Patent GB-758357-A (GB)
Title: Improvements in and relating to electrical apparatus for communication systems
IPC: H04N9/00
Filed: 1953-06-19
Abstract: 758,357. Television. PHILCO CORPORATION. June 19,1953 [June 20, 1952], No. 17046/53. Class 40 (3). In a colour-television receiver in which a synchronous relationship is maintained between the colour modulating signals and the scanning of the colour-screen of the picture reproducing cathode-ray tube by means of " indexing " signals (derived from the scanning of " indexing- stripe " members forming part of the screen), undesired phase variations which, necessary amplifiers and filters produce in the " indexing " signals are compensated by combining the " indexing signals with further signals in which similar phase variations are present in opposite sense. As shown in Fig. 1, the " indexing " signals comprising the upper sideband (52.5 Mc/s.) of the modulation components which are produced across, the load impedance 52 associated with the " indexing- stripe " members of the screen 24 when the latter is scanned by an electron beam modulated by means of grid 18 and oscillator 54 (operating at 45.5 Mc/s.) are supplied to an amplifier 56 which raises the level of the normally low intensity signals to a value suitable for subsequent use in synchronizing the colour scanning operations. Amplifier 56 which is required to amplify the 52.5 Mc/s. input signals at constant gain throughout the frequency range occupied by the " indexing " signals without phase distortion whilst excluding the other modulation products must in practice have a band-pass characteristic which is wider than that required by the desired signals, e.g. as shown at 58, and, due to this extended pass-band, undesired noise signals (due to differences in the secondary electron emission of the " indexing-stripes ") are translated by the amplifier. These noise signals are attenuated by a band-pass filter 60 the transmission characteristic 62 of which passes only the desired " indexing " signals but because of this narrow band-width the filter imparts a phase-shift which varies as a function of the frequency. This phase-shift is compensated, according to the invention, by combining, in substractive relationship, in mixer 72, the 52.5 Mc/s. output of filter 60 with a 45.5 Mc/s. signal which exhibits the same phase-shift. This latter signal is produced by combining in mixer 64, the 52.5 Mc/s. output of amplifier 56 with a 45.5 Mc/s. signal (derived from mixer 96 and exhibiting the phase and amplitude variations of the colour sub-carrier wave) and then combining, in mixer 66, the 7.0 Mc/s. output of mixer 64 with the 52.5 Mc/s. output of filter 60. The resulting 45.5 Mc/s. output of mixer 66 which is free from frequency variations due to the " indexing " information (since these variations are present in both inputs to mixer 66 and are cancelled by the substractive action) is then fed to mixer 72 via a (narrow) band-pass filter 68 which removes undesired noise signals. The 7 Mc/s. output signal of mixer 72 (which contains colour information in synchronism with the scanning action), is then combined in adder 88 with the brightness signals from low-pass filter 84 and fed to grid 16 which controls the intensity of the beam of tube 10 employed for picture reproduction. If the transmission system is such that the brightness information at the output of filter 84 and the colour information at the output of filter 86 are in terms of imaginary colour primaries modification of these signals, if necessary, to make them conform to the particular real primary colours (e.g. Red, Green, Blue) characteristic of the colour screen 24 may be accomplished by synchronously detecting the colour carrier at a particular phase and adding the detection products in correct relative amounts to the imaginary colour primary signals. This may be effected in the mixer 98 by supplying thereto the colour wave, which is derived from mixer 96 at the pilot oscillator frequency of 45.5 Mc/s., and a demodulating signal at this frequency derived from the pilot oscillator 54 the detected products so produced being supplied as an additional input to adder 88. The invention is stated to be also applicable to other communication systems employing time multiplex techniques, e.g. telephony. Specifications 715,506, 745,353, 753,451 and 758,346 are referred to.