Video display controller

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A Video Display Controller or VDC is an integrated circuit which is the main component in a video signal generator, a device responsible for the production of a TV video signal in a computing or game system. Some VDC's also generate a sound signal, but in that case it's not their main function.

VDC's were most often used in the old home-computers of the 80's, but also in some early video game systems.

The VDC is always the main component of the video signal generator logic, but sometimes there are also other supporting chips used, such as RAM's to hold the pixel data, ROM's to hold character font's, or perhaps some discrete logic such as shift registers were necessary to build a complete system. In any case, it's the VDC's responsibility to generate the timing of the necessary video signals, such as the horizontal and vertical synchronisation signals, and the blanking interval signal.

Most often the VDC chip is completely integrated in the logic of the main computer system, (its video RAM appears in the memory map of the main CPU), but sometimes it functions as a coprocessor that can manipulate the video RAM contents independently from the main CPU.

Video Display Controllers vs. Video Display Processors

The difference between a VDC and the more modern Video Display Processor (VDP) is not that the VDC's could not generate graphics, but they did not have the special hardware accelerators to create 2D and 3D images, while a typical 1990's VDP does have at least some form of hardware graphics acceleration. Also VDC's often had special hardware for the creation of "sprites", a function that in more modern VDP chips is done with the "Bit Blitter" using the "Bit blit" function.

One example of a typical Video Display Processor is the "VDP2 32-bit background and scroll plane video display processor" of the Sega Saturn. Another example is the Advanced Graphics Architecture (AGA) chip that was used for the improved graphics of the later generation Amiga computers.

This said, it is not completely clear when a "Video chip" is a "Video Display Controller" and when it is a "Video Display Processor". For example, the TMS9918 is sometimes called a "Video Display Controller" and sometimes a "Video Display Processor". In general however a "Video Display Processor" has some power to "Process" the contents of the Video RAM (filling an area of RAM for example), while a "Video Display Controller" only controls the timing of the Video synchronisation signals and the access to the Video RAM.

Types of Video Display Controllers

Video Display controllers can be (arbitrarily) divided in several different types (here listed from simple to complex);

  • Video shifters, or "Video shift register based systems" (there is no general agreed upon name for these type of devices) are the most simple type of video controllers, they are, (directly or indirectly) responsible for the video timing signals, but they normally do not acces the Video RAM directly. They get the video data from the main CPU, a byte at a time, and convert it to a serial bitstream (hence the technical name "Video shifter"). This serial data stream is then used, together with the synchronisation signals, to output a (colour) video signal. The main CPU needs to do the bulk of the work. Normally these chips only support a very low resolution Raster graphics mode.
  • A CRTC, or CRT Controller, generates the video timings and reads video data from a RAM attached to the CRTC, to output it via an external character generator ROM, (for text modes) or directly, (for high resolution graphics modes) to the video output shift register. Because the actual capabilities of the video generator depend for a large degree on the external logic, video generator based on a CRTC chip can have a wide range of capabilities. From very simple (text mode only) systems to very high resolution systems supporting a wide range of colours. Sprites however are normally not supported by these systems.
  • Video coprocessors have their own internal CPU dedicated to reading (and writing) their own video RAM, and converting the contents of this video RAM to a video signal. The main CPU can give commands to the coprocessor, for example to change the video modes or to manipulate the video ram contents. The video coprocessor also controls the (most often RAM based) character generator, the colour attribute RAM, Palette registers and the Spite logic (as long as these exist of course).

List of example VDC's

Examples of Video Display Controllers are:

Video shifters

CRT Controllers

  • The Intel 8275 CRT controller. Was not used in any mainstream system, but was used in some S100 bus systems.
  • The Motorola 6845 is a video address generator first introduced by Motorola and used for the Amstrad CPC, the BBC Micro. It was later used for all the video adapters for the PC such as the MDA, CGA and EGA adapters. In all later VGA compatible adapters the function of the 6845 is reproduced inside the Video Chip.

Video Interface Controllers

Video Coprocessors

Alternatives to using a VDC chip

Note that many older home-computer did not use a VDP-chip, but build the whole video display controller from a lot of discrete logic chips, (examples are the Apple II, PET, and TRS-80). Because these methods are very flexible the video display generators could be very capable, (or extremely primitive, depending of the quality of the design) but also needed a lot of components.

Others used some form of early programmable logic chips, (examples are the Sinclair Spectrum and ZX-81 systems). These systems could build a very capable systems with relatively few components, but the low transistor count of early programmable logic meant that the capabilities of these systems often were less impressive than those using video interface controllers or video coprocessors.

Later solutions

With Moore's law working, integrated circuits became more and more complex. The simple Video Display Controllers were slowly replaced by chips that had build in video processing logic such as Blitters and other logic to manipulate the video RAM contents to do things like drawing lines, filling areas, or drawing fonts. Later chips also got special hardware to draw triangles to support 3D images, gained hardware Z-buffers and many other methods to accelerate the drawing of 3D pictures. Current Video generator chips almost always are "Graphics processing units" (GPU's)

See also

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