Patent GB-1241374-A (GB)
Title: Keying arrangement
Inventor: MICHELS FRIEDRICH
IPC: H04N5/765
Filed: 1969-04-23
Abstract: 1,241,374. Educational appliances. FERNSEH G.m.b.H. 23 April, 1969 [23 April, 1968], No. 20697/69. Heading G5G. [Also in Division H4] In a television transmitting and receiving apparatus e.g. for a closed circuit teaching system, Fig. 1 (not shown), in which pre-recorded sound from a tape recorder, and video signals from a slide scanner, which is controlled by signals from the recorder, are sent via a cable to head phones and a monitor, the tape recorder also supplies signals which enable an indicating light spot to be reproduced on the monitor at a desired position determined by the lecturer during recording i.e. the lecturer controls a manipulator whereby he moves a light spot shown on his monitor to indicate points of interest and appropriate signals giving the instantaneous co-ordinates of the spot are recorded as digital or analogue signals. The apparatus is designed to overcome the fact that the picture scanning and tape recorder are not synchronized. Using analogue signals, Fig. 2, on a four track tape are recorded: (a) a frequency A, of 15,625 c/s minus a frequency fx indicative of the x co-ordinate, (b) a frequency B of 14,375 c/s. plus a frequency fy indicative of the y co-ordinate, (c) a reference frequency c of 15,000 c/s. and (d) a sound track D. During reproduction the signals are mixed and filtered to give the frequencies fx and fy and rectified at 15, 16 and 151, 16<SP>1</SP> respectively to give voltage outputs proportional in magnitude of fx and fy, which control the length of rectangular pulses produced from multivibrators 17, 17<SP>1</SP> in response to line sync pulses H and picture sync pulses V respectively. The pulses are differentiated to leave only the tracking edge, that derived from stage 17<SP>1</SP> being extended to last for the duration of one line, and the outputs are applied to the control systems of the monitor via a coincidence circuit 18 so that a spot of light is produced. Alternatively, Fig. 3, a digital word A<SP>+</SP> representing the time spacing between the picture sync pulse and a pulse applied to the monitor to produce the light spot may be recorded on a separate track of the magnetic tape or on the sound track itself if outside the sound band width. Thus to define any one of 400,000 points a word of 17 bits is amplitude modulated on to a carrier whose frequency is a harmonic of the bit frequency. At the receiver a demodulator 20 separates the digital signals and supplies the carrier to a divider and shaping circuit 21 which supplies rectangular pulses at the bit frequency to a shift register 22. The output of the register 22 and the digital bits are fed to "AND" gates 23, to 23n used to set a divider chain 24, to 24n. Pulse from an oscillator 25 activated on each vertical sync pulse are counted down by the divider until a pulse is produced at the last stage 24n, which is used to stop the oscillator and is applied to the monitor to produce the light spot. A blocking oscillator 28 may cause the pulse to be repeated over a plurality of picture periods. To ensure that the oscillator 25 is activated only after the code word has been stored, and also not at repeated intervals, the carrier frequency is rectified at 30 and differentiated at 31, 32. Since the carrier only lasts during transmission of a code word, each word produces a positive pulse, short circuited at 33, followed by a negative pulse which is stored on a condenser 35. The condenser 35 and vertical sync pulse 29 are connected to the oscillator 25 via a coincidence circuit 36. In a modification, Fig. 4 (not shown), the admission of the vertical sync pulse is prevented during a code word by feeding the rectified inverted carrier and the vertical pulses to an "AND" gate. The trailing edge of the inverted output of the "AND" gate resets a bi-stable multivibrator which is actuated by the rectified carrier pulse and thus produces an output pulse extending from the start of a code word until the rear flank of a vertical sync pulse. The output of the AND gate and the multivibrator pulse are fed to the oscillator 25 via a further "AND" gate. Two storage chains representing and y co-ordinates may be used together with a coincidence circuit. By using rapid sequences of co-ordinate signals, letters or numbers may be formed at the monitor.