The apparatus and method is shown for transmitting one or more audio frequency signals on a television video signal including an encoder comprised of a pulse modulator responsive to the audio frequency signals and a combiner to combine modulated pulses from the modulator with the video signal....http://www.google.com.tw/patents/US5675388?utm_source=gb-gplus-share專利 US5675388 - Apparatus and method for transmitting audio signals as part of a television video signal 
