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 VDCs also generate an audio signal, but in that case it is not their main function.
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 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
Video display controllers vs. video display processors and graphics processing units
The difference between a VDC and the more modern video display processor (VDP) is not that the VDCs could not generate graphics, but they did not have the special hardware accelerators to create 2D and 3D images, while a typical 1990s VDP does have at least some form of hardware graphics acceleration. Also 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 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.
The graphics processing unit (GPU) goes one step further than the VDP and normally also supports 3D functionality. It is the chip that is now used in modern personal computers.
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 generally 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 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 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 cathode ray tube 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 colours. 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 colour attributes and pallette registers (Color lookup tables) for the high-resolution and/or text-modes.
- 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 sprite logic (as long as these exist of course).
List of example VDCs
Examples of video display controllers are:
- The RCA CDP1861 was a very simple chip, built in CMOS technology (which was unusual for the mid '70's) to complement the RCA 1802 microprocessor, it was mainly used in the COSMAC VIP. It could only support a very low resolution monochrome graphic mode.
- The "Television Interface Adapter (TIA) is the custom video chip that is the heart of the Atari 2600 games console, a very primitive chip that relied on the 6502 microprocessor to do most of the work, also was used to generate the audio.
- The Intel 8275 CRT controller was not used in any mainstream system, but was used in some S100 bus systems.
- The Motorola 6845 (MC6845) is a video address generator first introduced by Motorola and used for the Amstrad CPC, and the BBC Micro. It was later used for almost all the early video adapters for the PC, such as the MDA, CGA and EGA adapters. The MDA an CGA use an actual Motorola chip, while the EGA has a custom IBM chipset of five LSI chips; one of those chips includes IBM's reimplementation of the CRTC, which operates like an MC6845 but differs in a few register addresses and functions so it is not 100% compatible. In all later VGA compatible adapters the function of the 6845 is still reproduced inside the video chip, so in a sense all current IBM PC compatible PCs still incorporate the logic of the 6845 CRTC.
Video interface controllers
- The Signetics 2636 and 2637 are video controllers best known for their use in the Interton VC 4000 and Emerson Arcadia 2001 respectively.
- The MC6847 is a video display generator (VDG) first introduced by Motorola and used in the TRS-80 Color Computer, Dragon 32/64, Laser 200 and Acorn Atom among others.
- The MOS Technology 6560 (NTSC) and 6561 (PAL) are known as the video interface controller (VIC) and used in the Commodore VIC-20.
- The MOS Technology 6567/8562/8564 (NTSC versions) and 6569/8565/8566 (PAL) were known as the VIC-II and were used in the Commodore 64.
- The MOS Technology 8563/8568 was used in the Commodore 128 to create the 80 column text mode, together with the normal VIC-II chip for the C64 compatible video modes.
- The MOS Technology 7360 text editing device (TED) was used in the Commodore Plus/4, Commodore 16 and Commodore 116 computers and had an integrated audio capability.
- The Philips semiconductors SCC66470 was a VSC (Video- and Systems Controller) used in conjunction with their 68070-Microcontroller e.g. in CD-i systems.
- The Picture Processing Unit was a video co-processor designed by Ricoh for Nintendo's use in the Famicom and Nintendo Entertainment System. It was connected to 2048 bytes of dedicated video RAM, and had a dedicated address bus that allowed additional RAM or ROM to be accessed from the game cartridge. A scrollable playfield of 256×240 pixels was supported, along with a display list of 64 OBJs (sprites), of which 8 could be displayed per scanline.
- The ANTIC (Alpha-Numeric Television Interface Circuit) was an early video system chip used in the Atari 8-bit family of microcomputers. It could read a "Display list" with its own built in CPU and use this data to generate a complex video signal.
- The TMS9918 is known as the Video Display Processor (VDP) and was first designed for the Texas Instruments TI-99/4, but was later also used in systems like the MSX (MSX-1), ColecoVision, Memotech MTX series, and for the Sega SG-1000 and SC-3000. The Sega Master System uses an enhanced VDP based on the TMS9918, and the Sega 315-5313 VDP used in the Sega Genesis and some arcade machines is a further advancement of the Master System VDP with the original (inferior) TMS9918 modes removed.
- The NEC µPD7220. Used in some high-end graphics boards for the IBM PC in the mid 80s, notably in products from Number 9 Computer Company.
- The Yamaha V9938 is an improved version of the TMS9918, and was mainly used in the MSX2.
- The Yamaha V9958 is the Video Display Processor (VDP) mainly used in the MSX 2+ and MSX turbo R computers.
Alternatives to a VDC chip
Note that many older home-computer 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 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.
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 are often very primitive, but later implementations could result 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 often were less impressive than those using the video interface controllers or video coprocessors that were available at the same time. Later PLA solutions, like 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 an 6845 in combination with an ASIC, that is the reason why all current VGA based video systems still use the hardware registers that were provided by the 6845.
With Moore's law working, integrated circuits became more and more complex. The simple video display controllers were slowly replaced by chips that had built-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 have special hardware to draw filled triangles to support 3D images, gained hardware Z-buffers and many other methods to accelerate the drawing of 3D pictures. Current video generator chips are almost always "graphics processing units" (GPUs). Entry-level PCs today commonly have the video display integrated into the motherboard chipset, which "steals" some system RAM for the display. The performance of such a system is not as good as one with dedicated video hardware.