Patent GB-1251782-A (GB)
IPC: G09G5/24
Filed: 1970-01-07
Abstract: 1,251,782. Cathode-ray tube displays. IN TERNATIONAL BUSINESS MACHINES CORP. 7 Jan., 1970 [13 Jan., 1969], No. 753/70. Heading H4T. Characters are formed on the screen of a cathode-ray display tube from a plurality of marked or unmarked segments the lengths of which are determined by coded signals defining each length without reference to signals defining the lengths of other segment. Thus, considering the character B (Fig. 1A) the vertical scan, portions of which, the marked portions, are shown in heavy black, is produced by blanking and unblanking the tube beam over lengths (expressed in terms of the number of grid squares) specified by the binary codes shown in either Fig. 1B or Fig. 1C which control the duration of the successive blank-unblank condition of the beam as it is deflected from the bottom upwardly. In the case of the code of Fig. 1B the tube beam scan is initially blanked for the length specified by the first segment count code (i.e. 000100 specifying two grid squares, the zero in the least significant position not being significant in the count) and is then unblanked and blanked alternately in response to the second and the remaining segment count codes (read upwardly): The last code read out includes a " 1 " in the least significant position which specifies end-of-scan (EOS) and thus is employed to synchronize the scan with the beam blank-unblank operation. The operation in the case of the code of Fig. 1C differs from that of Fig. 1B in that each segment count code includes a bit specifying the blank-unblank condition of the beam. Fig. 4 shows an embodiment utilizing the code of Fig. 1B in which the first code, loaded on to data bus 210 by a computer, is transferred via AND gates 213, conditioned by the computer over line 211, to a register 214 and thence via AND gates 217 to a counter 215. Initially counter 215 contains all zeros and this state is recognised by zero detect circuit 216 which supplies a signal to condition AND gates 217 to transfer the contents of register 214 to counter 215. When this has been done zero detect circuit 216 no longer has an output so that AND gates 217 are deconditioned and the drop in signal from circuit 216 then operates via delay 218 to signal the computer via line 219 to place the next (second) segment count on data bus 210 from which it is transferred to register 214 via AND gates 213 again conditioned by the computer line 211. A scan/unblank synchroniser 220 operates by supplying pulses over line 222, which occur whilst the beam is moving upwardly (one pulse per grid unit of Fig. 1A) to count down to zero the (segment length) count in counter 215. Zero detect circuit 216 then supplies a pulse which changes over flip-flop 224 to unblanks the (initially blanked) beam via driver 222 and conditions AND gates 217 to transfer the second segment count to counter 215. The operation continues in this manner with flip-flop changing over at the end of each count down of counter 215 (alternately blanking and unblanking the beam) until the last segment-count code is loaded into counter 215. This code contains an end of scan bit (EOS) which forms one input to AND gate 228 the other input to which is derived from the zero detect circuit 216 (when counter 215 has been counted down to zero) so that AND gate 228 produces an output and this output operates over line 230 to arrest the supply of pulses over line 222 until the next (upward) scan. Fig. 2 shows an embodiment utilizing the code of Fig. 1C and comprises two groups of identical segment counters A, B each of which contains nine counters each of six bit capacity. In operation one group of counters is counted out whilst the other group is being loaded with segment counts defining the next scan. The loading is effected by a computer controlled counter 32 which distributes the segment counts to specific counters in the groups A or B via AND gates 34 or 36 and AND gates 41 ... 49 or 51 ... 59, one group of counter being loaded after being counted-out by the occurrence of the end of scan signal EOS. The character producing operation in this embodiment differs from that of the embodiment of Fig. 4 in that the beam blanking is controlled by specific bit. Details of one of the groups of counters are best described with reference to Figs. 3A and 3B (not shown). The characters produced are imaged on photographic film in a photoprinting machine (no details given) and instead of indexing the beam in the horizontal (X) direction after each vertical scan a fixed position vertical scan may be produced and the film indexed horizontally. The operation of the photoprinting machine may be controlled by a specific code word not concerned with the segment scan control and the segment counting means is automatically inhibited when such code word is detected.