audio-guide

Low-level Audio Guidelines (Real-Time Software Mixing)

Engine-agnostic architecture for a software audio mixer: a real-time render thread feeds the device, voices are summed into the output buffer, and the game thread talks to the audio thread only through a lock-free queue. The lock-free queue itself is owned by lock-free-guide; preallocated pools by memory-management-guide; struct-of-arrays voice state by data-oriented-design-guide.

Essentials

• The callback is hard real-time - No locks, allocations, blocking, or syscalls on the audio path; bounded work per block, see references/audio-callback-thread.md
• Mix in float, ramp every gain - Accumulate voices in deinterleaved float through a gain matrix; never snap a gain or you click, see references/mixing-and-buffers.md
• One step does resample + pitch - Read each source with a fractional, per-voice playback step and interpolate, see references/resampling-and-dsp.md
• Fixed voice pool - Preallocate MAX_VOICES; cap concurrency, steal by priority, ramp before reusing a slot, see references/voice-management.md
• Talk to the audio thread by message - Post immutable commands through an SPSC ring; the audio thread owns voice state, see references/command-handoff.md

The render path

• Render quantum - Produce fixed-size blocks sized to fit the device buffer; bounds per-block cost, see references/audio-callback-thread.md
• Queue ahead, sleep on the event - Top up ~1.5 quanta, then wait on the device's buffer-low event, see references/audio-callback-thread.md
• Latency vs buffer size - Bigger buffers resist underrun but add latency; tune against the worst real device, see references/audio-callback-thread.md

Mixing and signal

• Gain matrix m[ic][oc] - Volume, pan, and spatialization are all one per-voice input->output matrix, see references/mixing-and-buffers.md
• Fit before output - Scale the summed float mix to range, then convert/interleave to the device's integer format, see references/mixing-and-buffers.md
• Interpolate fractional reads - Linear by default, cubic/Hermite where fidelity is worth it, see references/resampling-and-dsp.md

Voices and control

• Steal the least important - Full pool: cut the lowest-priority/quietest/oldest voice, never refuse silently, see references/voice-management.md
• Handles, not pointers - Hand out slot+generation so stale references to recycled voices are detectable, see references/voice-management.md
• Commands are value-complete - A command must not point at game-thread-mutable state, see references/command-handoff.md

Gotchas

• An underrun is instantly, harshly audible: one lock the game thread holds, one malloc slow path, or one first-touch page fault inside the callback and you get a click or dropout — far worse than a dropped render frame.
• Snapping any gain (start, stop, volume, pan, steal) inserts a step into the waveform that clicks; ramp every coefficient over tens of milliseconds, and keep ramping a stopping voice to zero before freeing its slot.
• Integer mixing overflows and wraps to a loud burst; accumulate in float so many loud voices coexist, then scale to fit at the end.
• A "stopped" voice still occupies its slot until its fade completes, so the free count lags — size the pool for that overlap, not just steady state.
• Accumulating a voice's fractional read position in float drifts a long sound out of tune; keep the cursor in double or fixed-point.
• A full command queue must never block the audio thread and must never malloc a bigger one; pick a producer-side drop/coalesce policy up front.
• Sizing the queue-ahead too tight starves on ordinary scheduler jitter and high-latency USB/Bluetooth devices; too loose adds latency you can hear in interactive sounds.
• This handoff is one producer, one consumer; multiple game threads posting audio commands need an MPSC queue or a single funnel thread (see lock-free-guide).

Progressive Disclosure

• Read references/audio-callback-thread.md - Load when the audio callback/render thread underruns, glitches, or you need its no-lock/no-alloc contract, render quantum, or buffer-size/latency tuning
• Read references/mixing-and-buffers.md - Load when summing voices, handling channels/format, debugging clicks/pops, or fitting/clipping the mix
• Read references/resampling-and-dsp.md - Load when converting sample rates, interpolating, or implementing per-voice pitch/playback speed
• Read references/voice-management.md - Load when allocating/stealing voices, sizing the voice pool, or managing voice lifetime and handles
• Read references/command-handoff.md - Load when passing play/stop/parameter changes from the game thread to the audio thread without locks