A differential amplifier (10) used as an IF filter and placed between an IF amplifier (2) and a first sound detector (9) in a television system. The IF amplifier produces IF signals for a sound carrier and a picture carrier at separate frequencies. The differential amplifier has frequency characteristics...http://www.google.com.tw/patents/US4630119?utm_source=gb-gplus-share專利 US4630119 - Sound detecting circuit
