Multi-keying circuitry for inserting a plurality of keys into a background television signal where at least the keying signal processing circuitry is time shared by control circuitry to effect the key insertions. The control circuitry includes dual port rams and RAM-DAC's to facilitate the time share...http://www.google.com.tw/patents/US5115314?utm_source=gb-gplus-share專利 US5115314 - Video keying circuitry incorporating time division multiplexing
