Ultra-low-latency in-ear monitoring for macOS.
Return routes your microphone to your headphones in real time so you can hear yourself while you speak, sing, stream, or record — without paying for a closed-source monitor utility.
It is designed for one of the smallest software delays among open-source projects with the same purpose: native Core Audio HAL I/O, 32-frame device buffers when the hardware allows, and a lock-free C ring buffer that keeps the software bridge tight.
Most “mic monitor” tools sit on higher-level audio APIs or keep larger safety buffers. Return goes lower:
| Design choice | What it does |
|---|---|
| Core Audio HAL units | Direct input/output paths, no AVAudioEngine graph overhead |
| Preferred 32-frame buffers | ~0.7 ms per buffer at 48 kHz when the device accepts it |
| Compact software bridge | Target fill ≈ 4× the larger device buffer (often ~128 frames) |
| Lock-free C ring buffer | Capture and render callbacks stay light and real-time safe |
On cooperative hardware, that stack is among the lowest-latency open-source software monitors available for macOS — close enough that the remaining delay is mostly the interface, converters, and headphones themselves.
Actual latency depends on your interface and sample rate. Return requests the smallest buffer size the device will take and restores the previous sizes when monitoring stops.
- Download
Return.dmgfrom the latest release. - Open the DMG and drag Return into Applications.
- Launch Return and allow Microphone access when macOS asks.
The app is a menu-bar accessory (LSUIElement). Click the mic icon to open
the panel.
The release is ad-hoc signed, not notarized. If Gatekeeper blocks the first launch: Control-click the app → Open → confirm.
- Plug in headphones (or choose them as the system output).
- Select your mic as the system input if needed.
- Click the menu-bar icon → turn Monitoring on.
- Adjust Volume so you hear yourself clearly without feedback.
Input and output must share the same sample rate (Return will refuse mismatched rates rather than introduce resampling latency).
- Real-time mic → headphones monitoring
- Menu-bar UI: monitoring toggle + volume
- Native Swift + Core Audio (macOS 14+)
- No account, no network, no telemetry
- Restores original device buffer sizes on stop
Requires macOS 14+ and Xcode command-line tools.
# Build, package Return.app, and write Return.dmg (no install)
./build.sh package
# Same, then install to /Applications
./build.shRun tests:
swift testMicrophone ──► AUHAL input ──► C ring buffer ──► AUHAL output ──► Headphones
(32 frames) (target fill) (32 frames)
Sources live under Sources/ (Swift UI + HAL bridge) and NativeAudio/ (C ring
buffer). See Package.swift for the SwiftPM layout.
MIT — free to use, modify, and distribute.