A macOS menu-bar app that lets you sing along to any Spotify song in your
range — shift the key up or down by semitones in real time, pitch without
tempo change. It stays out of the way: a small 𝄞 +2 / 𝄞 −3 / 𝄞 0 in the
menu bar, and a popup when you click it. Settings are remembered per song.
Install
You build it on your own Mac with one script. Because it's compiled locally, macOS doesn't quarantine it — no Apple Developer account, no notarization, no "unidentified developer" warning.
git clone https://github.com/evanhu1/transposify.git
cd transposify
./install.shThat builds the app, ad-hoc-signs it, installs it to /Applications, and
launches it. On first launch macOS asks for Microphone access — that's the
permission Core Audio uses to capture Spotify's audio; it never touches your
real microphone. Click Allow, then look for the 𝄞 in your menu bar.
Requirements: macOS 14.4 or later, the Spotify desktop app, and Apple's
command-line tools (xcode-select --install if swift isn't found).
Uninstall:
rm -rf /Applications/Transposify.app && tccutil reset Microphone com.evanhu.transposifyUsing it
Click the menu-bar item to open the popup:
- Now playing — current track + artist.
- Transpose —
−/ value /+, or the slider (±12 semitones). Reset (↺) appears when shifted. - Reduce vocals (karaoke) — experimental center-channel cancellation to duck the original lead vocal; works best on stereo tracks.
- Remember key for this song — on by default; your setting auto-applies when that track plays again. New songs start at the original key.
- Launch at login — register as a login item via
SMAppService.
The menu-bar item (treble clef + signed value) always reflects the current offset, at fixed width so it never shifts. The app auto-adjusts when you switch output devices (e.g. plug in headphones) and engages only while Spotify is playing.
How it works
Spotify's desktop client never exposes its decoded audio — it's DRM-protected, so no injected code (Spicetify etc.) can touch the stream for DSP. The only way to pitch-shift it is to capture Spotify's audio output and process it before your speakers:
Spotify ─► Core Audio process tap (muted-when-tapped) ─► ring buffer
│
popup sets semitones ─► [vocal reduce] ─► Rubber Band R3 ◄── source node
│
└─► default output device
- Capture: a Core Audio process tap (
AudioHardwareCreateProcessTap, macOS 14.4+) grabs only Spotify's audio — no virtual device, no extension. The tap is created muted-when-tapped, so Spotify's untouched output is silenced and you hear only the processed copy. - Transpose: the Rubber Band Library R3 ("finer") engine in real-time mode — a state-of-the-art music pitch shifter, run directly in the audio render callback. Latency is higher than Apple's built-in unit, but that's irrelevant here (you sing along to the output, so there's no monitoring loop). Verified accurate to <0.1%.
- Pristine passthrough at 0: the pipeline engages only while Spotify is
playing and there's something to do (
pitch ≠ 0or karaoke on). At 0 with karaoke off the tap is fully torn down, so Spotify plays untouched — bit-perfect, zero added latency. It also disengages when you pause. - Now playing / per-song memory: read from Spotify's
PlaybackStateChangeddistributed notification (instant, no polling), keyed by track ID.
Diagnostics & tests
swift probe.swift # OSStatus of each Core Audio tap call TRANSPOSIFY_SELFTEST=1 .build/debug/Transposify # headless engagement state-machine test TRANSPOSIFY_RBTEST=1 .build/debug/Transposify # Rubber Band pitch-accuracy check (440Hz +7st)
To build without installing: ./make-app.sh && open Transposify.app.
License
This project is GPLv2-or-later (see LICENSE). It must be — it
statically links the Rubber Band Library
by Particular Programs Ltd, which is GPLv2+. The Rubber Band source is vendored
under Sources/CRubberBand (single-file build, using
Apple's vDSP FFT). If you want to ship a closed-source build, obtain a
commercial Rubber Band licence from Breakfast Quay.