Patent GB-1256911-A (GB)
IPC: B41B19/01
Filed: 1969-01-14
Abstract: 1,256,911. Cathode-ray tube displays. HARRIS-INTERTYPE CORP. 14 Jan., 1969 [4 March, 1968], No. 2254/69. Heading H4T. In an arrangement in which characters produced on the screen of a cathode-ray tube are recorded on photographic film for use in a phototypesetting system the characters are formed on the tube screen by a plurality of light spots the spacings between which are increased or decreased to form characters of larger or smaller point size respectively and to ensure that the characters are recorded on the photographic film with the same optical density irrespective of point size the duration of the production of light by the spots on the tube screen is controlled as a function of the point size so that equal light energy density is produced by characters of different size. The invention is described in connection with the phototypesetting apparatus shown in Fig. 1 and further detailed in Figs. 2, 3 and 4 as described in Specifications 1,255,841 and copending Application 2252/69. In the operation pertinent to the present invention a control record 40 (Fig. 1) specifies a character to be produced together with its point size and other information such as leading information and the character and word spacing necessary to produce a justified line of type. The character to be produced is selected by circuit 15 from a character memory 10 comprising a record medium (Fig. 3) carrying digital signals representative of Character Generating Data (i.e. signals representing the location of a plurality of light spots which form the character as shown in Fig. 5) together with signals representative of the other indicated parameters as described in the Applications referred to above, for a plurality of founts of characters. In practice founts having five different point size viz. 5, 7, 10, 14 and 20 are recorded and characters of intermediate point size are produced by increasing or decreasing the spacing of the light spots which form the recorded character nearest to the desired point size. The characters in a single fount are recorded on the record medium with a predetermined height and width each of which may be expanded independently in response to commands from the control record 40 (Fig. 1) so that in addition to altering the point size justified lines of type may be provided by width expansion in conjunction with interword spacing, e.g. see Fig. 6, which shows the dot structure (a) expanded in the X direction (b) and in both the X and Y directions (c). When the area of the character is altered, for any purpose, it is necessary to modify the duration of dwell of each of the light spots which form the character on the tube screen as a function of the ratio of the area of the altered character to the normalized area of the character of nearest size on the record medium which is selected so that the optical density of the characters recorded on the photographic film is the same for all characters irrespective of character size. When the character area is expanded the modification requires a mathematical division i.e. the duration of dwell must be increased by a factor equal to the quotient of the area of the expanded character divided by the normalized area of the character. To effect division requires more apparatus (electronic computing equipment) than to effect multiplication and therefore, instead of recording the character area or the normalized point size on the record medium in the character memory 10 a number proportional to the reciprocal of the character point size is recorded. This number, indicated as PREDIPS in Fig. 3, Proportional REciprocal DIsc Point Size is, for founts of point size 5, 7, 10, 14 and 20, equal to an arbitrarily chosen number 140 divided by the normalized point size and is indicated by R in the computing arrangement, shown in Fig. 7 as a functional block diagram of the computations which are made by the actual circuitry shown in Fig. 3. The outputs produced by the circuit of Fig. 7-B comprise signals from registers 56 and 57 which are supplied to the Horizontal and Vertical Scale D/A converters 65 and 66 (Fig. 2) to control the spacing of the light spots on the screen of the display tube 30 and signals from the Exposure Factor Register 96 which are supplied to the beam control circuit 59 in which they are decoded and utilized to select the appropriate one of seven pulse generators of different pulse widths which determine the duration for which the tube beam is intensified on each spot location (Fig. 8, not shown). Thus the light energy per unit of area of the characters is the same irrespective of character size and characters of the same optical density are therefore produced on the photographic film 51 when this is exposed to the light images on the tube screen. Other features of the arrangement described which are not pertinent to the invention claimed are the same as those described in the applications referred to above.