| 摘要 |
693,895. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Sept. 27, 1950 [Oct. 19, 1949], No. 26786/49. Class 40 (vi). In an electronic switching circuit, particularly for inserting blanking pulses or other suppression pulses into a television waveform and comprising a diode valve which may be switched selectively to a conducting or to a non-conducting condition, means are provided for supplying a biasing current to the diode when in its conducting condition so that the application thereto of zero-amplitude signals, i.e. signals giving rise to an output equal to that obtained when the diode is non-conducting, does not cause the diode to operate at or near its cut-off point. In the arrangement shown, television picture signals from a source 4 are fed to a load comprising two equal resistances 3, 14 by transmission through diodes 5 and 6 which are periodically rendered non-conducting (e.g. at blanking intervals) under the control of switching pulses from source 13, which pulses render valve 9 conducting and thus the diodes 5, 6 non- conducting by causing the potential of the diode anodes to fall below that of earth (the line 12 being negative with respect to earth). To ensure that the diode 6 operates over a linear portion of its characteristic when the potential of the signals at the cathode of valve 2 falls to zero or lower, a further valve 11 is arranged to conduct when valve 9 is non-conducting (and vice versa), whereby its anode current, dividing between the equal resistances 3, 14, supplies a biasing current for diode 6 which causes it to operate on a linear portion of its characteristic. Specifications 682,159, [Group XL (b)], and 693,893 are referred to. |
