KeyASIO.Net

Low-Latency Audio Middleware for osu!

Audio latency as low as 0.6ms.
"I was hearing clicks before I even tapped." — player feedback

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🚀 Why KeyASIO?

osu!'s built-in audio stack typically sits around 30–40ms of latency. KeyASIO replaces it with an external ASIO / WASAPI Exclusive backend that talks past the Windows mixer, and reconstructs hitsounds in real time from the game's own memory.

• ⚡ Low latency: ~0.6ms on high-end hardware (verified by EmertxE), 8–15ms on generic setups.
• 🎮 Real game integration: reads game state via ReadProcessMemory (stable) or a named-pipe IPC bridge (lazer), so the hitsounds stay synchronized to the live beatmap and judgement state instead of just firing on keypress.
• 🎧 Full hitsound support: custom hitsounds, storyboard samples, dynamic volume, skin overrides.
• 🎹 Mania-aware: per-key sound processing matching the native game's behavior.

📸 Screenshots

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⚙️ Configuration Guide

KeyASIO has two main modes. Pick the one that matches your hardware.

🚩 Which mode should I choose?

• I have a dedicated soundcard / audio interface / mixer: → Hardware Mix (Recommended). Lowest latency, needs hardware setup.
• I'm on a laptop / integrated audio / no mixer: → Software Mix (FullMode). Easier setup, requires VB-CABLE.

🔧 Option A: Hardware Mix (FullMode DISABLED) — Recommended

For: users with more than one audio output (e.g. PC Line-out + dedicated soundcard) and a physical mixer.

1. Prerequisites
   • Your soundcard's ASIO driver, or ASIO4ALL.
   • A physical mixer to combine osu! (music) and KeyASIO (hitsounds).
2. Routing
   • osu! music → Device A (e.g. motherboard Line-out).
   • KeyASIO hitsounds → Device B (e.g. external soundcard).
   • Mix A + B → headphones.
3. In KeyASIO
   • Select your ASIO device.
   • Set Realtime Mode to TRUE.
   • Set FullMode (EnableMusicFunctions) to FALSE.
4. In osu!
   • Set Effect Volume to 0.
   • Adjust your audio offset (likely below -40ms) to sync.

💻 Option B: Software Mix (FullMode ENABLED)

For: users with a single soundcard (laptops / desktops without a mixer).

1. Prerequisites
   • Install VB-CABLE.
2. Routing
   • Set osu!'s output to VB-Cable Input.
   • (Optional) If you stream, capture VB-Cable Output in OBS.
3. In KeyASIO
   • Select your real output device (headphones/speakers) with WASAPI Exclusive on.
   • Set FullMode (EnableMusicFunctions) to TRUE.
   • This mode fully replaces osu!'s audio engine.

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Full options in appsettings.yaml (edit after the program is closed):

Item	Description
Debugging	Show debug logs.
Device	Device configuration (prefer configuring in the GUI).
HitsoundPath	Default hitsound path (relative or absolute).
Keys	Triggering keys. See System.Windows.Forms.Keys.
RealtimeMode	When true, memory scanning is enabled and hitsounds are played from the loaded beatmap.
RealtimeMode.AudioOffset	Offset applied when RealtimeMode is on (adjustable in GUI).
RealtimeMode.BalanceFactor	Balance factor.
RealtimeMode.IgnoreBeatmapHitsound	Ignore the beatmap's hitsounds and force the user skin.
RealtimeMode.IgnoreComboBreak	Suppress the combo-break sound.
RealtimeMode.IgnoreSliderTicksAndSlides	Ignore slider ticks and slides.
RealtimeMode.IgnoreStoryboardSamples	Ignore the beatmap's storyboard samples.
RealtimeMode.SliderTailPlaybackBehavior	Slider-tail playback. Normal: always play; KeepReverse: only on multi-reverse sliders; Ignore: never play.
SampleRate	Device sample rate (adjustable in GUI).
SkinFolder	Skin folder used when RealtimeMode is on.
VolumeEnabled	Software volume control. Disable for lowest latency when RealtimeMode is off.
Volume	Configured device volume.

❓ FAQ

Is this safe for my osu! account?

KeyASIO reads game state passively (like tosu, StreamCompanion or Gosumemory), an approach that has been accepted for similar tools. As always, use at your own risk.

Why do I hear no sound in Auto mod?

In Exclusive mode osu! can lose control of the audio device and its internal clock desyncs — this is expected when bypassing the Windows Mixer. Select a different audio device in osu!.

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🛠️ For Developers

This section is for anyone who wants to build KeyASIO locally, write a plugin, or understand the internals. The user-facing sections above are all most people need; the rest is optional.

Build

• .NET 10 SDK (net10.0, x64). The solution file is KeyAsio.Net.slnx.
• Package versions are centralized in Directory.Packages.props.
• Some dependencies are vendored as projects under dependencies/ (a trimmed NAudio.Asio, Milki.Extensions.*, and OverlayAPI.LazerProtocol for lazer IPC).
• Native AOT is supported via the Satori.targets import; assembly signing runs through Signer.ps1 when a private key is present.
• SIMD benchmarks live in benchmarks/SimdBenchmarks/ (BenchmarkDotNet).

dotnet build -c Release
dotnet run --project src/Apps/KeyAsio -c Release

Solution layout

src/
Apps/KeyAsio/ Avalonia + Suki GUI, OS integration, DI composition root
Common/
KeyAsio.Application/ Plugin host, skin discovery, localization, session adapters
KeyAsio.Configuration/ appsettings.yaml, migration, self-documenting YAML comments
KeyAsio.Plugins.Contracts/ The only API surface plugins compile against (trimmable)
KeyAsio.Plugins.Abstractions/
KeyAsio.Common/ Implementation-independent utilities
KeyAsio.Shared/
KeyAsio.Sentry/ Crash reporting glue
KeyAsio.Secrets/
Core/
KeyAsio.Core.Audio/ ASIO/WASAPI/DirectSound engine, mixing, caching, limiters
KeyAsio.Core.Memory/ Process memory scanning, signature patterns, declarative reader
KeyAsio.Core.OsuAudio/ Beatmap hitsound analysis, timeline, nightcore beat synthesis
KeyAsio.Core.OsuPlayback/
KeyAsio.Sync/ Game-state acquisition, state machine, hitsound sequencing

Dependency direction is inward, toward contracts and primitives. KeyAsio.Sync does not depend on KeyAsio.Application; neither plugin contracts nor application services depend on Avalonia/Suki. See docs/architecture.md for the full boundary rules and the plugin contract migration table.

How it works

Game state acquisition. Two sources implement IGameSyncSource and are selected by priority:

• StableMemoryGameSyncSource opens the osu! process with PROCESS_VM_READ and resolves pointers declaratively from osu_memory_rules.json (signatures → pointer chains → typed values). Reads run on two cadences: timing data at ~2ms (500Hz), general data (mods, beatmap, combo, username) at ~50ms. The reader is single-threaded (LongRunning) to avoid pool switching; timeBeginPeriod(1) raises timer resolution and power throttling is disabled on the reader thread.
• LazerIpcGameSyncSource talks to lazer over two named pipes (timing + events) using the OverlayAPI delta protocol, and resolves beatmap files from lazer's hash-based store.

Both feed SyncSessionContext, which mirrors the game state and computes a predicted, monotonic PlayTime anchored on the last memory read and extrapolated by the playback rate (1.5× for DT/Nightcore, 0.75× for HT). Backward motion is classified as a seek (>100ms) or a freeze (<100ms), so the 2ms read jitter never reaches the audio engine.

Sync loop.SyncController runs a LongRunning 500Hz loop driven by Stopwatch. Each tick dispatches into a GameStateMachine (Playing / Browsing / Results / NotRunning). PlayingState runs hitsound sync at up to 1000Hz, drives the autoplay/manual playback queues, and handles slider-loop pause/resume on IsAudioPaused edges.

Hitsound sequencing. Per-ruleset sequencers (Standard / Taiko / Mania / Catch) implement IHitsoundSequencer. BeatmapHitsoundLoader parses the .osu with Coosu.Beatmap, builds a timeline of SampleEvent (one-shot) and ControlEvent (loop / volume / balance), and maintains a 13-second sliding precache window so sounds are decoded before they're needed. Playback is dispatched through SfxPlaybackService into the effect mixer.

Audio engine.AudioEngine owns a dedicated STA thread (AboveNormal priority) for all device calls. The mixer graph is:

EffectMixer ──┐
├─→ RootMixer (QueueMixingSampleProvider) ─→ Volume ─→ Limiter ─→ output device
MusicMixer ──┘

• Backends: ASIO (vendored NAudio.Asio), WASAPI (exclusive or shared, event-driven), DirectSound.
• Caching: audio is decoded once to 16-bit PCM in native memory (UnmanagedByteMemoryOwner), keyed by a Blake3 hash of the file content, and converted to float on playback via tiered SIMD (SimdAudioConverter: AVX-512 → AVX2 → Vector<short> → scalar).
• Hot path:QueueMixingSampleProvider mixes with TensorPrimitives.Add (SIMD), queues add/remove operations, and recycles finished sources into a shared pool. No allocations on the audio thread beyond pool rentals.
• Rate / pitch:PlaybackTimelineClock applies rate scaling with pitch-preservation position compensation; IPlaybackRateProcessor is the time-stretch extension point.
• Limiters: selectable per config — peak, soft (polynomial), hard, or off.

Plugin system

Plugins compile only against KeyAsio.Plugins.Contracts and must not reference the host, Sync, Configuration, or Avalonia/Suki. The host loads each plugin into a collectible AssemblyLoadContext, shares the contracts assembly with the default context (so types stay identity-equal), and resolves plugin-private dependencies beside the plugin DLL.

Key contract interfaces:

Interface	Purpose
IPlugin / ISyncPlugin	Lifecycle (Startup/Shutdown/Unload) and sync callbacks (OnTick, OnStatusChanged, OnBeatmapChanged).
IPluginContext	LoggerFactory, IAudioEngine, IPluginSettings, IGameplaySession, IPluginInteractionService.
ISyncContext	Read-only game-state view: PlayTime, PlayMods, OsuStatus, Statistics, HitErrors, Beatmap.
IGameStateHandler	Per-status handler with Priority and a HandleResult that can block lower-priority handlers or the base logic.
IAudioEngine	Narrow audio port: open files, add/remove music inputs, acquire seekable sample providers.
IGameplaySession	Resolve beatmap resources, create cached audio providers.

SyncPluginCoordinator adapts SyncSessionContext to ISyncContext and dispatches handler calls by priority; HandleResult.BlockLowerPriority / BlockBaseLogic lets a plugin override or suppress the built-in sync logic.

Configuration

appsettings.yaml (YamlDotNet, camelCase) is self-documenting — [Description] attributes are emitted as YAML comments on save. Legacy configs are auto-migrated, including enum renames (MidSide → ProMixFocus, BinauralMix → LinearStereoPan, CrossMix → ConstantPower, Polynomial → Soft, Master → Peak). Top-level sections: General, Input, Paths, Audio, Logging, Performance, Sync, Update.

Notable knobs: Sync.Scanning.{General,Timing}ScanInterval, Sync.Playback.{NightcoreBeats, LimiterType, BalanceMode}, Sync.Filters.{DisableBeatmapHitsounds, DisableStoryboardSamples, DisableSliderTicksAndSlides}, Performance.EnableAvx512, Update.Channel (Stable/Beta).

🚧 Roadmap (v4 Preview)

A complete rewrite (KeyASIO v4) is in progress. Per-version changelogs live in docs/Updates/.

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Made for the osu! community.