Explainer · 6 min read
How TONO works
TONO receives the sound of every app through Core Audio process taps – the interface macOS has provided since version 14.2 for capturing other apps’ audio – processes it in memory with a few milliseconds of latency, and plays it on your output. It installs no kernel extension and needs no virtual output device. If TONO quits or crashes, macOS gives every app its original sound back at once. Only the optional virtual microphone uses a small audio driver, which runs in user space.
Signal path
1.5 ms
The limiter’s look-ahead – most of the fixed delay TONO adds to your sound.
TONO 2.3.0macOS 15+Apple siliconUpdated
The signal path
- Apps play as always. Spotify, Safari, Music, games and calls send their sound to your current output. You don’t change anything in them.
- A process tap hands TONO the sound. TONO asks Core Audio for a tap on your default output. macOS delivers the mixed stereo sound of the apps playing there, and mutes their original output only while TONO holds the tap. TONO’s own process, known audio relays and apps that record from the same device – a call on your AirPods’ or headset’s mic, for example – are excluded and play untouched, so nothing loops back.
- TONO processes it on the audio thread. A private aggregate device joins your output and the tap. Each buffer – 256 samples on wired outputs, 512 over Bluetooth and AirPlay – runs through TONO’s chain in memory, with no allocation or locking on the audio thread.
- The result goes to your output. Speakers, headphones, USB, HDMI, DisplayPort or AirPlay. The volume keys still work – they sit after TONO. Switch devices, and TONO follows.
The chain, in order
- Preamp and auto-gain – keeps a boosted curve from simply getting louder.
- Bass enhancer, then the equalizer: up to 24 bands with filters matched to their analog shape (no cramping near the top of the spectrum), cuts from 6 to 96 dB per octave, each band on both channels, left, right, mid or side.
- Air, leveler, stereo width and crossfeed – the quick controls and presets use them.
- 3D and 8D – a binaural model of the head (interaural time and level differences, the pinna notch) and a room reverb.
- Output – gain up to +18 dB, balance, mono; then your hearing profile, per ear.
- Look-ahead limiter – 4× oversampled true-peak detection; while the limiter is on (the default), the output never exceeds the ceiling.
- Listening guard – the sleep timer’s fade and Hearing Guard’s volume cap come last.
The curve on screen is computed from the same filter coefficients the audio runs through, so what you see is what you hear.
Latency
The limiter looks 1.5 ms ahead; with its true-peak filter it delays the sound by about 1.6 ms at 44.1 or 48 kHz. 3D and 8D add about 0.3 ms. Together with the output’s own buffers, TONO sits a few milliseconds behind the music – not noticeable in music, video or calls. HEAR ORIGINAL delays the untouched signal by exactly the same amount, so switching between the two never jumps in time.
When TONO quits or crashes
The taps belong to TONO’s process. When it ends – normally or in a crash – macOS removes them and unmutes every app at once, on the device it was playing to. You never get stuck in silence. Settings, presets and profiles are stored as files and come back at the next launch; a recording in progress is recovered from its working file.
One fader per app
With the App Mixer on, TONO opens a tap per app next to one for “Everything else”, mixes them with their own faders and sends the sum through the chain. Apps sent to another output play through their own path to that device, with their own processing. The equalizer applies there too; 3D and 8D are left out on speakers, HDMI, DisplayPort and AirPlay.
Recording
Recording uses its own private tap, so it can capture system audio, one app (all its processes, on any device) or a call plus your microphone. The audio thread only copies into a lock-free ring; a separate writer thread encodes to M4A, WAV or FLAC and keeps a journal, so a crash, a full disk or sleep always leaves a playable file.
Mic Studio and TONO Mikro
Mic Studio is a separate chain on your microphone: high-pass, gain, noise filter (AI uses Apple’s voice-isolation audio unit on the Mac, Light is TONO’s own spectral filter), gate, tone, compressor, de-esser, effect and a limiter at −1 dBFS. The result feeds TONO Mikro, a virtual input provided by an audio server plug-in that runs in its own user-space process. The session runs only while an app reads TONO Mikro; if TONO stops, TONO Mikro reads silence, never old audio.
Displays and windows
- Brightness: macOS’s own display services for the built-in and Apple displays, DDC/CI for external monitors (each checked once with a validated read), and a software overlay for everything else and for the range below the minimum. The overlay joins every Space, including full-screen apps.
- Warmth: color (gamma) tables where the display supports them, a warm tint elsewhere.
- Windows: the Accessibility interface moves and resizes windows; their positions come from the window server. TONO never reads window titles or content and never records the screen.
Permissions
| Permission | When macOS asks | What it’s for |
|---|---|---|
| Screen & System Audio Recording | The first time you switch the sound on (required) | The process taps – TONO processes live and records only when you press record |
| Microphone | A call recording or Mic Studio | Your voice |
| Head movement | Head tracking in 3D & Space | AirPods head position, used live, never stored |
| Accessibility | The Windows module | Moving and resizing other apps’ windows |
| Location | Optional, for Auto Rules | Exact sunset times; without it, TONO uses your time zone |
Network and privacy
- License: activation and a check about once a week with Lemon Squeezy – the license key and a name for this Mac that is only its model, for example “MacBook Pro (MacBookPro18,1)”; the name you gave your Mac is never sent. Pro keeps working offline for 30 days after the last check, 60 days with a lifetime license.
- Updates: once a day Sparkle looks for new versions on TONO’s website, sending only TONO’s version (you can switch it off under Tools → Updates); every update is signed and verified before it installs.
- Nothing else: no account, no analytics, no ads. Your sound, recordings, hearing profile and rules stay on your Mac.
Why TONO isn’t in the Mac App Store
TONO installs an audio driver for Mic Studio’s virtual microphone and uses system interfaces that App Store apps can’t – for example for display brightness. So TONO is distributed directly from its website.
Built to fail safe
- Invalid samples are zeroed before any filter, and a faulty block is muted and the chain restarts from a clean state within 50 ms.
- Every sample written to your output is clamped; with the limiter on (the default), its ceiling is tested never to be exceeded.
- Muted microphones are released when TONO quits; after a crash, TONO restores them at the next launch.
Questions
What is a Core Audio process tap?
A macOS interface, available since macOS 14.2, that lets an app receive the audio of other processes – for example everything playing on an output – with the user’s permission. It replaced the need for virtual audio drivers such as Soundflower or BlackHole for this job.
Does TONO use a kernel extension?
No. The sound goes through process taps, and the only driver – TONO Mikro, for Mic Studio – is an audio server plug-in running in user space, installed only if you set it up.
Why does macOS call the permission “Screen & System Audio Recording”?
That’s the name macOS uses for any access to other apps’ audio. TONO never reads your screen; it processes the sound live and records only when you press record.
Does TONO work offline?
Yes. Everything runs on the Mac. A Pro license is checked about once a week and keeps working offline for 30 days, or 60 days with a lifetime license.
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