Patent GB-819218-A (GB)
Title: Color-image-reproducing apparatus
IPC: H04N3/32
Filed: 1956-12-03
Abstract: 819,218. Cathode-ray tube circuits. HAZELTINE CORPORATION. Dec. 3, 1956 [Dec. 12, 1955], No. 36879/56. Class 40 (7). [Also in Group XL (b)] Colour television signals comprising a luminance component and a colour component are employed to reproduce an image on a cathoderay tube of the type having a screen formed of a repeating sequence of vertical red, green and blue phosphor stripes by applying the luminance component, which is derived via amplifier 14, to modulate the tube grid and the colour component, which is derived as a 3.6 Mc. subcarrier via band-pass filter 15, to control auxiliary line-direction deflection coils 27. The image detail is reproduced normally in response to the grid modulation, whilst the colour is determined by varying the velocity of line scan so that the beam dwells appropriately on the various colour stripes. The cathode-ray tube screen incorporates indexing stripes whereby a signal is obtained indicating the instantaneous beam position and this signal is employed to maintain the scanning of the colours and the signals representing the colours in step, so as to ensure correct colour reproduction, either by controlling the sweep slope of the line-scan generator 19, or, as illustrated, by controlling the effective rate at which the colour representative signals appear. The indexing signal, which occurs at a frequency of 7 Mc., but subject to variation due to scanning and phosphor strip irregularity, may be derived at 7 Mc. or, as illustrated, as the lower side band at 45 Mc. of a 52 Mc. pilot signal applied to the cathode-ray tube from oscillator 34. The tube is shown incorporating a second grid for the pilot signal, although reference is also made to the use of a single-grid tube. The 45 Mc. signal is derived via an amplifier 29 and applied to a modulator 30 where it combines with a 52 Mc. signal from oscillator 34 and a 3.6 Mc. sub-carrier reference signal from a synchronized oscillator 28 to produce outputs at 7 Mc. and 10.6 Mc., both carrying the indexing information. The 10.6 Mc. output is then applied to a modulator 31 where it combines with the 3.6 Mc. colour sub-carrier signal from band-pass filter 15 to produce finally a 7 Mc. signal for application to coils 27. The 7 Mc. signal carries as modulation the colour information but varies in frequency in accordance with the indexing information and thus ensures correct colour reproduction. A buffer stage 33 is included between modulator 31 and coils 27. The 7 Mc. output from modulator 30 is multiplied to 21 Mc. in a multiplier stage 35 and combined with the output of modulator 31 in a further modulator 36 to produce a second harmonic colour signal at 14 Mc. This is also applied to coils 27 and serves to improve the accuracy of colour reproduction particularly for highly saturated colour. The invention is described as applied to a receiver operating with a signal according to the American N.T.S.C. standards, a sub-carrier modifier and Y-to-M converter stage 13 (see Proc. IRE., Jan., '54) being included to convert the signal to the dotsequential form. An inverse modulator 32, of the type, for example, described in Specification 727,161, [Group XL (b)], is also included between band-pass filter 15 and modulator 31 in order to produce a colour representative signal which is independent of luminance.