Jon Relay's Apple II Info Archives

Video Display Theory

Robert Munafo's Response

From: munafο@gcctеch.cοm
Newsgroups: comp.emulators.apple2, comp.sys.apple2.programmer
Subject: RGB values for the standard Apple ][ colors
Date: Thu, 12 Oct 2000 03:13:33 GMT

I am giving a table of all the lores and hires colors with their values in RGB. First, though there is a bit to explain...

The Apple ][ hardware generates a NTSC composite (luma plus chroma) signal complete with a color burst on the "back porch" of each scanline. The dots for all display modes (text, lores and hires) come from a master 14.31818-MHz clock, which is divided by 4 to produce the color burst. Dots for 40-col text and hires graphics come out at 7.15909 MHz. Dots for 80-col text, lores graphics, and double hires graphics come out at 14.31818 MHz. Although NTSC defines the scanline as 227.5 cycles per scanline (so the phase alternates on alternate scanlines, producing a dithering effect), the Apple rounds it up to 228. (This is why solid bright colors on a monochrome monitor appear as vertical stripes rather than the "tiny checkerboard" pattern you see when watching a TV program on the same monochrome monitor.)

To understand the NTSC standard fully, there are four coordinate systems you have to deal with:

chroma phase, chroma amplitude and luma
YIQ
Y B-Y R-Y
RGB

The "color burst" signal I just described, plus the positions of some of the coordinate axes of the other systems, are all defined by relative phase angles with respect to each other. I'll skip all the details, but you are invited to read any of the URLs I'm listing at the end if you want to know more (some are dead but cached on Google).

The important things to know for our purposes are:

1. On the Apple ][, all pixels and the color burst are in phase with the master clock. However, they come out as square waves, and the effect of the TV's circuitry (or a composite monitor) is to turn the square wave into the closest matching sine wave.

2. Because all the pixels generated are in phase with the color burst signal, the phase of the sine wave is always 0, 45, 90, 135, 180, 225, 270, or 315. The color burst itself is phase 180. For example, "green" HCOLOR=1 and "light green" COLOR=12 are 50% duty cycle square waves centered at phase 225.

3. The 4 hires colors are all 50% square waves, which gives a chroma amplitude higher than anything in a real TV broadcast. This is why they look so garish. Contrary to what was recently said in the article "Jon Relay's Apple II Info Archives" in the Aug 2000 GS WorldView, these four colors are not the same as the NTSC "+I, +Q, -I and -Q" colors. They are actually 12 degrees lower in phase -- probably not enough to notice, since your tint control is probably a bit off anyway (-:

4. The lores colors are derived a similar way, although two of them (the two grays) are square waves of twice the chroma frequency, and these are "averaged out" to a flat 50% gray by the TV. All of the lores colors are binary combinations of the four primary lores colors, which are the four that appear "dark gray" on a monochrome monitor:

                       duty cycle  phase
  Red        COLOR=1    45 to 135    90
  Dark-blue  COLOR=2   315 to 45      0
  Dark-green COLOR=4   225 to 315   270
  Brown      COLOR=8   135 to 225   180

If we combine COLOR 4 and COLOR 8, we get COLOR 12: 4 plus 8 = 12. This color is "on" from 135 to 315, which makes a 50% square wave centered on 225. Thus, it appears the same as the HCOLOR=1 described above.

Here is the table. It was used by taking the chroma/luma values and transforming to R-Y and B-Y, then transforming to YUV and finally to RGB. The last step requires a gamma correction for display on an RGB monitor, I used Y to the power of -0.4. For reference, the NTSC "color bars" test pattern colors and the YIQ axis colors are also given.

                 --chroma--
 Color name      phase ampl luma   -R- -G- -B-
 black    COLOR=0    0   0    0      0   0   0
 gray     COLOR=5    0   0   50    156 156 156
 grey     COLOR=10   0   0   50    156 156 156
 white    COLOR=15   0   0  100    255 255 255
 dk blue  COLOR=2    0  60   25     96  78 189
 lt blue  COLOR=7    0  60   75    208 195 255
 purple   COLOR=3   45 100   50    255  68 253
 purple   HCOLOR=2  45 100   50    255  68 253
 red      COLOR=1   90  60   25    227  30  96
 pink     COLOR=11  90  60   75    255 160 208
 orange   COLOR=9  135 100   50    255 106  60
 orange   HCOLOR=5 135 100   50    255 106  60
 brown    COLOR=8  180  60   25     96 114   3
 yellow   COLOR=13 180  60   75    208 221 141
 lt green COLOR=12 225 100   50     20 245  60
 green    HCOLOR=1 225 100   50     20 245  60
 dk green COLOR=4  270  60   25      0 163  96
 aqua     COLOR=14 270  60   75    114 255 208
 med blue COLOR=6  315 100   50     20 207 253
 blue     HCOLOR=6 315 100   50     20 207 253
 NTSC Hsync          0   0  -40      0   0   0
 NTSC black          0   0    7.5   41  41  41
 NTSC Gray75         0   0   77    212 212 212
 YIQ +Q             33 100   50    255  81 255
 NTSC magenta       61  82   36    255  40 181
 NTSC red          104  88   28    255  28  76
 YIQ +I            123 100   50    255  89  82
 NTSC yellow       167  62   69    221 198 121
 Color burst       180  40   0       0   4   0
 YIQ -Q            213 100   50     51 232  41
 NTSC green        241  82   48     12 234  97
 NTSC cyan         284  88   56     10 245 198
 YIQ -I            303 100   50      0 224 231
 NTSC blue         347  62   15     38  65 155