Patent GB-615660-A (GB)
Title: Improvements in d.c. re-insertion networks
IPC: H04N5/16
Filed: 1946-03-18
Abstract: 615,660. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 18, 1946, No. 8348. Convention date, March 17, 1945. [Class 40 (iii)] A D.C. re-insertion network for use in television apparatus comprises a D.C. restoration circuit for restoring the low-frequency and D.C. components of applied signals, means affording a low-pass filter for the restored signals and a high-pass filter for applied signals and means for combining the outputs of the said high and low-pass filter means. A conventional D.C. restoration circuit consisting of a capacitor 7, diode 9 and resistor 11 has a filter network comprising capacitor 13 and resistor 15 connected across the capacitor 7, Fig. 1, such that when received television signals, such as shown in Fig. 2, are applied to the input terminals 3, 5, output signals which are a substantially true replica of the original signal are obtained at the output terminals 17, 19. The action of the circuit of Fig. 1 is as follows : The conventional operation of the components 7, 9, 11 produces at the point b a waveform such as shown in Fig. 3, in which the signal pulses are brought to a uniform level 29 except during the vertical synchronizing periods v when the signal peaks are depressed below this level due to the greater percentage of time for which the diode conducts. The spurious component introduced by this action which, although repeated at frame frequency is comprised principally of much higher harmonic frequency characteristics, is effectively eliminated by means of the network 13, 15 which, for signals transmitted from b to c, functions as a low-pass filter. The signals of Fig. 2 which have a spurious low-frequency component (i.e. lack of D.C. correction) are transmitted through the network 13, 15, from a to c, the network in this case operating as a high-pass filter, and combined with the signals transmitted from b to c to produce a corrected waveform at the output terminals 17, 19. In a modification, Fig. 4 (not shown), separate high-pass and lowpass filter networks, the frequency characteristics of which are made complementary, are substituted for the common filter network of 'Fig. 1. In a further embodiment, Fig. 5, signals derived from one of the valves 31 of a conventional television receiver are applied through a filter 33 (for attenuating noise components), a capacitor 7 and a low-pass filter 37 to a diode 9 shunted by a high-resistance voltage divider 11<1>, the cathode of the diode supplying the required D.C. correction voltage to the control electrode 41 of a cathode-ray tube 43 through a filter resistor 15 and the received video signals being applied to the control electrode through the high-pass filter network 13, 15. A network consisting of capacitor 13<1> and resistor 15<1> having a highpass and low-pass filter action (similar to 13, 15 of Fig. 1) supplies desired high-frequency components of the received signal from a voltage divider 35 and the low-frequency components of the D.C. insertion signal, derived from the voltage divider 11<1>, to the control grid of a first limiter amplifier 47 which is cathode coupled to a second limiter amplifier 49 the output from which controls conventional deflecting circuits 51. The circuit thus provides D.C. restoring action for both the image and synchronizing signals. Specification 425,177 is referred to.