GB519727A - Improvement in television receivers - Google Patents

Abstract

519,727. Multiplex wireless signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 28, 1938, No. 28169. Convention date, Sept. 29,1937. [Classes 40 (iii) and 40 (v)] In a superheterodyne television receiver for the reception of associated picture and sound signals, in which the sound I.F. signals are transferred to a superheterodyne broadcast receiver, a trap circuit is employed to prevent the local oscillator of the broadcast receiver interfering with the vision I.F. signals. The input signals are supplied from the aerial via the tunable circuits 12 which have a wide pass range to the first detector 10, which is also fed by the local oscillator 22. The vision I.F. amplifier 11 is connected to the first detector over a band pass coupling circuit consisting of tuned primary and secondary circuits with a coupling coil 28. The primary circuit consists of adjustable inductance 26 and the valve and distributed capacity 36. Similarly, the secondary circuit which is connected through the blocking condenser 25 consists of the adjustable inductance 27 and the valve input capacity 37. The primary and secondary circuits are so coupled and tuned as to pass the band of picture signals and to reject the sound signals. The broadcast receiver 24 is supplied with sound signals by a conductor 41 leading from the high potential end of coil 28 through a parallel resonant circuit 42, 43 and a condenser 44 which tunes the whole circuit to be series resonant to the sound I.F. The parallel circuit 42, 43 is tuned to the local oscillator frequency of the receiver 24 and voltage from this local oscillator, whose frequency falls within the vision I.F. band, is thus prevented from reaching the amplifier 11. Switches 51, 56, and conductor 53 are provided to enable the broadcast receiver to be disconnected from the band pass coupling and connected to the mid-point of the input coil 16, thus utilising the television antenna and transmission line for normal broadcast reception. The switches 51, 56 are preferably ganged with the television receiver power switch. The first detector valve 10 is of the pentode type and has the screen and suppressor grids connected together to increase the neutral conductance. For the same reason the vision I.F. amplifier has its screen and suppressor electrodes connected, but a resistor 38 is included to prevent oscillation.