is there an app that writes sheet music as you play?

The main split is between writing notation while you play and transcribing a recording afterwards. The picks below follow from that.

If you want to play straight into notation

ScoreCloud Studio is notation software for Windows and Mac where you write and arrange music by playing into it, through a microphone or a MIDI keyboard. Once your playing or singing is notation, you can edit it and arrange it as a full score. It is free for up to 10 saved songs.

What you give up is a quick, no-install route: you need the desktop software. After 10 saved songs you are into paid plans, and the free plan prints with a watermark.

If you want to hum or whistle an idea on your phone

ScoreCloud Express, the iPhone app, lets you play, sing or whistle a single-line melody and instantly turn it into notation. It costs £2.99 up front on the UK App Store, which also lists a month of PLUS at £4.99 as an in-app purchase.

The limits are real. It handles one melody line only and up to 10 saved songs, it needs a free account and an internet connection because recordings are analysed in the cloud, and its UK App Store listing shows 1.4 out of 5 from 38 ratings, with the latest version dated June 2018.

If you play an instrument other than piano

Klangio Transcription Studio transcribes over 25 instruments, including guitar, bass, vocals, drums, strings and wind. You record on your phone's microphone, or give it an MP3 or a YouTube link, and it returns sheet music and TAB you can export as PDF, MIDI, MusicXML or Guitar Pro. It runs on iOS and the web.

The catch is that it transcribes after you record, not live. Free demo transcriptions cover short recordings, and the website transcribes the first 20 seconds free. Klangio says upgrading to Pro brings full-song transcription, unlimited exports and more formats, so the free demos are limited on exports too. Studio Pro is £17.99 a month or £99.99 a year on the UK App Store.

If you want to try it free in a browser

Songscription is a website that converts an uploaded or recorded clip, or a YouTube link, into sheet music and guitar tabs, with a built-in editor and piano roll for fixing what it gets wrong. Its free plan gives unlimited 30-second transcriptions to view, and there is a 14-day trial for 2-minute transcriptions with no card required.

Sheet music is only half the story, though, since plenty of people just want the chords of a song and nothing more. If that's you, I've put together a page on identifying chords from a song. It covers Chord ai, Chordify and Moises, which all have free versions, plus Yamaha's Chord Tracker, which is free but only reads songs stored on your device.

You give up length, since 30 seconds is not a whole song, and you cannot download anything on the free plan: PDF, MusicXML, MIDI and Guitar Pro export start on Plus. You also give up some control over your audio, covered below. Plus gives 60 minutes of transcription a month, but the Plus and Pro prices did not load in the page text we read, so check them yourself.

If you do not want a subscription

AnthemScore 6 is desktop software for Windows, Mac and Linux that converts audio files such as MP3s into sheet music, PDF or MIDI, with editing tools to correct the result. It is a one-time purchase from €30.63, usable on up to 4 computers with no recurring charges, and it works offline after activation.

The trade is that it works from audio files, not live playing, and there is no free edition. The trial gives unlimited 30-second transcriptions, up to 100 in total, limited to the first 30 seconds of each song. The Lite edition at €30.63 saves PDF only; MIDI and MusicXML export start with Professional at €41.20, and Studio is €104.57. Those prices were shown to us in euros and moved by a few cents between two reads on the same day, so the site may show another amount or currency where you are.

When paying is worth it

For ScoreCloud, pay for Plus at US$5.99 a month once you need more than 10 saved songs or want to print without a watermark. Songwriter, at US$11.99, adds lead sheets from singing and playing at once, and Pro is US$20.99 a month. For AnthemScore, pay for Professional rather than Lite if you need MIDI or MusicXML for other software. If you are only trying the idea out, the free plans and trials cover short pieces.

Automatic transcription makes mistakes. Klangio's piano app, Piano2Notes, says it will not detect 100% of notes and will make some wrong detections, and that recording several instruments at once gives bad results, although Transcription Studio says it separates multiple instruments. Piano2Notes also says it does not show live note recognition: it takes time to analyse, then shows the finished sheet music. Treat any score you get back as a draft to check.

ScoreCloud Express depends on its cloud analysis, so it will not work without a connection, and its App Store listing shows no update since June 2018.

Songscription may use your audio and transcriptions to improve its AI by default. Only Plus and Pro subscribers can opt out, which is worth weighing before you upload music you have not released.