Patent GB-841160-A (GB)
Title: Signal translating apparatus
IPC: G06K7/10
Filed: 1956-11-27
Abstract: 841,160. Valve non-linear circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. Nov. 27, 1956 [Nov. 30, 1955], No. 36233/56. Class 40(6). [Also in Group XIX] A valve circuit for producing a uniformamplitude pulse train from an input signal generated by a light-sensitive device scanning a sheet bearing graphic data such as printed characters which input signal may vary both as regards its amplitude limits and mean level comprises a discriminating device which delivers an output having one or other of two values according as whether the input signal is more or less positive than a control signal which is derived from a signal developed across a capacitor in accordance with the mean level of the input signals by means of a circuit comprising a plurality of gates arranged between the input to the valve circuit and the capacitor and under the control of gating pulses developed from the output. In the arrangement illustrated the input signal at terminal 10 is applied to a discriminating circuit 47 which comprises a thermionic valve, Fig. 4 (not shown), so biassed that it conducts only when the input thereto exceeds a level determined by the output from a D.C. amplifier circuit 39 having as its input the signal developed across capacitor Cl in accordance with the mean signal level. The circuit 39 comprises two cathode followers arranged in cascade, Fig. 2 (not shown), and having an overall gain of less than unity so that the discriminating or clipping level determined by the output of the circuit is always closer to a "centre-value" voltage than is the input to the circuit, as shown in Fig. 6, the "centre-value" voltage being that voltage which applied as an input to the circuit 39 results in an equal valued output. The output from the circuit 47 is applied to a gate generator 21 which delivers firm outputs to gating circuits 11, 12, 13 and 14 respectively. The outputs from the generator are given such polarities that gates 11 and 12 are opened when the output level from circuit 47 indicates that the photo-sensitive device is scanning a character and gates 13 and 14 are opened when the output level shows that the background between characters is being scanned. Furthermore, gates 11, 13 are only able to conduct when the input signal is less positive than the potential developed at the input to circuit 39 and gates 12 and 14 are only able to conduct when the input signal is more positive than this value. The time constants associated with gates 11 and 14 are made short compared with those associated with gates 12 and 13. Thus it will be seen by reference to Fig. 6 that during the scanning of the background, gate 13 conducts to permit capacitor Cl to discharge slowly towards the background level. If, however, there is any interference or noise going more positive than the upper side of capacitor C1, but not exceeding the clipping level, gate 14 conducts instead to charge capacitor Cl rapidly. Thus the potential of Cl tends to establish itself at about the upper limit of noise or interference peaks. When a character is scanned gate 12 conducts and capacitor Cl is charged slowly so that ultimately capacitor Cl takes up a charge dependent both upon the background and character levels in the input signal. Gate 11 serves a function similar to that of gate 14 when negative-going noise or interference occurs during the scanning of a character but will only become operative when both the background and characters are darker than shown in Fig. 6 so that the potential of capacitor C1 and the clipping level are positive with respect to the "centre-value" voltage, Fig. 7 (not shown).