Patent GB-1458778-A (GB)
Title: Electron beam recording of signals on thermoplastic film
IPC: H04N5/83
Filed: 1975-04-14
Abstract: 1458778 Forming record grooves by electron beams DECCA Ltd 14 April 1975 [23 April 1974] 15270/75 Addition to 1373511 Heading G5R In a method as disclosed in Specification 1,373,511 by which a modulated groove, particularly a spiral on a disc record, is formed in a thin electrically resistive, thermoplastics film (11), Fig. 4 (not shown), on an electrically conducting substrate (12) by means of an electron beam intensity modulated by a signal, e.g. video, to be recorded and which beam deposits electric charge along a track on the relatively moving film (11) such that the deposited charge density is substantially greater in the middle of the track width than at the edges thereof, softening of the film by heating allowing electrostatic forces to deform the film surface into a groove of depth varying in accordance with said signal, whereafter the film is cooled to "freeze" the groove, the beam is controlled so that its effective diameter at the film, in a sense along the track, is substantially less than the shortest wavelength, as recorded, in the signal but the width of said track is significantly wider than said effective diameter. This allows playback by a stylus of such grooves in a video disc rotating at 1500 r.p.m. The required groove form may be achieved by deliberately introducing astigmatism into the focus of the electron beam, e.g. by suitable adjustment of a conventional stigmotor control (52), Fig. 10 (not shown), on an electron column (40), so that the beam spot at the film is elongate transversely to the track. Alternatively a circular beam spot may be wobbled laterally of the track at a sufficiently high frequency to ensure charge deposition at least twice at every point within the width of the track, e.g. as shown by the path 61, 62 ... 65, 66 in Fig. 6 wherein lines L3, L4 denote the track edges and L1, L2 denote the extremities of spot wobble, at which extremities the electron beam is blanked. For disc recording, the wobble frequency may be varied in accordance with the tangential velocity V T  at the focus location. In addition to beam intensity modulation by the signal being recorded, it is also varied in accordance with the waveform of Fig. 7i which occurs at the spot wobble frequency and is the sum of waveforms in Fig. 7f, 7g and 7h, which respectively determine the groove bottom, the groove side walls, and electron beam blanking. In a control circuit, Fig. 8, for a spot-wobbled electron column, a voltage controlled oscillator 103 produces a signal at eight times the wobble frequency and which is varied in accordance with disc tangential velocity by ramp generator 104. Circuit 101 frequency modulates a carrier with a video signal inputed at 100, a multiplier 105 multiplies the square of the video signal with the reciprocal of the output of generator 104 and circuit 106 amplitude modulates the output of modulator 101 with the output of multiplier 105, circuit 107 amplitude modulating the output of generator 103 with the output of modulator 106. A network 109 receives the output of modulator 107 and gives two outputs 90 degrees out of phase with one another, one such output passing via divider 116, 117, 119 and generator 134 to supply wobble plates 45 of the electron column with a triangle waveform at the wobble frequency. The two outputs of network 109 are processed by dividers 116, 117, inverters 114, 115, 118, 140, gating circuit 113, OR gates 120, 121, 122, and bistables 123, 124, 125, so that amplifiers 126, 127, 128 have outputs corresponding to Figs. 7f, 7g, 7h respectively, such outputs being combined with the output of modulator 101, recovered by envelope detector 110 and low pass filter 111, for supply to a beam intensity control grid 44. The outputs of amplifiers 126, 127 are amplitude modulated in accordance with said tangential velocity variation by a signal corresponding to the output of ramp generator 104 and recovered by a frequency discriminator 129 and amplifier 130. Current feed for the cathode of the electron column may be controlled by a generator 132 controlled by a processor 131 giving a D.C. signal proportional to the reciprocal of said tangential velocity.