Patent GB-849475-A (GB)
Title: Improvements relating to correction of transmission variations in television signal system converters
Inventor: JAMES IVANHOE JOHN PENFOUND
IPC: H04N7/01
Filed: 1955-12-14
Abstract: 849,475. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 14, 1956 [Dec. 14, 1955], No. 35851/55. Drawings to Specification. Class 40(3). Transmission variations due to non-uniformity in the sensitivity of a photo-sensitive storage surface of a pick-up tube and fluorescent surface of a cathode-ray display tube employed in a television standards converter system are corrected by an arrangement in which the image to be transmitted is impressed on the storage surface as modulation of a pattern of electric variations of high frequency relative the intensity variations of the image. The signals derived from the storage surface comprise a modulated carrier wave due to the pattern, which is detected to derive a video signal corresponding to the image, and a low frequency signal which corresponds to the transmission variations. This latter signal is then employed to control the gain of a channel transmitting the image video signal so as to reduce the effect of the transmission variations. In a first embodiment, Fig. 1 (not shown), the invention is described as applied to correcting variations appearing in a channel of a colour television systems converter for converting field sequential signals of 150 fields per second to simultaneous signals at 50 fields per second. The sequential signals are displayed on a cathoderay tube and this is viewed by a storage-type pick-up tube operating at the 50 fields per second rate. In accordance with the invention, the original signals, which are restricted in bandwidth to 4À5 mc/s., before application to the cathode-ray tube are first fed to a modulation system from which they appear as modulation of a 6 mc/s. carrier. The signal produced by the pick-up tube then comprises a modulated carrier signal of 2 Œ 1À5 mc/s. and a low frequency signal of 0-0À5 mc/s. representing the transmission variations. The signals are separated by filters, the carrier demodulated to derive the image signal, and the low frequency signal applied to control the gain of an amplifier through which the image signal is passed. Due to non- linear distortion the low frequency signal may contain components due to the image signal but these may be cancelled by an antiphase image signal. It is necessary that the input side of the system should be linear and to this end the cathode-ray display tube is associated with gamma correcting means, either in the form of a preceding non-linear circuit or a negative feed back arrangement to linearise the tube characteristic. In a second embodiment, Fig. 2 (not shown), the need to linearise the tube is avoided by applying the image signal to modulate the tube directly whilst the beam is pulsed on and off at 6 mc/s. With this arrangement the low frequency signal derived from the pick-up signal includes a component due to the mean value of the 6 mc/s. pulses and this is first eliminated by an antiphase image signal. The invention may be applied to colour system converters other than for sequential to simultaneous conversion and to monochrome systems converters.