Video display controller

A video display controller or VDC (also regularly called display engine, display interface) 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 an audio signal, but that is not their main function.
VDCs were used in the home computers of the 1980s and also in some early video picture systems.
The VDC is the main component of the video signal generator logic, responsible for generating the timing of video signals such as the horizontal and vertical synchronization signals and the blanking interval signal. Sometimes other supporting chips were necessary to build a complete system, such as RAM to hold pixel data, ROM to hold character fonts, or some discrete logic such as shift registers.
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.
Video display controller vs. graphics processing unit.
The difference between a display controller, a graphics accelerator, and a video compression/decompression IC is huge, but, since all of this logic is usually found on the chip of a graphics processing unit and is usually not available separately to the end-customer, there is often much confusion about these very different functional blocks.
GPUs with hardware acceleration started appearing during the 1990s. VDCs 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 Lisa (AGA) chip that was used for the improved graphics of the later generation Amiga computers.
That 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 synchronization signals and the access to the video RAM.
The graphics processing unit (GPU) goes one step further than the VDP and normally also supports 3D functionality. This is the kind of chip that is used in modern personal computers.
Types.
Video display controllers can be divided in several different types, listed here from simplest to most complex;
List of example VDCs.
Examples of video display controllers are:
Video shifters
CRT Controllers
Video interface controllers
Video coprocessors
Alternatives to a VDC chip.
Note that many early home computers did not use a VDP chip, but built 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, video display generators could be very capable (or extremely primitive, depending of the quality of the design), but also needed a lot of components.
Many early systems used some form of an early programmable logic array to create a video system; examples include the ZX Spectrum and ZX-81 systems and Elektronika BK-0010, but there were many others. Early implementations were often very primitive, but later implementations sometimes resulted in fairly advanced video systems, like the one in the SAM Coupé.
These systems could thus build a very capable system with relatively few components, but the low transistor count of early programmable logic meant that the capabilities of early PLA-based systems were often less impressive than those using the video interface controllers or video coprocessors that were available at the same time. Later PLA solutions, such as those using CPLDs or FPGAs, could result in much more advanced video systems, surpassing those built using off-the-shelf components.
An often-used hybrid solution was to use a video interface controller (often the Motorola 6845) as a basis and expand its capabilities with programmable logic or an ASIC. An example of such a hybrid solution is the original VGA card, that used a 6845 in combination with an ASIC. That is why all current VGA based video systems still use the hardware registers that were provided by the 6845.
Modern solutions.
With the advancements made in semiconductor device fabrication, more and more functionality is implemented as integrated circuits, often licensable as semiconductor intellectual property core (SIP core). Display controller SIP blocks can be found on the die of GPUs, APUs and SoCs.
They support a variety of interfaces: VGA, DVI, HDMI, DisplayPort, VHDCI, DMS-59 and more. The PHY includes LVDS, TMDS and Flat Panel Display Link, OpenLDI and CML.
For example, a VGA-signal, which is created by the display controller, is being transported over a VGA-cable to the display. Both ends of the cable end in a VGA connector. Laptops and other mobile computers use different interfaces between the display controller and the display. A display controller usually supports multiple computer display standards.
KMS driver is an example of a device driver for display controllers and AMD Eyefinity is a special brand of display controller with multi-monitor support.
RandR (resize and rotate) is a method to configure screen resolution and refresh rate on each individual outputs separately and at the same time configure the settings of the windowing system accordingly.
An example for this dichotomy is offered by ARM Holdings: they offer SIP core for 3D rendering acceleration and for display controller independently. The former has marketing names such as Mali-200 or Mali-T880 while the latter is available as Mali-DP500, Mali-DP550 and Mali-DP650.
History.
In 1982, NEC released the NEC μPD7220, one of the most widely used video display controllers in 1980s personal computers. It was used in the NEC PC-9801, APC III, IBM PC compatibles, DEC Rainbow, Tulip System-1, and Epson QX-10. Intel licensed the design and called it the 82720 graphics display controller.
Previously, graphic cards were also called graphic adapters, and the chips used on these ISA/EISA cards consisted solely of a display controller, as this was the only functionality required to connect a computer to a display. Later cards included ICs to perform calculations related to 2D rendering in parallel with the CPU; these cards were referred to as graphics accelerator cards. Similarly, ICs for 3D rendering eventually followed. Such cards were available with VLB, PCI, and AGP interfaces; modern cards typically use the PCI Express bus, as they require much greater bandwidth then the ISA bus can deliver.