Difference between revisions of "Atari 7800 Color Documentation"
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A second set of artifact colors can be produced by alternating pairs of light pixels with pairs of dark pixels. This second set of artifact colors will be less intense, and striped in luminance. | A second set of artifact colors can be produced by alternating pairs of light pixels with pairs of dark pixels. This second set of artifact colors will be less intense, and striped in luminance. | ||
| − | The specific artifact colors produced can vary from console to console, depending on hardware modifications, and likely varying between MARIA revisions. Its also worth mentioning that if a console has been video-modified to produce s-video output, it won't be able to display artifact colors, since the pixel luminance signal will no longer be mixed with the chroma signal. | + | The specific artifact colors produced can vary from console to console, depending on hardware modifications, and likely varying between MARIA revisions. Its also worth mentioning that if a console has been video-modified to produce s-video output, it won't be able to display artifact colors, since the pixel luminance signal will no longer be mixed with the chroma signal. Similarly, if you turn off the colorburst signal using the CTRL register, artifacting will be eliminated, along with the rest of the colors. |
If you're designing graphics and fonts for 320 mode display, and you wish to minimize artifacting, you need to ensure that any bright or dark pixels are horizontally followed by 1 or more pixels of the same luminance. | If you're designing graphics and fonts for 320 mode display, and you wish to minimize artifacting, you need to ensure that any bright or dark pixels are horizontally followed by 1 or more pixels of the same luminance. | ||
Revision as of 14:24, 20 November 2015
Contents
NTSC and PAL Palettes
The 7800 uses very similar palettes for NTSC and PAL consoles, with some minor differences with the color quality at any one register. With some careful selection of color indexes, most 7800 games should be able to use a common palette between the 2 platforms.
Technical Overview of Color Generation
The the color system in the 7800 is designed to efficiently generate color signals that are PAL or NTSC ready, with very little silicon and support circuitry. To better understand 7800 colors, it helps to have an understanding of NTSC and PAL color encoding.
Both NTSC and PAL TV standards have a short colorburst signal near the beginning of each scanline. The colorburst serves 2 purposes. Its presence signals to the TV that the scanline will contain color information, but it also serves as a phase reference. If the same frequency is later detected by the TV during the display of a pixel, the TV will determine the frequency's phase relative to the original colorburst, and use the result as an index to a hue in the YIQ colorspace(NTSC) or YUV colorspace(PAL).
To drive this system, 7800 color registers are split into 2 nibbles. The top nibble determine the amount of phase-shift (or hue value), and the bottom nibble determines the voltage level (or luminance) of the pixel.
The 7800 color system uses a uniform level of saturation for all of its color values, except for color values from $00 to $0F, which are displayed without any saturation. This allows for the display of black, shades of grey, and white.
Consistency Issues
There are a number of factors that result in inconsistency between the palettes of 7800 consoles:
- Each 7800 console has a tunable inductor inside, which is used to modify the colorburst frequency. On any given console this inductor may be adjusted a number of different ways, with at least 2 historic standards in play - one with hues ~25.7 degrees apart, and another with hues ~26.7 degrees apart.
- As a 7800 console warms up, so do the components that produce the colorburst frequency, and the distance between the various hues in the palette stretch out. When cold, the console's hue values have ~24.7 degrees of separation, and as the console warms up, up to ~27.7 degrees of separation can be seen between the hues. This effect is more pronounced with higher hue values; the $Ex hue can shift from being green to yellow, and $Fx will shift from being yellow to
- The NTSC and PAL standards don't consistently display the same colors from TV to TV, and almost all TVs allow the viewer to tune colors to their liking.
The largest factor of these is typically the warm-up issue. Since the latter hues shift more than the earlier ones, a 7800 game designer should generally avoid the last 2 $Ex and $Fx hues for critical items that need faithful color. e.g. using $Ex for green grass, or $Fx for for gold coins.
Color Artifacting
320 pixel modes on the 7800 are prone to artifacting colors. The 7800 pixel clock operates at twice the colorburst frequency, so from the TV's perspective, any alternating pattern of light and dark 320 mode pixels is indistinguishable from color information. If the light pixels are on even positions, one color is produced, and if dark pixels are on even positions, another color is produced.
A second set of artifact colors can be produced by alternating pairs of light pixels with pairs of dark pixels. This second set of artifact colors will be less intense, and striped in luminance.
The specific artifact colors produced can vary from console to console, depending on hardware modifications, and likely varying between MARIA revisions. Its also worth mentioning that if a console has been video-modified to produce s-video output, it won't be able to display artifact colors, since the pixel luminance signal will no longer be mixed with the chroma signal. Similarly, if you turn off the colorburst signal using the CTRL register, artifacting will be eliminated, along with the rest of the colors.
If you're designing graphics and fonts for 320 mode display, and you wish to minimize artifacting, you need to ensure that any bright or dark pixels are horizontally followed by 1 or more pixels of the same luminance.
