Patent GB-1130635-A (GB)
Title: Improvement relating to electronic display systems
Inventor: BARKER KEITH STANSFIELD
IPC: G06F3/153
Filed: 1966-02-08
Abstract: 1,130,635. Cathode-ray tube displays. FERRANTI Ltd. 6 Feb., 1967 [8 Feb., 1966], No. 5529/66. Heading H4T. In an arrangement for displaying characters, e.g. alpha-numeric characters or electronic circuit elements, on the screen of a cathode-ray tube by controlling the beam deflection so as to trace such characters, each of the latter is defined by a block of digital information stored at a respective address in a first storage means from which it is abstracted and supplied to digital-to-analogue conversion means, providing the required beam deflection signals, in accordance with digital information in a second storage means defining the addresses, in the first storage means, at which the information relating to the character to be displayed is stored. Fig. 1 shows the general scheme in which the character information comprising a twenty-four bit word is written into the first storage means 5 from an input device such as a computer 7, a second computer or a suitably adapted typewriter 8 being utilized to write (also as a twenty-four bit word), the address information together with a number (three) of control words into the second storage means 6. When it is desired to display information a control word together with an initial address (in store 6) is entered into a control register 9 by means of a control logic circuit 10 (described in block diagram form in Figs. 2, 3 and 4, not shown). The initial address contains a second control word signifying that a display is to be produced together with information defining the number of characters per line and the number of lines in the display, termed matrix size. Logic circuit 10 then causes the information of the next address in store 6 to be read into register 9 such information containing a third control word signifying that the display is to be e.g. tabular, together with information defining the position of the first character in the display. The position information is then decoded by digital-to-analogue means 11 which supplies the required deflection signals to the deflection coils 2, 3 via scan amplifiers 12, 13. Logic circuit 10 now operates to read the information at the next address in store 6 into register 9 such information specifying the initial addresses in store 5 of the first three characters to be displayed together with information specifying the size of the character-i.e. whether full-size or half-size. Logic circuit 10 then causes the information of the initial address in store 5 to be read into register 9 word-byword. Each of these words defines two lines to be drawn and this information is decoded by means 11 to cause the required character to be drawn. The word defining the last line of a character also contains a termination bit which, when detected, causes the beam to move to the position for the next character and the information at the next address in store 5 to be read in to register 9. When the third character has been drawn the information of the next address in store 6 is read into register 9 and if this contains the addresses in store 5 of three further characters they are drawn as above described. After the address (in store 6) of the last character to be drawn the first control word together with an address is inserted in store 6, these forming the control word and initial address referred to above so that the whole display of the characters defined by the information in store 5 is cyclically repeated. Changes in the display may then be effected by input devices 7 and 8. The character de-coding means 11 (described in block diagram form in Fig. 5, not shown) operates firstly to position the beam at the start position under the control of eight bits of digital information which allow the selection of any one of 256 positions in each of the X and Y co-ordinate directions. For the actual tracing of character lines the deflections for each line of a character are controlled by twelve bits (one half of the twenty-four bit word which specifies two lines) the least significant of which is normally a "zero" but is set to "one" to indicate the last line of a character and the second least significant of which is normally a "zero" but which is set to "one" to blank out the trace when required during character tracing. The remaining bits control the actual beam deflection, the operation being such that, depending on the angle at which a line is to be shown, a deflection of "maximum gradient" is applied to the beam in one co-ordinate direction so that it moves with a maximum velocity component in this direction whilst, simultaneously, a deflection having a selected one of sixteen possible gradients up to the "maximum gradient" is applied in the other co-ordinate direction. These deflections are controlled by the six most significant bits and the remaining four bits are utilized to determine the length of the line traced by controlling the time for which the deflections are operative. In this method of tracing the characters the digital words define lines joining points on the character outline. In an alternative method (described in block diagram form in Fig. 11, not shown), the digital words define points on the character outline between which the beam is deflected in a straight line. In this method of operation each line is traced in a fixed time irrespective of its length so that, it is necessary to control the beam intensity to produce a trace of uniform brightness and this is achieved by utilizing three bits in each twelve bit word feeding an additional digital-to-analogue converter connected to the beam brightness control. In each method of operation the beam deflection waveforms are supplied to the beam deflection means via size control circuits which in response to digital information cause the character to be traced in either full or half-size.