The device between the interference signals in a receiver for television signals modulated in the negative direction

摘要

886,739. Non-linear discharge tube circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. July 8, 1958 [July 10, 1957], No. 21854/58. Class 40(6). [Also in Group XL(b)] A limiter circuit for separating out interference signals, particularly for use in a television receiver in which the synchronizing pulses are positive and the picture signals negative going, comprises a negatively biased valve to the control electrode of which the detected signal is applied with the sync. pulses having positive polarity. The signal is also applied to a peak detector circuit having a network of time constant high relative to one period of the sync. signal and a limiting impedance connected in series with the rectifier the output voltage being applied to the control electrode of the limiter so that the peaks of the sync. pulses are at a level corresponding to the cut-off voltage of the limiter. In this way effective removal of the interfering pulses is produced even when the amplitude of the incoming signals varies considerably in either direction. Fig. 3 shows an interference separator circuit in which the detected input signal 22 has interference signals 19, 20, 21, the limiter tube 8 has a negative bias voltage which is approximately equal to the cut-off voltage of the tube. Capacitor 2 is charged via the second control grid 5SP1/SP of tube 6SP1/SP so that the peaks of the sync. pulses are substantially at earth potential, the time constant of the network 2, 3 being large compared with one line period. Only during the occurrence of interference pulses does anode current flow to produce an output signal 19SP1/SP, 20SP1/SP, 21SP1/SP. The limiting resistor 4 ensures that no current flows during interfering pulses of short duration. No grid current flows in tube 8 unless the interfering signals are of such amplitude that they exceed the cut-off voltage of the tube, this effect being restricted by limiting resistor 35. Whatever amplitude variation of the video input signal may occur, for example due to A.V.C. variation, only the interference pulses appear at the output of tube 8. The interference signals separated out in tube 8 are supplied to the first control grid 30 of tube 61 via time constant circuit 28, 29, the interference signals 27 being applied with a polarity to cut-off the tube. The supply voltage for the screen grids 31, 32 is so chosen that the output signal 34 is substantially free of interference, the pulses 19, 20, 21 being removed from the output signal owing to the counteraction of signal 27.