Patent GB-1171609-A (GB)
Title: Electron beam recorder
IPC: H04N3/38
Filed: 1966-11-02
Abstract: 1,171,609. Television image recording. MINNESOTA MINING & MFG. CO. 2 Nov., 1966 [3 Nov., 1965], No. 49212/66. Heading H4F. [Also in Division HI] In an arrangement in which television images are recorded on an electron sensitive film 48 by direct bombardment with an electron beam 82, the film is advanced intermittently at the rate of 24 frames per second whilst the electron beam is scanned and modulated at a standard television rate of 30 frames (60 interlaced fields) per second, the arrangement including a frame conversion control circuit 174 which responds to 24 c/s. pulses obtained from the film advance mechanism and horizontal synchronizing pulses separated from the television signal and functions to produce blanking signals to suppress the electron beam from the film for selected periods such that the number of frames which are scanned and recorded on the film in each second corresponds to the number of film frames advanced in each second. In one way of carrying out the invention, which is described in detail, the beam is blanked for half a field after every frame, the film advance occurring during the blank interval. In another way (not described in detail) the beam is blanked for a whole field after every two frames. Reference is also made to the use of the invention with signals where only one field is recorded on each film frame. The frame conversion control circuit 174, Fig. 5 (not shown), incorporates a counter which responds to horizontal synchronizing pulses to count the number of lines in a field (525). The count operation starts in response to a 24 c/s. pulse from the film advance mechanism and at the completion of the count the beam is blanked until the next following 24 c/s. pulse. Normal horizontal and vertical blanking signals are applied via circuit 174 so that the beam is also blanked in the customary manner between lines and fields. The drive for the film advance is synchronized with the television vertical synchronizing pulses by a servo control system 176 which receives the synchronizing pulses and 60 c/s. pulses from film advance mechanism and applies a speed control signal to the drive motor 32. The 24 c/s. and 60 c/s. pulses related to the film advance rate are generated photo-electrically with aid of an apertured disc 38 on the motor shaft (see Fig. 4, not shown). Electron beam blanking is effected by deflector plates 100, 102 which divert the beam to an electrode 98. The video signal is applied to grid 86 in the electron gun via amplifier 136, the signal being clamped to black level by a clamp circuit 130 operated in response to each horizontal synchronizing pulse by signals from a driver stage 132. Following each synchronizing pulse, during the back porch period of the blanking interval, a beam servo control circuit 150 applies a pulse to video amplifier 136 to cause the beam to assume a reference level. Since the beam is blanked this causes a corresponding signal to appear from electrode 98. This signal is then compared with a reference level signal in servo control 150 to produce a corroction signal which is applied, by way of a lamp 154 and photo-resistor 152, to control a bias supply 128 and alter the potential to which clamp 130 restores. This beam servo control system is described more fully in Specification 1,171,608. The electron beam passes through a cylindrical assembly 110 of rods 160 which is supplied from a high-frequency oscillator synchronized by the horizontal synchronizing pulses. The effect is to produce focus modulation of the beam, whereby each scanned line is broken up into a stationary array of dots. A ring electrode which surrounds the electron beam in the immediate vicinity of the film 48 provides fine focus of the beam and also generates a signal in response to secondary electrons which is applied to a picture monitor 168.