| 摘要 |
809,396. Television. STANDARD TELEPHONES & CABLES Ltd. April 18, 1957 [May 25, 1956], No. 12717/57. Class 40 (3). The anode voltage and, consequently, the cut-off characteristic, of a valve V2 which operates as a limiter to separate synchronizing signals from combined video and synchronizing signals derived from the final video stage V1 and supplied with positively-going synchronizing signals to the grid of V2, is varied directly as the amplitude of the combined signal input to the latter valve by including the contrast control potentiometer R2 in a potential dividing network R5, R6 from a point P on which the anode voltage for V2 is derived. This arrangement ensures that synchronizing signals free from video components are derived in the anode circuit of V2 for all levels of the input signal thereto. Instead of having the contrast control potentiometer R2 in the video stage V1 it may, alternatively, be employed to control either the H.F. or I.F. amplifier stages. In arrangements employing screen-grid or pentode valves for V2, the combined signals to be separated may be supplied to the second grid and the first grid utilized to effect noise limiting. |
