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# Audio Output Design — Full 5-Channel Mixer + cpal Backend
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## Overview
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Add real audio output to the desktop NES emulator client. This involves two independent pieces of work:
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1. **Full APU mixer** — replace the current DMC-only mixer with proper 5-channel mixing (Pulse 1, Pulse 2, Triangle, Noise, DMC) using NES hardware-accurate formulas.
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2. **cpal audio backend** — replace the stub `AudioSink` in the desktop client with a real audio output using `cpal`, connected via a lock-free ring buffer. Add a volume slider to the GTK4 header bar.
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## 1. Full APU Mixer
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### Current State
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`AudioMixer::push_cycles()` in `src/runtime/audio.rs` reads only `apu_regs[0x11]` (DMC output level) and generates a single-channel signal. All other channels are ignored.
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### Design
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#### 1.1 Channel Outputs Struct
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Add to `src/native_core/apu/`:
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```rust
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#[derive(Debug, Clone, Copy, Default)]
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pub struct ChannelOutputs {
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pub pulse1: u8, // 0–15
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pub pulse2: u8, // 0–15
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pub triangle: u8, // 0–15
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pub noise: u8, // 0–15
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pub dmc: u8, // 0–127
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}
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```
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#### 1.2 New APU Internal State
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The current `Apu` struct lacks timer counters and sequencer state needed to compute channel outputs. The following fields must be added:
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**Pulse channels (×2):**
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- `pulse_timer_counter: [u16; 2]` — countdown timer, clocked every other CPU cycle
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- `pulse_duty_step: [u8; 2]` — position in 8-step duty cycle sequence (0–7)
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**Triangle channel:**
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- `triangle_timer_counter: u16` — countdown timer, clocked every CPU cycle
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- `triangle_step: u8` — position in 32-step triangle sequence (0–31)
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**Noise channel:**
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- `noise_timer_counter: u16` — countdown timer, clocked every other CPU cycle
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- `noise_lfsr: u16` — 15-bit linear feedback shift register, initialized to 1
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These must be clocked in `Apu::clock_cpu_cycle()`:
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- Pulse and noise timers decrement every **2** CPU cycles (APU rate, tracked via existing `cpu_cycle_parity`)
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- Triangle timer decrements every **1** CPU cycle
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- When a timer reaches 0, it reloads from the period register and advances the corresponding sequencer
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#### 1.3 APU Method
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Add `Apu::channel_outputs(&self) -> ChannelOutputs` that computes the current output level of each channel:
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- **Pulse 1/2:** Output is 0 if length counter is 0, or sweep mutes the channel, or duty cycle sequencer output is 0. Otherwise output is the envelope volume (0–15).
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- **Triangle:** Output is the value from the 32-step triangle waveform lookup at `triangle_step`. Muted (output 0) if length counter or linear counter is 0.
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- **Noise:** Output is 0 if length counter is 0 or LFSR bit 0 is 1. Otherwise output is the envelope volume (0–15).
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- **DMC:** Output is `dmc_output_level` (0–127), already tracked.
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#### 1.4 Save-State Compatibility
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Adding new fields to `Apu` changes the save-state binary format. The `save_state_tail()` and `load_state_tail()` methods must be updated to serialize/deserialize the new fields. This is a **breaking change** to the save-state format — old save states will not be compatible. Since the project is pre-1.0, this is acceptable without a migration strategy.
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#### 1.5 Bus Exposure
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Add `NativeBus::apu_channel_outputs(&self) -> ChannelOutputs` to expose channel outputs alongside the existing `apu_registers()`.
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#### 1.6 Mixer Update
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Change `AudioMixer::push_cycles()` signature:
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```rust
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// Before:
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pub fn push_cycles(&mut self, cpu_cycles: u8, apu_regs: &[u8; 0x20], out: &mut Vec<f32>)
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// After:
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pub fn push_cycles(&mut self, cpu_cycles: u8, channels: ChannelOutputs, out: &mut Vec<f32>)
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```
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Mixing formula (nesdev wiki linear approximation):
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```
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pulse_out = 0.00752 * (pulse1 + pulse2)
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tnd_out = 0.00851 * triangle + 0.00494 * noise + 0.00335 * dmc
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output = pulse_out + tnd_out
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```
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Output range is approximately [0.0, 1.0]. Normalize to [-1.0, 1.0] by: `sample = output * 2.0 - 1.0`.
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**Known simplifications:**
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- This uses the linear approximation, not the more accurate nonlinear lookup tables from real NES hardware. Nonlinear mixing can be added later as an enhancement.
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- The current `repeat_n` resampling approach (nearest-neighbor) produces aliasing. A low-pass filter or bandlimited interpolation can be added later.
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- Real NES hardware applies two first-order high-pass filters (~90Hz and ~440Hz). Without these, channel enable/disable will cause audible pops. Deferred for a future iteration.
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#### 1.7 Runtime Integration
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Update `NesRuntime::run_until_frame_complete_with_audio()` in `src/runtime/core.rs` to pass `ChannelOutputs` (from `self.bus.apu_channel_outputs()`) instead of the register slice to the mixer.
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## 2. Lock-Free Ring Buffer
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### Location
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New file: `src/runtime/ring_buffer.rs`.
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### Design
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SPSC (single-producer, single-consumer) ring buffer using `AtomicUsize` for head/tail indices:
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- **Capacity:** 4096 f32 samples (~85ms at 48kHz) — enough to absorb frame timing jitter
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- **Producer:** emulation thread writes samples after each frame via `push_samples()`
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- **Consumer:** cpal audio callback reads samples via `pop_samples()`
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- **Underrun (buffer empty):** consumer outputs silence (0.0)
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- **Overrun (buffer full):** producer **drops new samples** (standard SPSC behavior — only the consumer moves the tail pointer)
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```rust
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pub struct RingBuffer {
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buffer: Box<[f32]>,
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capacity: usize,
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head: AtomicUsize, // write position (producer only)
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tail: AtomicUsize, // read position (consumer only)
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}
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impl RingBuffer {
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pub fn new(capacity: usize) -> Self;
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pub fn push(&self, samples: &[f32]) -> usize; // returns samples actually written
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pub fn pop(&self, out: &mut [f32]) -> usize; // returns samples actually read
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pub fn clear(&self); // reset both pointers (call when no concurrent access)
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}
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```
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Thread safety: `RingBuffer` is `Send + Sync`. Shared via `Arc<RingBuffer>`.
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## 3. Desktop cpal Audio Backend
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### Dependencies
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Add to `crates/nesemu-desktop/Cargo.toml`:
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```toml
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cpal = "0.15"
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```
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### CpalAudioSink
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```rust
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pub struct CpalAudioSink {
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_stream: cpal::Stream, // keeps the audio stream alive
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ring: Arc<RingBuffer>,
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volume: Arc<AtomicU32>, // f32 bits stored atomically
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}
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```
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- Implements `nesemu::AudioOutput` — `push_samples()` writes to ring buffer
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- Created when a ROM is loaded; the ring buffer is cleared on ROM change to prevent stale samples
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- cpal callback: reads from ring buffer, multiplies each sample by volume, writes to output buffer
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- On pause: emulation stops producing samples → callback outputs silence (underrun behavior)
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- On ROM change: old stream is dropped, ring buffer cleared, new stream created
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### Error Handling
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If no audio device is available, or the requested format is unsupported, or the stream fails to build:
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- Log the error to stderr
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- Fall back to `NullAudio` behavior (discard samples silently)
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- The emulator continues to work without sound
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The cpal error callback also logs errors to stderr without crashing.
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### Stream Configuration
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- Sample rate: 48,000 Hz
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- Channels: 1 (mono — NES is mono)
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- Sample format: f32
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- Buffer size: let cpal choose (typically 256–1024 frames)
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### Volume
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- `Arc<AtomicU32>` shared between UI and cpal callback
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- Stored as `f32::to_bits()` / `f32::from_bits()`
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- Default: 0.75 (75%)
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- Applied in cpal callback: `sample * volume`
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## 4. UI — Volume Slider
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### Widget
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`gtk::Scale` (horizontal) added to the header bar:
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- Range: 0.0 to 1.0 (displayed as 0–100%)
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- Default: 0.75
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- `connect_value_changed` → atomically update volume
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### Placement
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In the header bar, after the existing control buttons (open, pause, reset), with a small speaker icon label.
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## 5. Threading Model
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- **GTK main thread:** runs emulation via `glib::timeout_add_local` (~16ms tick), UI events, volume slider updates
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- **cpal OS thread:** audio callback reads from ring buffer — this is the only cross-thread boundary
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- The ring buffer (`Arc<RingBuffer>`) and volume (`Arc<AtomicU32>`) are the only shared state between threads
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## 6. Data Flow
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```
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CPU instruction step (GTK main thread)
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→ APU.clock_cpu_cycle() [updates internal channel state]
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→ AudioMixer.push_cycles(cycles, apu.channel_outputs())
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→ mix 5 channels → f32 sample
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→ append to frame audio buffer (Vec<f32>)
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Per frame (GTK main thread):
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→ FrameExecutor collects audio_buffer
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→ CpalAudioSink.push_samples(audio_buffer)
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→ write to Arc<RingBuffer>
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cpal callback (separate OS thread):
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→ read from Arc<RingBuffer>
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→ multiply by volume (Arc<AtomicU32>)
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→ write to hardware audio buffer
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```
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## 7. Files Changed
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| File | Change |
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|------|--------|
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| `src/native_core/apu/types.rs` | Add `ChannelOutputs` struct, new timer/sequencer fields to `Apu` and `ApuStateTail` |
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| `src/native_core/apu/api.rs` | Add `channel_outputs()` method, update `save_state_tail`/`load_state_tail` |
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| `src/native_core/apu/timing.rs` | Clock new timer/sequencer fields in `clock_cpu_cycle()` |
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| `src/native_core/bus.rs` | Add `apu_channel_outputs()` |
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| `src/runtime/audio.rs` | Rewrite mixer with 5-channel formula |
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| `src/runtime/ring_buffer.rs` (new) | Lock-free SPSC ring buffer |
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| `src/runtime/core.rs` | Pass `channel_outputs()` to mixer in `run_until_frame_complete_with_audio()` |
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| `src/runtime/mod.rs` | Export `ring_buffer`, `ChannelOutputs` |
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| `crates/nesemu-desktop/Cargo.toml` | Add `cpal` dependency |
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| `crates/nesemu-desktop/src/main.rs` | Replace stub AudioSink with CpalAudioSink, add volume slider |
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## 8. Testing
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- Existing tests in `tests/public_api.rs` must continue to pass (they use NullAudio). **Note:** the regression hash test (`public_api_regression_hashes_for_reference_rom`) will produce a different audio hash due to the mixer change — the expected hash must be updated.
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- Unit test for ring buffer: push/pop, underrun, overrun, clear
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- Unit test for mixer: known channel outputs → expected sample values
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- Manual test: load a ROM, verify audible sound through speakers
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