GB779019A - Improvements in or relating to carrier-wave transmission systems for television signals - Google Patents

Abstract

779,019. Multiplex radio signalling. NAAMLOOZE VENNOOTSCHAP PHILIPS GLOEILAMPENFABRIEKEN. Nov. 2. 1954 [Nov. 5, 1953], No. 31666/54. Class 40(5) In a television system two video signals # 1 , # 2 corrected to avoid cross - modulation as in Specification 776,989 modulate respectively the positive and negative half cycles of a subcarrier in turn modulating a main carrier, so that to zero values of the subcarrier there corresponds a predetermined amplitude γ of the main carrier greater than the maximum amplitude A\ of the negative half cycles of the subcarrier. To this end # 1 , # 2 after correction may be separately multiplied by (1 + 2 cos w 0 t) and (1 - 2 cos w 0 t) where w 0 is the angular frequency of the subcarrier, then the outputs filtered to lie within a frequency band with upper angular frequency w 0 , i.e. the filtered products are then respectively # 1 + # 1 (transposed to the subcarrier frequency) and # 2 - # 2 (transposed to the subcarrier frequency). The difference of the two filtered products is then taken and is therefore such that during alternate half cycles of the subcarrier frequency the maximum amplitudes of the subcarrier are # 1 and # 2 respectively so that detecting alternate peaks of the subcarrier give # 1 and # 2 . This composite subcarrier signal (difference of the above filtered products) then modulates the main carrier to a depth subject to the aforesaid restriction (to avoid distortion due to over-modulation). Pulses of amplitude Œγ in synchronism with the. line or frame sync pulses A 9 of the two video signals, or one of them may be inserted in the composite subcarrier during instants of zero amplitude by diode or other circuits to fix the modulation level of the subcarrier but are preferably removed during modulation of the main carrier. The lower level modulation # 2 , (Fig. 2a) of the main carrier may be displayed on a cathode ray tube and the modulation depth thereby corrected manually or automatically by a, photo-electric cell coacting with an apertured mask on the screen. At the receiver the upper level # 1 and lower # 2 (Fig. 2a) modulation are separated by threshold diode circuits reversely connected in parallel.