feat(mixer): 5-channel APU mixing with linear approximation formula

src/runtime/audio.rs

@@ -1,3 +1,4 @@
+use crate::native_core::apu::ChannelOutputs;
 use crate::runtime::VideoMode;
 
 #[derive(Debug)]
@@ -25,15 +26,53 @@ impl AudioMixer {
         self.sample_accumulator = 0.0;
     }
 
-    pub fn push_cycles(&mut self, cpu_cycles: u8, apu_regs: &[u8; 0x20], out: &mut Vec<f32>) {
+    pub fn push_cycles(&mut self, cpu_cycles: u8, channels: ChannelOutputs, out: &mut Vec<f32>) {
         self.sample_accumulator += self.samples_per_cpu_cycle * f64::from(cpu_cycles);
         let samples = self.sample_accumulator.floor() as usize;
         self.sample_accumulator -= samples as f64;
 
-        // Current core does not expose a final mixed PCM stream yet.
-        // Use DMC output level as a stable interim signal in [-1.0, 1.0].
-        let dmc = apu_regs[0x11] & 0x7F;
-        let sample = (f32::from(dmc) / 63.5) - 1.0;
+        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 mixed = pulse_out + tnd_out;
+        let sample = mixed * 2.0 - 1.0;
+
         out.extend(std::iter::repeat_n(sample, samples));
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn mixer_silent_channels_produce_negative_one() {
+        let mut mixer = AudioMixer::new(44_100, VideoMode::Ntsc);
+        let channels = ChannelOutputs::default();
+        let mut out = Vec::new();
+        mixer.push_cycles(50, channels, &mut out);
+        assert!(!out.is_empty());
+        for &s in &out {
+            assert!((s - (-1.0)).abs() < 1e-6, "expected -1.0, got {s}");
+        }
+    }
+
+    #[test]
+    fn mixer_max_channels_produce_positive() {
+        let mut mixer = AudioMixer::new(44_100, VideoMode::Ntsc);
+        let channels = ChannelOutputs {
+            pulse1: 15,
+            pulse2: 15,
+            triangle: 15,
+            noise: 15,
+            dmc: 127,
+        };
+        let mut out = Vec::new();
+        mixer.push_cycles(50, channels, &mut out);
+        assert!(!out.is_empty());
+        for &s in &out {
+            assert!(s > 0.0, "expected positive sample, got {s}");
+        }
+    }
+}