:
+        //   pulse_out = 95.88 / (8128 / (p1 + p2) + 100)
+        // TND channels use a separate lookup:
+        //   tnd_out = 159.79 / (1 / (tri/8227 + noise/12241 + dmc/22638) + 100)
+        // Both formulas produce 0.0 when all contributing channels are silent.
+        let p_sum = f32::from(channels.pulse1) + f32::from(channels.pulse2);
+        let pulse_out = if p_sum == 0.0 {
+            0.0
+        } else {
+            95.88 / (8128.0 / p_sum + 100.0)
+        };
+        let tnd_sum = f32::from(channels.triangle) / 8227.0
+            + f32::from(channels.noise) / 12241.0
+            + f32::from(channels.dmc) / 22638.0;
+        let tnd_out = if tnd_sum == 0.0 {
+            0.0
+        } else {
+            159.79 / (1.0 / tnd_sum + 100.0)
+        };
+        let sample = pulse_out + tnd_out + channels.expansion;
 
         if samples == 0 {
             return;
@@ -118,6 +134,7 @@ mod tests {
             triangle: 15,
             noise: 15,
             dmc: 127,
+            expansion: 0.0,
         };
         let mut out = Vec::new();
         mixer.push_cycles(50, channels, &mut out);
@@ -143,6 +160,7 @@ mod tests {
                 triangle: 15,
                 noise: 15,
                 dmc: 127,
+                expansion: 0.0,
             },
             &mut out,
         );