DSA — Data-oriented Streaming Audio

Open-source FMOD replacement built on DSO patterns: arena allocators, SoA (Struct of Arrays) layout, tick-aware mix loop.

Drop-in replacement for libfmod.so / libfmodstudio.so via LD_PRELOAD. Tested on Slay the Spire 2 (Steam).

Architecture

┌────────────────────────────────────────────┐
│  DSA_System  (owns arenas for each type)   │
│  ┌──────────────────────────────────────┐  │
│  │  Arena: "channel"                    │  │
│  │  ├─ volume[]    pitch[]    pan[]     │  │  ← SoA layout
│  │  ├─ frequency[] sound_id[] state[]   │  │
│  │  └─ pos[]       vel[]               │  │
│  ├─ Arena: "sound"      — PCM + metadata │
│  ├─ Arena: "dsp"        — DSP graph nodes│
│  ├─ Arena: "channelgrp" — group trees    │
│  └─ Arena: "soundgroup" — sound groups   │
│                                           │
│  Tick loop: mix_channel() → DSP chain →   │
│  output (ALSA)                            │
└────────────────────────────────────────────┘

DSO Patterns Used

Pattern	Implementation
Arena allocator	Per-type bump allocator with 64KB cache-aligned blocks, bulk-free at tick
SoA layout	Channels stored as parallel float/int arrays for SIMD-friendly mixing
Tick-aware	Mixer runs on audio callback boundary; arena epoch tracked per tick
Provenance	Hash-chain audit trail on every allocation
Semantic typing	Arena type_name classifies every allocation by object type

DSP Chain

mix_channel() runs each channel's DSP graph before summing to the output buffer. Every DSP node carries its own DSA_DSPState (allocated/freed via dsa_dsp_bind_process / dsa_dsp_free_state), so effects are stateful and tick-stable.

15 effects implemented in src/mixer.c:

Effect	Notes
lowpass / highpass / itlowpass	one-pole and IT-style resonant filters
parameq	parametric EQ (center freq + bandwidth)
distortion	waveshaper + output gain
flanger	modulated delay (LFO on delay time)
tremolo	amplitude LFO
echo	feedback delay line
delay	single-tap delay
reverb / sfxreverb	algorithmic + SFX reverb
compressor	peak compressor with threshold/ratio
normalize	peak-normalize pass
pitchshift	resampling-based pitch shift
oscillator	generated waveform source

3D Spatialization

Real 3D positioning in mix_channel():

• Distance attenuation — inverse-rolloff model (min_distance / max_distance SoA fields; DSA_Channel_Set3DMinMaxDistance writes them). Verified curve: near peak ≈ 0.25, 200 m ≈ 0.0025.
• Azimuth pan — per-channel pan computed from listener forward / up / right vectors and the channel's 3D position.

Build

Requirements: gcc, make, libasound2-dev (ALSA), libmpg123-dev (decoder).

make           # builds libdsa.so + libfmod.so + libfmodstudio.so + example
LD_LIBRARY_PATH=. ./example   # run test (hear 440Hz sine wave)

Drop-in FMOD Replacement

Replace FMOD in any Linux game without modifying the game:

DSA_DIR=/path/to/dsa
export LD_LIBRARY_PATH="$DSA_DIR"
export LD_PRELOAD="$DSA_DIR/libfmod.so:$DSA_DIR/libfmodstudio.so"
steam steam://rungameid/<appid>

Or use the helper script:

./steam-with-dsa.sh <appid>

Example: Slay the Spire 2 (appid 2861150):

FMOD Sound System: Successfully initialized

API Coverage

All core FMOD API functions implemented. Covers:

• System: Create/Init/Close/Release/Update/Version/Driver info/3D settings/Advanced settings
• Sound: Create/GetMode/SetMode/GetLength/GetInfo/Release
• Channel: PlaySound/SetVolume/SetPitch/SetPan/IsPlaying/Stop/3D attributes/Frequency/Position/AddDSP
• ChannelGroup: GetMaster/AddDSP/SetVolume/GetPitch/GetNumChannels/GetChannel/Stop
• DSP: CreateByType/GetInfo/SetParameter/GetParameter/AddDSP/Remove/SetActive
• DSPConnection: SetMix/GetMix
• SoundGroup: Create/SetVolume/GetVolume/GetNumPlaying
• Geometry: Create/Release
• Studio System: Create/Release/Init/Update/LoadBank/GetCoreSystem + C++ ABI

C++ ABI (_ZN4FMOD*) symbols exported: 205 symbols covering:

• FMOD::System, FMOD::Sound, FMOD::Channel, FMOD::ChannelGroup
• FMOD::ChannelControl, FMOD::DSP, FMOD::DSPConnection
• FMOD::Studio::System, FMOD::Studio::EventInstance, FMOD::Studio::EventDescription
• FMOD::Studio::Bank, FMOD::Studio::Bus, FMOD::Studio::VCA

License

Apache 2.0