Atari 8-bit TIA

The Atari 8-bit family of computers use a series of custom Television Interface Adapter (TIA) chips called CTIA and GTIA respectively. These chips are the successors to the TIA chip used in the Atari 2600. According to Joe Decuir, George McLeod designed the CTIA (Colleen TIA) in 1977. He later designed the GTIA (George's Television Interface Adapter) as well.

The CTIA is used in early Atari 400/800 home computers. It was replaced by the GTIA in later revisions of the 400 and 800, and in all subsequent members of the Atari 8-bit family. No CTIA-equipped Atari computers were shipped to Europe.Fact|date=February 2008 By 1981, all Atari computers were equipped with the GTIA chip.Fact|date=February 2008 The GTIA chip was also used in the Atari 5200.

Functions

The Atari 8-bit TIA 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.
* 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 reads the console keys (Start/Select/Option) and joystick triggers.
* It also 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.

GTIA enhancements over CTIA

The GTIA chip was backward compatible to 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.

One minor backward incompatibility between GTIA and CTIA is that programs that relied on color artifacts in high-resolution monochrome modes would reverse their colors. This was due to differences in how the CTIA and the GTIA interface with the television.

Atari owners can determine if their machine is equipped with the CTIA or GTIA by executing the following 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"
*C014805 — NTSC. "North America NTSC".
*C014889 — PAL. "European PAL".
*C020120 — FGTIA. "French SECAM video".
*C020577 — CGIA. "Combined ANTIC and GTIA".

Bugs

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. The only solution is to replace the chip.

ee also

* ANTIC
* List of home computers by video hardware

External links

* [http://www.atariarchives.org/dere/ De Re Atari] by Chris Crawford
* [http://www.atariarchives.org/mapping/ Mapping the Atari, Revised Edition] by Ian Chadwick

* [http://krap.pl/mirrorz/atari/homepage.ntlworld.com/kryten_droid/Atari/800XL/atari_hw/gtia.htm GTIA Chip data sheet]
* [http://krap.pl/mirrorz/atari/homepage.ntlworld.com/kryten_droid/Atari/800XL/atari_hw/cgia.htm CGIA Chip data sheet]
* [http://www.atarimax.com/jindroush.atari.org/achgtia.html jindroush site(archived)] GTIA info