CTIA and GTIA

Color Television Interface Adaptor^[1] (CTIA) and its successor Graphic Television Interface Adaptor^[1] (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.^[2][3]

Color Television Interface Adaptor and Graphic Television Interface Adaptor are names of the chips as stated in the Atari field service manual.^[1] Various publications named the chips differently, sometimes using the alternative spelling Adapter^[4][5] or Graphics,^[3] or claiming that the "C" in "CTIA" stands for Colleen/Candy^[4] and "G" in "GTIA" is for George.^[3][4][5][6]

History

The CTIA was designed in 1977 as part of the chipset for use in an improved successor of the Atari 2600 console.^[7] 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 sprites) 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.^[7] 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".^[8]

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.^[4][5] From this point, all new Atari units were equipped with the new chip, now called GTIA, that supported the new graphics modes.^[5][9]

The Atari computers' operating system ROM supported the 16-color graphic modes from the start,^[9] 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.^[5][6]

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 ANTIC 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 sprites (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 joysticks' 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 compatible 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.^[4]

Atari owners can determine if their machine is equipped with the CTIA or GTIA by executing the BASIC 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^[10]
• C014805 — NTSC GTIA^[11]
• C014889 — PAL GTIA^[11]
• C020120 — French SECAM GTIA (FGTIA)^[11]

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^[11]
• C021737 — KERI^[11][12][13]

Bugs

The last Atari XE computers made for the Eastern European market were built in China. Many if not all have a buggy PAL GTIA chip. The luma values in Graphics 9 and higher are at fault, appearing as stripes. Replacing the chip fixes the problem.

See also

• ANTIC
• List of home computers by video hardware

References

1. ^ ^{a b c} "I. Theory of Operation" (PDF). Atari Home Computer Field Service Manual - 400/800. Atari, Inc.. p. 1-10. http://www.digitpress.com/library/techdocs/Atari_400-800_Service_Manual.pdf. Retrieved 2010-09-10. 
2. ^ Neubauer, Doug (2009-06-20). "The Atari Years, by Doug Neubauer. Star Raiders, Solaris and Pokey". DougNeubauer.com. http://dougneubauer.com/atari/#headnumber3. 
3. ^ ^{a b c} Sherer, Robin Alan (6 1988). "GTIA Joystick Painter - Powerful Atari Animation Tool". ANTIC 7 (2): 37. ISSN 0113-1141. http://www.atarimagazines.com/v7n2/GTIAJoystickPainter.html. Retrieved 2011-01-26. 
4. ^ ^{a b c d e} Patchett, Craig; Sherer, Robin (1984). "Special Chips and ROM". The Master Memory Map for the Atari. Reston, Va.: Reston Publishing Company. ISBN 0-8359-4242-2. http://www.atariarchives.org/mmm/special_chips.php. Retrieved 2011-01-26. 
5. ^ ^{a b c d e} Mace, Scott (1982-03-15). "Atari quietly switches to a 16-color graphics chip". InfoWorld (Palo Alto, CA: Popular Computing) 4 (10): 3–4. ISSN 0199-6649. http://books.google.com/books?id=gD4EAAAAMBAJ&pg=PA3. Retrieved 2011-02-01. 
6. ^ ^{a b} Chamberlain, Craig (7 1982). "Atari Video Graphics And The New GTIA". Compute! (26): 124. ISSN 0194-357X. http://www.atarimagazines.com/compute/issue26/Atari_Video_Graphics_And_The_New_GTIA.php. Retrieved 2011-01-24. 
7. ^ ^{a b} Joe Decuir, "3 Generations of Game Machine Architecture", CGEXPO99
8. ^ Carl Zahrt and Orson Scott Card, "Screenbyter", Compute!'s Second Book of Atari Graphics
9. ^ ^{a b} Small, David; Small, Sandy; Blank, George (May 1983). "Design Philosophy and GTIA Demos". The Creative Atari. Creative Computing Press. ISBN 978-0916688349. http://www.atariarchives.org/creativeatari/Design_Philosophy_and_GTIA_Demos.php. Retrieved 2011-01-26. 
10. ^ Switzer, Steve (10 1983). "Atari Clinic". ANTIC 2 (7): 103. ISSN 0113-1141. http://www.atarimagazines.com/v2n7/atariclinic.html. Retrieved 2011-02-01. 
11. ^ ^{a b c d e} Boris, Dan. "Atari Chips". Dan B's Home Page. http://www.atarihq.com/danb/AtariChips.shtml. Retrieved 2011-02-01. 
12. ^ Vendel, Curt. "Atari 800XLCR". AtariMuseum.com. http://www.atarimuseum.com/computers/8bits/xl/800xl/800xlcr.html. Retrieved 2011-02-01. 
13. ^ Vendel, Curt. "KERI Performance Tester". AtariMuseum.com. http://www.atarimuseum.com/computers/8BITS/XL/keri.html. Retrieved 2011-02-01. 

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