Abstract:
A video circuit which enables three full color VDU&#39;s (6a, 6b, 6c) to be driven by only one color graphics generator (2). The Circuit has many uses including the generation of head down displays for flight simulators.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to video circuitry and has a particular application to the generation of color displays for instrument panels used on flight simulators. 
     It is usual when generating full color displays for simulator use, to employ one color graphics generator per visual display unit (VDU). However, graphics generators are expensive items of equipment to purchase and maintain and therefore where several displays are required, this practice has the disadvantage of high cost. 
     This invention aims to provide a means whereby only one color graphics generator is required in order to drive a multiplicity of full color VDU&#39;s, thereby significantly reducing the cost of visual display generation. 
     SUMMARY OF THE INVENTION 
     The invention thus comprises a video circuit for connection to a color graphics generator which has red, green and blue video output channels of variable amplitude, the video circuit consisting of a first circuit, whose outputs are connected to a second circuit, in which the first circuit assigns a color value to the incoming signal on each video channel, dependent on the instantaneous amplitude of said signal, and the second circuit converts the assigned color values to red, green and blue video signals for connection to VDU&#39;s. 
     Hence three full color VDU&#39;s can be driven from just one color graphics generator. Each display can be unique by programming the graphics generator to generate three different formats in each of the three primary colors, red, green and blue. Color variations within each format are achieved by varying the intensity of each primary color as desired. 
     The first circuit may, for example, consist of a three-channel analogue to digital converter (ADC) and a color palette. The color palette may be controlled by a microprocessor so that one set of assigned colors may be quickly and conveniently changed to another set, if desired. 
     The second circuit may consist of a three channel video digital to analogue converter (DAC). 
     The graphics generator could be programmed to generate a single format using all three primary colors. In this case all three VDU&#39;s display the same format. However, the colors on the VDU&#39;s could differ depending on how the color palette was programmed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An embodiment of the invention will now be described, by way of example only, with reference to the drawing which is a block diagram of a video circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The diagram shows an 8-bit three channel, high speed ADC 1 whose inputs are the red, green and blue output video channels from a graphics generator 2. The outputs from the ADC 1 are connected to a colour palette 3 which is under the control of a microprocessor 4. A three-channel high speed DAC 5 converts the outputs from the color palette 3 to red, green and blue video signals which in turn are connected to VDU&#39;s 6a, 6b, and 6c. A sync separator 7 and clock generator 8 ensure synchronisation between the rest of the circuitry. 
     In use the graphics generator 2 is programmed to generate three distinct formats in each primary color. The intensities of each color are varied across the formats as desired. 
     The colour palette 3 is programmed by the microprocessor 4 so that the afore-mentioned intensity variations can be translated by the palette into the desired colors for final display on each VDU 6a, 6b, and 6c. 
     Signals from the red, green and blue video output channels from the graphics generator 2 are then digitised by the ADC 1. The ADC 1 produces 8-bit data corresponding to the instantaneous amplitude of the video signal on each channel. The ADC 1 is clocked at the appropriate pixel rate by the clock generator 8. 
     The clock generator 8 is in turn synchronised to the video output channels by the sync separator 7 which receives its sync signal from the green video channel. Alternatively, the sync signal could be supplied from a separate output of the graphics generator 2. 
     Digital data from the ADC 1 is passed on to the palette 3. The palette allows selection of any 256 colors out of the 262,144 colors which are available on current commercially produced palettes. 
     Data in the palette relating colors to digitised amplitude values of the incoming video signals is controlled by the microprocessor 4 which itself is under operator control. 
     Signals relating to discrete colors are fed from the output of the color palette 3 to the inputs of the three channel DAC 5. The DAC 5 reconstructs the signals on its inputs into three video channels each having red, green and blue components and sync. 
     Input sync, blanking data and a clock signal are supplied to the DAC 5 by the clock generator 8. 
     Each channel is fed to a VDU 6a, 6b or 6c. Thus each of the three formats originally generated by the graphics generator are displayed on one of the VDU&#39;s 6a, 6b and 6c. Each VDU presents a full color display.