GB863042A - Method of indicating the tuning condition of a television receiver - Google Patents

Abstract

863,042. Radio receiving circuits. STANDARD TELEPHONES & CABLES Ltd. June 20, 1958 [June 22, 1957], No. 19829/58. Class 40(5) In a television receiver the picturetube is used for tuning indication, the indication modifying the size of a narrow vertical strip across the centre of the picture. Line frequency oscillations from line A are passed through a filter 2 to select the fundamental frequency, which is fed via a phase-shifting network 3 to a frequency-doubler 4. The output of the doubler comprises a train of positive-going "spikes" (Fig. 5, not shown) of twice line repetition frequency which is applied via a blocking condenser to the grid of an amplifier 6 (also Fig. 6, not shown). To this grid is also applied, from terminal C, a D.C. voltage which is dependent upon the accuracy of tune, the output of the amplifier comprising voltage pulses (Fig. 7, not shown) whose height and width is proportional to the magnitude of the D.C. voltage. These voltage pulses are fed via a condenser to a clipper 7 (also Fig. 8, not shown), the output of which comprises current pulses of uniform amplitude but whose width is proportional to the D.C. voltage. These are combined with the video signal, either by superposition in the video rectifier circuit or by application to an otherwise unused electrode in a video amplifier or the picture tube : a vertical bar of variable width is produced, across the centre of the picture (Fig. 1, not shown). The sensitivity may be increased by width modulating the pulses with a saw-tooth wave of frame repetition frequency, in which case the indication comprises a triangular area of variable height and width (Fig. 2, not shown). The modulating wave may alternatively be triangular or paraboloid in shape, and may give a pair of triangular indicating areas of variable separation (Fig. 3, not shown). The modulating waveforms are applied to the grid of amplifier 6 via line C<SP>1</SP> and blocking condenser K. The indicating pulses which occur during the line retrace periods-the frequency being double line repetition frequency-may be suppressed, either by the normal receiver blanking or by applying a line-frequency blanking pulse to stage 6 or 7. Alternatively the output of the frequencydoubler 4 may be combined with a waveform of the fundamental line frequency (Fig. 10, not shown), obtained over a phase-shifting network 8.