Im giving four stars to this app to underscore that the underlying technology is doing more than you think it is.
There are negative reviews in that the app is not perfectly identifying all chords and cant handle jazz chords. Well, Ill play you single tone for the key ("do"), play the audio for you, and ask you to aurally identify and write down the chords of that jazz piece. This is traditional ear training and it isnt easy not even for the trained musician. And jazz is exponentially more difficult. I cant do it.
If you play along with a current song on iTunes and follow the supplied guitar chords, they will not always match the actual song. Sometimes they may be off a lot. It depends on the recording, the complexity of the music and who knows what else. However, consider this a starting point--a learning device to get your musical ear and guitar playing to a higher level. I dont play guitar. I am using Jamn to see the chords easily. I can easily transcribe my pieces by listening and writing them down but why not give it a headstart.
As for the technology that may be used in Jamn? Not long ago, MIDI transcription could barely identify a single solo melody line and transcribe it into readable music notation. It still cant do that well. Play an AUDIO file that is a COMPLEX ARRANGEMENT and analyze and transcribe the audio into chords? I give credit to this app that it does as well as it does -- and sometimes comes close. At the minimum, it is at least a start to figuring out the chords.
To test this app, I played several songs that I wrote and recorded (home only) 20 and 30 years ago. Singing, drums, horn lines, lots of filler. Jamn Player did a nice job. Not perfect but a good start--ok, they werent complicated jazz pieces. However, I never wrote these pieces down. I improvised into my MIDI software and just layered tracks. I never wrote the chord progressions down and layered tracks by ear as I played. There were mistakes. Sometimes I couldnt remember what I played and what was coming next. The chords often had leading tone activity and were playing rhythms -- and Jamn often correctly identified the basic progression.
For the future, it would be nice to save a Jamn file as a MIDI or text file where I could then copy the chords if I wanted to transcribe or rerecord the pieces. It would be nice if it also did this for iTunes songs--but the copyrights are probably the legal reason for not doing this.
So, play, get your musical ears going, and use Jamn Player for getting those basic progressions down.