The Main Classes of Video Hardware
There are two main categories of solutions for a home computer to generate a video signal.
- a custom design, either build from discrete logic chips, or with some kind of Programmable logic device.
- a system using some form of Video Display Controller (VDC), a VLSI chip that contained most of the logic circuitry needed to generate the video signal.
Systems in the first category were the most flexible, and could offer a wide ranges of (sometimes unique) capabilities, but generally speaking the second category could offer a much more complex system for a comparable lower price.
Note that for completeness, systems that did not really have "Video" hardware in the conventional sense, but used 7-segment displays as a visual output device have been included.
The VDC based systems can be divided into four sub-categories.
- Simple video shift register based solutions, have a simple "video shifter chip", and the main CPU doing most of the complex stuff. Only one example of such a chip for a home computer exists, the RCA CDP1861 used in the COSMAC VIP. It could only create a very low resolution monochrome graphic screen. The chip in the Sinclair ZX-81 also is a video shifter, but is not a dedicated chip but a programmable chip, a ULA. The CDP1861 however was specially designed for this task only. Dedicated Video shifter chips did have some use in very early game systems, most notable the Television Interface Adapter chip in the Atari 2600. Note that although one of the chips in an Atari ST is also called a "video shift register" it does not fall into this class, mainly because the IC's in this class depend on the main CPU to feed them with picture data. They do nothing more than generate the sync signals and convert parallel data into a serial video data stream. The Atari ST's chip used a DMA system to read out video data independent of the main CPU, and contained a palette RAM, and resolution/color mode switching logic.
- CRTC (Cathode Ray Tube Controller) based solutions. A CRTC is a chip that generates most of the basic timing and control signals. It must be complemented with some "Video RAM" and some other logic for the "arbitration", so that the CPU and the CRTC chip can share access to this RAM. To complete the design, a CRTC chip also needs some other support logic. For example a ROM containing the bitmap font for text modes, and logic to convert the output from the system into a video signal.
- Video interface controllers were a step up on the ladder, these were true VLSI chips that integrated all of the logic that was in a typical CRTC based system, plus a lot more, into a single chip. The VIC-II chip is probably the best known chip of this category.
- Video co-processor chips are at the highest end of the scale; Video interface controllers that can manipulate, and/or interpret and display, the contents of their own dedicated Video RAM without intervention from the main CPU. These chips are highly flexible offering options and features with minimal CPU involvement that on other systems are impossible or at best difficult to produce, requiring extensive CPU overhead. The Atari ANTIC/GTIA and Amiga OCS/ESC/AGA are well known examples of this high-feature category. But note that not all video co-processors are powerful, some are even simpler than many Video interface controllers, notably the primitive SAA5243 which is still technically a co-processor.
Read more about this topic: List Of Home Computers By Video Hardware
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