Transposify

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.sh

That 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.transposify

Using 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 ≠ 0 or 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 PlaybackStateChanged distributed 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.