Apparatus suitable for use with teletext display apparatus which is arranged to operate in a plurality of display modes, including a graphic display mode, in response to digital coded control signals, and to display in each mode digital coded data signals which provide the display information for the viewer. The digital coded control signals are interspersed among the digital coded data signals, with the result that no display information is normally available when a digital coded control signal is present, but the apparatus is so arranged that, in the graphics display mode, the digital coded, data signal which arrived immediately before the digital coded control signal is held over, and used to fill in the blank space that would correspond to the presence of the digital coded control signal. `Hold over` is effected by arranging that a current data signal is held in a data store, and that the stored data is pushed out only by a subsequent data signal.
A data transmission of the videotex type in which coded information can be transmitted from a central information store to a plurality of terminals. The coded information includes groups of bits representing characters and groups of bits representing control codes. In the transmission technique used in the system a character attribute can be represented by a display control code which is transmitted in association with a cursor backspace code. By using this technique characters of different attributes can be displayed without a space being required between the characters on the display.
A logic system is provided in a video system for accommodating the display of both video data characters and graphic characters, and the application of visual attributes to such characters whether occurring singularly or in fields.
In a display signal generating means in a display device for both the character display and the graphic display, address conversion means is provided for converting the addresses of one character in a plurality of lines of one character section of a memory field corresponding to a display panel into a predetermined address or addresses.
The pixel contents of a frame buffer scan line segment are moved from a source location to a destination location by reading fields of the source segment into a source data shift register and fields of the destination segment into a destination data shift register. The contents of the shift registers may then be rotated relative to one another to account for different starting locations within source and destination fields. The contents of the two shift registers are then simultaneously and synchronously shifted into an ALU where a replacement rule may be used to create a modified stream of pixels shifted into the emptying portion of the destination data shift register. At the conclusion of the shifting for that field the destination data shift register contains the proper pixels to be written back to the destination location in the frame buffer. Then the process is repeated for any remaining fields in the segment being moved. In the special case of scrolling a vertical column the fields are read directly into the destination data shift register and immediately rewritten to the destination location.
