VIDC1

The VIDC1 was a Video Display Controller chip created as an accompanying chip to the ARM CPU used in Acorn Archimedes computer systems.[1][2] Its successor, the VIDC20, was later used in RiscPCs.[2]

Video

The VIDC1 offers colour depths of 1, 2, 4 or eight bits per colour, allowing for 2, 4, 16 and 256 colour displays (the VIDC20 can offer up to approximately 16 million colours). A colour lookup table or palette register set of 16 12-bit words was provided, offering a range of 4096 colours for each of the colours in those displays or modes employing up to 16 colours. The 12 bits were split in three 4-bit RGB values, with a 4-bit high speed D/A converter for each of the three primary colours. However, in 256 colour modes, 4 bits of the colour data were hardware derived and could not be adjusted. The net result was 256 colours, covering a range of the 4096 available colours.[2]

Since the device had no horizontal sync interrupt, it was difficult to display additional colours by changing the palette for each scan line, but not impossible, thanks to the 2 MHz IOC timer 1.[3] Many demos managed to display 4096 colours on screen, or in a sense more through dithering.[4]

The timing generator was fully programmable, and could be clocked with an 8 to 24 MHz clock. Resolutions that could be supported were 1024x1024 in monochrome, 640x512 in 16 colors, or 640x256 in 256 colors.[5]

It had also one hardware 32-pixel wide sprite with unlimited height (by default used for the mouse pointer), where each pixel is coded in two bits: value 0 is for transparency, and the three others are freely chosen from the 4096 colour palette.[6][7]

Acorn also used the VIDC chip in its laser printer interface podule, which featured in its Technical Publishing System solution. The VIDC was used to generate a high-resolution monochrome signal driven by "a gated form of the synchronised laser dot clock", assisted by a proprietary video laser interface chip, VLASER6. In the Technical Publishing System, the podule was "configured specifically to drive a 300 dpi Canon CX/SX print engine directly". Unlike conventional video, each raster line produced by the print engine effectively corresponded to a single video frame having only a single scanline, with vertical synchronisation occurring repeatedly over the course of generating a single page. An A4 page could have a resolution of 2432 dots horizontally, reproduced in 3440 lines vertically, requiring a total of over 8 million pixels.[8]

Sound

The VIDC also supported eight-channel stereo logarithmic 8-bit PWM sound.

References
  1. Acorn VIDC datasheet (PDF). Acorn Computers Limited. 1986.
  2. "Riscos : Specifications of Legacy RISC OS machines". www.riscos.org. Retrieved 2020-09-15.
  3. "Google Groups". groups.google.com. Archived from the original on 2012-07-08. Retrieved 2020-12-21.
  4. "Demos - Arcade BBS Filebase Listing". 2015-06-11. Archived from the original on 2015-06-11. Retrieved 2022-12-22.
  5. Pountain, Dick (August 1987). "Acorn Archimedes" (PDF). Personal Computer World. pp. 98–102, 104. Retrieved 7 March 2021.
  6. "Acorn Archimedes A3000". Retro Treasures. April 10, 2009.
  7. "RISC OS News, Software and Information". Archived from the original on 2012-07-13. Retrieved 2012-03-13.
  8. Acorn Technical Publishing System Technical Reference Manual (PDF). Acorn Computers Limited. July 1988. pp. 137–138, 150. Retrieved 27 May 2023.
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