Video Display Controller

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 VDCs also generate a sound signal, but in that case it's not their main function.

VDCs were most often used in the old home-computers of the 80s, 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 to hold the pixel data, ROM to hold character fonts, 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 synchronization 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 co processor that can manipulate the video RAM contents independently from the main CPU.

Types of Video Display Controllers

The few types of controllers are discussed below :

Video shifters, or "Video shift register based systems" are the most simple type of video controllers; they are responsible for the video timing signals, but they normally do not access the Video RAM directly. They get the video data from the main CPU, a byte at a time, and convert it to a serial bit stream (hence the technical name "Video shifter"). This serial data stream is then used, together with the synchronization signals, to output a (color) 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 to 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 colors. Sprites however are normally not supported by these systems.

Video interface controllers are much more complex than CRT controllers, and the external circuitry that is needed with a CRTC is embedded in the video controller chip. Sprites are often supported, as are (RAM based) character generators and video RAM dedicated to color attributes and palette registers (Color lookup tables) for the high-resolution and/or text-modes.

Video co processors 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 co processor, for example to change the video modes or to manipulate the video ram contents. The video co processor also controls the (most often RAM based) character generator, the color attribute RAM, Palette registers and the Spite logic (as long as these exist of course).

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