Patent GB-814092-A (GB)
Title: Data sensing system
IPC: G06V30/10
Filed: 1955-12-20
Abstract: 814,092. Automatic gain control; limiters. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 20, 1955 [Dec. 29, 1954], No. 36442/55. Classes 40 (5) and 40 (6). [Also in Group XL (b)] Apparatus for producing data signals having a constant signal contrast amplitude irrespective of different density or colour or reflectivity of the background utilizes stabilizing means for maintaining a substantially constant amplitude difference between signals when the background is sensed and a reference level and means controlling the level of a pass-band filter receiving the data signals in accordance with a sample of data signals received a predetermined time previously. General operation.-A lens system 24 moving to and fro as indicated by the arrows images the characters marked on a travelling record sheet 22 on a slit 26. A shutter disc 42 breaks the modulated beam passing through the slit into regular pulses on its way to a photo multiplier 29 the gain of which is automatically controlled to compensate for differences in the lightreflecting properties of different record sheets. To provide the optimum signal/noise ratio a clipping circuit 36 acting as a band-pass amplitude filter is provided, the level at which clipping occurs being adjusted in accordance with the result of an earlier part of the scan and of the previous scan. This is achieved, with the aid of a shifting register (Fig. 1B, not shown) to which signal impulses due to the presence of a mark on sheet 22 after inversion at 41 are applied, from a gate 40 operating synchronously with the slotted disc 42. Assuming that impulses are received on a given number (five as shown) of consecutive positions at the beginning of a scan, the negative potential of line 57 is reduced by the application of negative potentials to the lower ends of all the normally conducting diodes and the apparatus 72 then lowers the clipping voltages to lower the amplitude pass band and increase the effective sensitiveness of the arrangement. This condition may be retained while signal impulses continue to come in in consecutive positions. By this means the signal level at which. limiter pentode 220, Fig. 3, begins to conduct is lowered in one or two steps when signals received indicate that data marks are or are likely to be encountered in the current and/or coming scans. The register has one more stage than the number of pulses occurring during the scanning of a letter and a signal to reduce the clipping level is applied to line 64 if any of the last three of the register stages is turned on since this indicates the presence of a mark laterally adjacent to the point now being scanned. If both lines receive a signal, a further lowering of the clipping level is produced over line 70. Automatic gain control.-Signals from the photo multiplier 29 after amplification in noisesuppressing amplifiers with clamping to ground are passed on by a cathode follower 146 (Fig. 2b) rectified at 156 and opposed by a pre-set voltage at 161 corresponding to the signal level desired. Any difference in the voltages is amplified at 160 and clamped by diode 182. Positive-going signals pass via a further diode 188 and an amplifying and smoothing stage 198 to the control electrode of the tube 29 to reduce the gain until the rectified signal voltage and the potential divider voltage at 161 are equal. Automatic sensitivity control.-The bias for limiter tube 220, Fig. 3, is provided by a potential divider comprising resistor 254 and the anode/cathode spaces of four pentodes in parallel. Normally these pentodes conduct, but a signal on 57 or on 64 as described in the penultimate paragraph suppresses 246 or 248 and a signal on both wires suppresses all four pentodes. Specification 806,457, [Group XIX], is referred to.