George's Television Interface Adapter
Encyclopedia
Color Television Interface Adaptor (CTIA) and its successor Graphic Television Interface Adaptor (GTIA) are custom chips used in the Atari 8-bit family
of computers and in the Atari 5200
console. In these systems, a CTIA or GTIA chip works together with ANTIC
to produce video display. The chips were designed by George McLeod with technical assistance of Steve Smith.
Color Television Interface Adaptor and Graphic Television Interface Adaptor are names of the chips as stated in the Atari field service manual. Various publications named the chips differently, sometimes using the alternative spelling Adapter or Graphics, or claiming that the "C" in "CTIA" stands for Colleen/Candy and "G" in "GTIA" is for George.
console. The 2600 used a chip known as the Television Interface Adaptor (TIA). In terms of graphics support, the design of the CTIA followed that of the TIA - it had a "playfield" layer for background graphics along with several "players" and "missiles" (today known as sprite
s) for moving foreground objects. However, the sprites were improved in number, from two players and two missiles to four of each. A fifth sprite in the TIA, the "ball", was removed, as some of its capabilities were combined into the missiles. The four missiles could alternately be combined to form a fifth player, by setting a register
.
During development, the home computer
revolution started in earnest in the later half of 1977. In response, Atari decided to release two versions of the new machine, a low-end model as a games console, and a high-end version as a home computer. In either role, a more complex playfield would be needed, especially support for character graphics in the computer role. For this purpose the new ANTIC
chip was introduced to handle the storage and interpretation of a bitmap
framebuffer
, which the TIA did not support. Under the new model, the ANTIC would feed the CTIA with data, which would then be colored and sent into the video circuitry. This had the added advantage of greatly reducing programming complexity, compared to the "racing the beam" system used in the 2600.
As a result of these changes, the number and selection of graphics modes on the new models was greatly improved over the TIA. Instead of a single playfield mode with 20 or 40 bits of resolution, the CTIA/ANTIC had eight modes with various resolutions and color depths, allowing the programmer to select a mode with the minimum memory needs they required for their display. Resolutions varied from 40 to 320 pixels horizontally, 24 to 192 vertically, and 2 to 4 colors per line. A particularly common mode for gaming purposes was a 160 x 192 x 4 color mode later known as "graphics 7.5".
The original design of the CTIA circuit also included support for three additional 16-color graphics modes. This feature was ready before the computers' November 1979 debut, but was delayed so much in the development cycle that Atari had already ordered a batch of about 100,000 CTIA chips with the graphics modes missing. Not wanting to throw away the already-produced chips, the company decided to use them in the initial release of the Atari 400 and 800 models in the US market. The CTIA-equipped computers, lacking the 3 graphics modes, were shipped until October–November 1981. From this point, all new Atari units were equipped with the new chip, now called GTIA, that supported the new graphics modes.
The Atari computers' operating system ROM supported the 16-color graphic modes from the start, which allowed for easy replacement of the CTIA with the GTIA once it was ready. Atari authorized service centers would install a GTIA chip in CTIA-equipped computers free of charge if the computer was under warranty; otherwise the replacement would cost $62.52.
GTIA was also mounted in all later Atari XL and XE computers and Atari 5200 consoles.
into the signal that goes to the television. It also performs the following additional functions:
It also performs a few additional minor tasks:
with the CTIA, and added 3 new graphics modes. All 3 modes were 80x192 pixels, with the difference being in the colors allowed. With the CTIA chip, the Atari was limited to a maximum of 4 colors in graphics, unless special programming techniques were used. The new modes allowed the following:
Of these modes, Graphics 9 is particularly notable. It enabled the Atari to display gray-scale digitized photographs, which despite their low resolution were very impressive at the time. Additionally, by allowing 16 shades of a single hue rather than the 8 available in other graphics modes, it increased the amount of different colors the Atari could display from 128 to 256. Unfortunately this feature was limited for use in this mode only, which due to its low resolution was not widely used.
The GTIA also fixed an error in CTIA that caused graphics to be misaligned by "half a color clock". The side effect of the fix was that programs that relied on color artifacts in high-resolution monochrome modes would reverse their colors.
Atari owners can determine if their machine is equipped with the CTIA or GTIA by executing the BASIC
command
Atari, Inc. intended to combine functions of the ANTIC and GTIA chips in one integrated circuit to reduce production costs of Atari computers and 5200 consoles. Two such prototype circuits were being developed, however none of them entered production.
GTIA chip. The luma values in Graphics 9 and higher are at fault, appearing as stripes. Replacing the chip fixes the problem.
Atari 8-bit family
The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips...
of computers and in the Atari 5200
Atari 5200
The Atari 5200 SuperSystem, commonly known as the Atari 5200, is a video game console that was introduced in 1982 by Atari Inc. as a higher end complementary console for the popular Atari 2600...
console. In these systems, a CTIA or GTIA chip works together with ANTIC
ANTIC
Alphanumeric Television Interface Controller is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s. The chip was patented by Atari, Inc. in 1981...
to produce video display. The chips were designed by George McLeod with technical assistance of Steve Smith.
Color Television Interface Adaptor and Graphic Television Interface Adaptor are names of the chips as stated in the Atari field service manual. Various publications named the chips differently, sometimes using the alternative spelling Adapter or Graphics, or claiming that the "C" in "CTIA" stands for Colleen/Candy and "G" in "GTIA" is for George.
History
The CTIA was designed in 1977 as part of the chipset for use in an improved successor of the Atari 2600Atari 2600
The Atari 2600 is a video game console released in October 1977 by Atari, Inc. It is credited with popularizing the use of microprocessor-based hardware and cartridges containing game code, instead of having non-microprocessor dedicated hardware with all games built in...
console. The 2600 used a chip known as the Television Interface Adaptor (TIA). In terms of graphics support, the design of the CTIA followed that of the TIA - it had a "playfield" layer for background graphics along with several "players" and "missiles" (today known as sprite
Sprite (computer graphics)
In computer graphics, a sprite is a two-dimensional image or animation that is integrated into a larger scene...
s) for moving foreground objects. However, the sprites were improved in number, from two players and two missiles to four of each. A fifth sprite in the TIA, the "ball", was removed, as some of its capabilities were combined into the missiles. The four missiles could alternately be combined to form a fifth player, by setting a register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
.
During development, the home computer
Home computer
Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
revolution started in earnest in the later half of 1977. In response, Atari decided to release two versions of the new machine, a low-end model as a games console, and a high-end version as a home computer. In either role, a more complex playfield would be needed, especially support for character graphics in the computer role. For this purpose the new ANTIC
ANTIC
Alphanumeric Television Interface Controller is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s. The chip was patented by Atari, Inc. in 1981...
chip was introduced to handle the storage and interpretation of a bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
framebuffer
Framebuffer
A framebuffer is a video output device that drives a video display from a memory buffer containing a complete frame of data.The information in the memory buffer typically consists of color values for every pixel on the screen...
, which the TIA did not support. Under the new model, the ANTIC would feed the CTIA with data, which would then be colored and sent into the video circuitry. This had the added advantage of greatly reducing programming complexity, compared to the "racing the beam" system used in the 2600.
As a result of these changes, the number and selection of graphics modes on the new models was greatly improved over the TIA. Instead of a single playfield mode with 20 or 40 bits of resolution, the CTIA/ANTIC had eight modes with various resolutions and color depths, allowing the programmer to select a mode with the minimum memory needs they required for their display. Resolutions varied from 40 to 320 pixels horizontally, 24 to 192 vertically, and 2 to 4 colors per line. A particularly common mode for gaming purposes was a 160 x 192 x 4 color mode later known as "graphics 7.5".
The original design of the CTIA circuit also included support for three additional 16-color graphics modes. This feature was ready before the computers' November 1979 debut, but was delayed so much in the development cycle that Atari had already ordered a batch of about 100,000 CTIA chips with the graphics modes missing. Not wanting to throw away the already-produced chips, the company decided to use them in the initial release of the Atari 400 and 800 models in the US market. The CTIA-equipped computers, lacking the 3 graphics modes, were shipped until October–November 1981. From this point, all new Atari units were equipped with the new chip, now called GTIA, that supported the new graphics modes.
The Atari computers' operating system ROM supported the 16-color graphic modes from the start, which allowed for easy replacement of the CTIA with the GTIA once it was ready. Atari authorized service centers would install a GTIA chip in CTIA-equipped computers free of charge if the computer was under warranty; otherwise the replacement would cost $62.52.
GTIA was also mounted in all later Atari XL and XE computers and Atari 5200 consoles.
Functions
The CTIA/GTIA is a television interface chip. It converts the digital commands from ANTICANTIC
Alphanumeric Television Interface Controller is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s. The chip was patented by Atari, Inc. in 1981...
into the signal that goes to the television. It also performs the following additional functions:
- It is responsible for adding color to the display.
- It draws spritesSprite (computer graphics)In computer graphics, a sprite is a two-dimensional image or animation that is integrated into a larger scene...
(known as Player/Missiles) over the background graphics (known as playfield). - It checks for collisions among the sprites as well as between the sprites and the background.
It also performs a few additional minor tasks:
- It is responsible for reading state of joystickJoystickA joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. Joysticks, also known as 'control columns', are the principal control in the cockpit of many civilian and military aircraft, either as a center stick or...
s' triggers (bottom buttons only in case of Atari 5200 controllers). - It contains four input/output pins that are used in different ways depending on the system:
- In Atari 8-bit computers, three of the pins are used to read state of the console keys (Start/Select/Option). The fourth pin controls the speaker built into the Atari 400/800, used to generate keyboard clicks. On later models there is no internal speaker, but the keyclick is still generated by GTIA and mixed with the regular audio output.
- In the Atari 5200, the pins are used as part of the process that reads state of joysticks' keyboards.
GTIA enhancements
The GTIA chip was backward compatibleBackward compatibility
In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...
with the CTIA, and added 3 new graphics modes. All 3 modes were 80x192 pixels, with the difference being in the colors allowed. With the CTIA chip, the Atari was limited to a maximum of 4 colors in graphics, unless special programming techniques were used. The new modes allowed the following:
- One mode, known as Graphics 9 to BASIC programmers, can display 16 shades of a single hue (there are 16 possible hues).
- The next mode, named Graphics 11, allows for 16 hues with a single shade/luminance value.
- Finally, Graphics 10 allows for 9 colors of any hue/luminance, from a palette of 128 colors.
Of these modes, Graphics 9 is particularly notable. It enabled the Atari to display gray-scale digitized photographs, which despite their low resolution were very impressive at the time. Additionally, by allowing 16 shades of a single hue rather than the 8 available in other graphics modes, it increased the amount of different colors the Atari could display from 128 to 256. Unfortunately this feature was limited for use in this mode only, which due to its low resolution was not widely used.
The GTIA also fixed an error in CTIA that caused graphics to be misaligned by "half a color clock". The side effect of the fix was that programs that relied on color artifacts in high-resolution monochrome modes would reverse their colors.
Atari owners can determine if their machine is equipped with the CTIA or GTIA by executing the BASIC
BASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
command
POKE 623,64. If the screen blackens after execution, the machine is equipped with the new GTIA chip. If it stays blue, the machine has a CTIA chip instead.Versions
by part number- C012295 — NTSC CTIA
- C014805 — NTSC GTIA
- C014889 — PAL GTIA
- C020120 — French SECAM GTIA (FGTIA)
Atari, Inc. intended to combine functions of the ANTIC and GTIA chips in one integrated circuit to reduce production costs of Atari computers and 5200 consoles. Two such prototype circuits were being developed, however none of them entered production.
- C020577 — CGIA
- C021737 — KERI
Bugs
The last Atari XE computers made for the Eastern European market were built in China. Many if not all have a buggy PALPAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
GTIA chip. The luma values in Graphics 9 and higher are at fault, appearing as stripes. Replacing the chip fixes the problem.
External links
- De Re Atari by Chris Crawford
- Mapping the Atari, Revised Edition by Ian Chadwick
- GTIA Chip data sheet
- CGIA Chip data sheet
- jindroush site(archived) GTIA info