The problem my Arduino project would try to solve would be applicable mostly to songwriters. I want to create an Arduino that would take in someone’s vocals and give an output of what the actual notes they’re singing are. For example, you could sing a pop song into the Arduino and have it tell you what each note of the melody is. Of course this assumes that you are a good enough singer to stay on-key. This could also apply to songwriters who work by coming up with a melody simply by singing and want to have an accurate representation of their notes in order to come up with chord structures, harmonies, accompaniments, etc. This would require a tuner that chooses a starting note so the singer doesn’t just pick a random frequency that’s not really a note. If someone was too far away from the actual frequency of a note, the Arduino would read it as an error.
This person has created a library to measure frequency: http://interface.khm.de/index.php/lab/experiments/frequency-measurement-library/
And here is a basic set-up of an Arduino audio input: http://www.instructables.com/id/Arduino-Audio-Input/
While you could do some basic pitch analysis on the arduino, you probably would be better off using something like fiddle~ or Tristan Jehan’s Analyzer~ objects for MaxMSP (http://web.media.mit.edu/~tristan/maxmsp.html ) if you wanted accuracy. Which is certainly possible, the projects don’t have to be arduino based.The only reason to use an arduino would be if you have a specific scenario that you wanted to use the thing where a computer would be impractical. So you might want to clarify exactly where/when this would be used (pocket sized pitch training tool for traveling musicians or really durable batter powered gadget that a voice teacher could use to teach children how to sing on key would both be good examples) You could combine some basic sound output stuff too, so the arduino could could make tones at the pitch and then you have to sing/play back the same one, like an audio version of that simon game from the 1980s http://www.youtube.com/user/vat19com?v=XrNt1XM8xik