Patent GB-1258524-A (GB)
IPC: H04N19/90
Filed: 1969-11-14
Abstract: 1,258,524. Television. FERNSEH G.m.b.H. 14 Nov., 1969 [16 Nov., 1968], No. 55865/69. Heading H4F. The control voltage for a controllable threshold value circuit, 2, Fig. 1, which converts a television signal into binary form, comprises a periodic stepped voltage 4 having, for example, four threshold values each of one frame, field, line or smaller duration, and it is arranged that the television signal corresponding to each position in the television picture is compared with a different step of the periodic voltage in successive television pictures (frames). The resulting binary signal may be transmitted or recorded and the received or recorded signal may be displayed directly on a CRT 10 since as the bright parts of the picture are transmitted and thus displayed more frequently than the dark parts they are correctly interpreted by the human eye due to its integration capability. Noise and amplitude fluctuations resulting from transmission or storage may be suppressed by a further threshold value circuit 8 in the receiver before the video amplifier 9 to which is fed a constant voltage or a voltage corresponding to one half the amplitude of the binary signals. The controlled threshold value circuit 2 which includes a black level control diode is described with reference to Fig. 2 (not shown). A circuit for generating the control voltage is described with reference to Fig. 3 (not shown), in which horizontal and vertical synchronizing pulses are fed via two frequency dividers (flip-flops) and four voltage dividers to an adder. To reduce flickering the amplitude steps (threshold values) of the control voltage may be arranged as shown at 4<SP>1</SP> so that if the television signal exceeds only the two lowest steps it is transmitted twice with the same time spacing within the period T of the control voltage whereas with control voltage 4 the time spacings are unequal. Flickering may be further reduced by arranging that the lower steps occur more frequently than the upper steps in one period of the control voltage. Where such threshold value is maintained for one line period, the resulting line structure is reduced by arranging for each line to be associated with a different threshold value in successive pictures. In a modification of the above arrangement, a triangular alternating voltage is superimposed on the stepped voltage (Fig. 4, not shown), the peak-to-peak value of which is equal to the step height and the frequency of which is greater than the switching frequency of the steps by between one to two orders of magnitude. This alternating voltage, which may instead be added to the television signal, enables all the brightness values of the television signal to be transmitted since a pulse width modulated signal is formed for values of the television signal which lie between the stepped threshold values. In principle the frequency of the alternating voltage may correspond to the duration of one picture point at the television signal, but for this the bandwidth of the transmission channel must be increased to maintain a large signal-to-noise ratio for the binary transmission. Thus preferably the frequency is selected so that it lies below the highest frequency of the television signal, and to avoid disturbing effects during reproduction it is coupled to the line and frame frequency. Graduation distortion may be effected by selecting a different height for the individual steps of the stepped voltage and super-imposing a triangular waveform having a parabolic shape or a sinusoidal or rectified sinusoidal oscillation shape.