Semitones, keys and cents
A semitone is the smallest step in Western music, the distance between two neighbouring keys on a piano. Moving a song up two semitones turns a song in C into a song in D; twelve semitones is a full octave. If you know the key of the song, choose it in Song key and the tool shows the new key as you change the pitch.
Cents divide a semitone into a hundred parts. Use them to match a recording that is slightly out of tune with your instrument, for example an old tape that runs a little fast, or a track tuned to A4 = 432 Hz (about -32 cents from 440 Hz).
Same tempo, different pitch
Simply playing audio faster raises the pitch, like a record at the wrong speed. To keep the tempo, the pitch changer uses a phase vocoder: the sound is split into short overlapping slices of about 90 milliseconds, each slice is analysed by frequency, stretched in time and then resampled to its original length. Phase locking keeps the components of each sound together, which avoids most of the hollow, echoing sound simple pitch shifters have.
Small changes of one to three semitones sound natural on most music. Large shifts are audible: drums lose some punch, and voices sound like a different, smaller or larger person, because the character of a voice (its formants) moves together with the pitch. Untick Keep tempo for the classic record effect, where speed and pitch change together and the sound stays perfectly clean.
Formats and quality
MP3 plays everywhere, M4A (AAC) is the native format of Apple devices and WAV is uncompressed for further editing. MP3 and M4A are saved at 192 kbit/s, which sounds the same as the original for most listeners. Files up to 20 minutes can be processed; a four minute song usually takes a few seconds.