Apparatus including amplifier and peak-detecting circuits receives an electrical signal representative of an aural tone containing a pitch to be determined and produces therefrom a pulse train having a pulse reoccurrence frequency (PRF) substantially equal to the pitch of the tone. The pulse-to-pulse...http://www.google.com/patents/US4273023?utm_source=gb-gplus-sharePatent US4273023 - Aural pitch recognition teaching device
