It depends greatly on the musical content you want to work with - extracting the pitch of a monophonic recording (i.e. single instrument or voice) is not the same as extracting the pitch of a single instrument from a polyphonic mixture (e.g. extracting the pitch of the melody from a polyphonic recording).
For monophonic pitch extraction there are various algorithm you could try to implement both in the time domain and frequency domain. A couple of examples include Yin (time domain) and HPS (frequency domain), link to further details on both are provided in wikipedia:
- http://en.wikipedia.org/wiki/Pitch_detection_algorithm
However, neither will work well if you want to extract the melody from polyphonic material. Melody extraction from polyphonic music is still a research problem, and there isn't a simple set of steps you can follow. There are some tools out there provided by the research community that you can try out (for non-commercial use only though), namely:
- MELODIA: http://mtg.upf.edu/technologies/melodia
As a final note, when synthesizing your output I'd recommend synthesizing the continuous pitch curve that you extract (the easiest way to do this is to estimate the pitch every X ms (e.g. 10) and synthesize a sine wave that changes frequency every 10 ms, ensuring continuous phase). This will make your result sound a lot more natural, and you avoid the extra error involved in quantizing a continuous pitch curve into discrete notes (which is another problem in its own).
