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Would never say its impossible, in fact its getting more likely all the time.
Seasoned musicians can easily pick up on the key, and from that and their musical knowledge will have a good idea of how to play along with a band even if they don't know the individual piece.
For a device it will depend on how complicated the piece being played is and the instruments.
For solo instruments especially those that aren't playing chords its relativity easy.
Add chords - a bit more difficult but still fundamentally an FFT, a bit of timing and logic.
Where it starts to get complicated is multiple instruments and techniques for example bends etc., but add some AI and possible directional information of the sound sources not impossible.
Would it be useful, probably not, although have to admit wouldn't mind something that could write out music scores as I played it from a guitar or scored the quality and correctness of a chord!
Although already have a device which judges musical quality - its called a partner!
I was a beta tester for some software that did it a long long time ago. It "listened" to a .wav file and turned it into a MIDI file. It didn't make any attempt to separate out the tracks, so it produced a really ugly file, all the notes squished onto one track...but he mananged to get the notes all there.
We won't sit down.
We won't shut up.
We won't go quietly away.
If anyone is working on this, it would probably be the major music notation software companies like Notion, Make Music's "Finale", or Sibelius. Most of them already have this functionality for monophonic input through a microphone, but it's designed more for playing a wind instrument or singing rather than wav or mp3 input. Google "Finale MicNotator HyperScribe" to see some examples of how to do it with Finale.
There is also a company called SeventhString that publishes a program called "Transcribe!" that can help you do your own transcription from an mp3 or wav file by marking the wav display with it's best guess as to what the notes and chords are, but it doesn't do direct audio-to-notation conversion. However, it does have a "piano roll view" that looks promising.
I think this is usually done in two steps: audio>midi and then midi>notation. Some of the audio>midi software works OK for a single-note (monophonic) line, but the recognition of midi note pitches from polyphonic (multi-note, multi-instrument) audio is inevitably a lot less accurate (to put it mildly). A lot depends on the instrumentation in the audio file. Complex instruments with lots of harmonics are harder to work with than e.g. a simple pure flute sound. Maybe Intelliscore, Wavemid or WIDI?
I agree. As an amateur musician (full time software engineer), if the software can get me 80% or better on the notes, I can sort out the instruments from hearing the song if the software can't get it. As for the key signature, if I can't spot a key signature and key change from the accidentals in a C key signature, then I should not be arranging music.
There have been many attempts to create software that can accurately transcribe polyphonic music, going back at least to 1998 (WidiSoft). All of the ones I have tried have produced quite bad results, with a mess of very short notes and lots of "noise" notes that it's falsely detected from overtones or whatnot.
There's a pretty comprehensive list of existing software here.
As it's still a research problem without a general satisfactory solution, you can also find lots of papers on Google Scholar[^]. I tried a couple of programs released with research papers with mixed results (mostly on piano music, but some chip music too). Let me know if you find something that works well!
That word brings back the memory of when I sat opposite two brawny Scotsmen in kilts, sitting directly opposite me across a not large enough meeting hall. Alas, they didn't think to keep their legs crossed or knees together and I thereby know the answer to that age-old question, accompanied by a long-lasting and severe trauma