Virtual choirs exist, but as one-off productions: an organiser collects hundreds of videos and an editor spends weeks aligning them by hand. Karaoke and duet apps let you sing along with one other person. World Choir Weekly makes the global choir a weekly ritual anyone can join in 30 seconds: one short line each, aligned automatically, and a finished choir every Sunday whether 40 people sing or 40,000.
The data it runs on
Songs: only public domain melodies (traditional folk tunes, or composers who died long ago), taken from scores at the Mutopia Project and IMSLP and checked for public domain status in major markets. We write our own arrangement and our own new lyric lines every week, so no modern arrangement or lyric copyright is touched.
Voices: users' own recordings. In the real app the browser's MediaRecorder captures a 5 to 8 second take while a guide tone plays in headphones; an audio file upload works too (this demo reads your file locally and draws its real waveform).
How it is accessed: takes upload to object storage with the singer's country (from their profile, never GPS). A worker runs pitch tracking, alignment and correction, then the Sunday job mixes the week.
Limits and gaps: phone mics, room noise and Bluetooth latency (often 150 to 300 ms) vary a lot; some takes are badly off key; some countries will have one singer or none at first.
Fallback: takes that fail alignment are re-timed but mixed lower, or the singer is asked for one more try with the guide. If a line has too few voices, a soft synthesized pad holds it up so the choir never sounds empty.
1. The mission
Make singing together a weekly habit for the whole planet. You give one line from your kitchen in Manila or Warsaw, and on Sunday you hear thousands of strangers finishing your sentence.
2. Why people can't resist it
Tiny ask, huge payoff. Seven syllables of effort; a choir of the world in return.
Find yourself on the map. Your dot lights up. "First singer from Tbilisi this week" is a powerful little badge.
Weekly ritual. A new song every Monday, a finished choir every Sunday, a streak of stamps in your archive.
3. How it works for real
Song of the week: a frontier LLM (Claude or GPT via OpenRouter) drafts simple original lyric lines that fit the melody's syllables in 6 languages; a human checks them. The arrangement is generated as MIDI.
Record: MediaRecorder in the browser with a guide tone and click; a fast model (Whisper or Gemini Flash audio) checks the right words were sung and nothing abusive.
Analyse: open-source pitch tracking (pYIN or CREPE) reads the sung notes; onset detection finds each syllable.
Align: dynamic time warping snaps syllables to the beat grid; latency is removed.
Correct: a vocoder (WORLD) or Rubber Band shifts pitch toward the target notes, keeping as much of the human voice as the "Natural" setting allows.
Mix: on Sunday, voices are grouped per line, panned by longitude, levelled, and given a room reverb. Harmony parts come from singers who chose "low" or "high" voice.
4. Cost per use
Audio alignment runs on a cheap server: pitch tracking and correction take about 1 to 2 seconds of CPU per 5 second take, so one modest CPU machine (around $40 a month) handles tens of thousands of takes a week. Speech checks with a cheap model cost a fraction of a cent per take. The lyric draft is one LLM call a week.
Free: sing every week, hear the full choir, your dot on the map. Choir Plus ($2 a month): sing harmony parts, download your week's choir with your voice brought forward, a printed year poster of every city you sang with. Schools and community groups get a private choir room.
5. Growth loop
The Sunday share card says "I sang line 4 with 1,285 people in 41 countries". Friends tap it, hear it, and see which countries are still dark. "Light up your country" pushes singers to bring the next one from their city.
6. Risks and guardrails
Privacy: country and city only, chosen by the singer; no GPS, no exact location. First names only on the map.
Consent: a clear licence screen: your take may appear in that week's public choir; delete it any time before Sunday.
Abuse: every take is speech-checked and loudness-checked; reported voices are muted from the mix.
Copyright: only verified public domain melodies, our own arrangements and our own lyrics. Children under 13 join only through a school room.
7. MVP build order
One song page with guide tone, MediaRecorder capture and upload.
Python worker: pYIN plus DTW alignment, simple pitch shift, output a WAV per take.