Rocktron Unity Tuner - Manual (Page 12)

Calibration:
To match the pitch of any instrument or audio source (like an old piano or tape
player) press the “Calibrate” button while simultaneously playing the note on
the instrument. (Note: you might have to first tune one string on the guitar or
bass by ear to match the actual note if the piano is out of standard tuning or if
the tape player you are playing along with runs slower or faster than normal).
Once you have one note on the guitar tuned to the source - simply play that
note on the guitar - while the note is “ringing” press the Calibrate button. The
tuning pitch of the tuner will automatically be shifted to match the pitch of the
audio source. Release the button and tune the remaining strings as normal with
the tuner. The tuner at this point has automatically adjusted the tuning range to
match that pitch. To reset the tuner to standard 440Hz pitch turn off the tuner
and then back on again with the footswitch (see controls - point 6).