When a display device, whether it is a TV camera, a VHS or DVD or a game console is manipulating the phase of the color signal directly, we call this "direct NTSC color." The phase shifting in this case is not only intentional but directly caused by the display device. Also, a consumer-grade standard definition CRT television has a horizontal definition of about 250 horizontal TV lines, so the actual horizontal resolution of a CRT is not that detailed. The luma signal has greater definition and, because color is being overlaid onto luma, can give great color detail than the limited phase shifting of the color signal can on its own. You might think the resulting color would have poor resolution, but the human eye is much more sensitive to changes in brightness than color. The color NTSC standard defines each line as having 227.5 color clocks or phase shifts per line, and this includes portions of the screen which are not visible and have no corresponding luma signal. (Some diagrams like the one above refer to the color burst as phase 0). The phase shifts can be plotted on a color wheel according to the sine of the phase, so 180 degrees is something of a greenish-yellow, 270 is close to cyan, 360 is blue and 90 is light red. Then as the line traveled across the screen, the sine wave would phase shift relative to the burst to define the color that the display should be displaying at that point on the line.
(Phase is defined as the sine of an angle, if you remember your basic) This was to signal the TV that color information would be displayed on this line. In color NTSC, each line would begin with a "back porch" and during this back porch a color burst signal of approximately 8 cycles of a 3.58MHz sine wave with a phase of 180 degrees would be transmitted. This interlaced display would alternate continually between odd and even fields, 60 times per second (Hz) for monochrome NTSC and 59.94Hz for color NTSC. NTSC defined a system of 525 lines being displayed at 60Hz with the odd lines an image being displayed the first field of 262.5 lines and the even lines being displayed the second field of 262.5 lines. the color or hue of a chroma signal was determined by the phase of the 3.58MHz sine wave relative to a "color burst" reference phase and the saturation of the color was determined by the amplitude of the sine wave. While the luma signal was an analog waveform of varying voltage levels corresponding to the brightness of the image as captured. In color NTSC, a 3.58MHz color subcarrier sine wave is transmitted alongside a bandwidth-unlimited luma signal (generally 4.2MHz due to over the air broadcast limitations). Color was added to by the NTSC to be backwards compatible with the black and white standard in 1953. Brightness and sync were present when the NTSC defined as the black and white television standards in 1941. A color NTSC signal is made up of three components, brightness/luma, color/chroma and sync. In this article, let's take a deep dive into how artifact color works on the Apple II and how it was adapted for systems where artifact color could not exist and how artifacts can change according to the display technology inside a display.Īrtifact color is based on a quirk of the NTSC method of decoding color. But the color method used did not translate to PAL countries and later improvements to color filtering could modify the colors shown. The design of the Apple II was so solid that its color works rather well on almost anything that can accept a composite signal, even today.
It worked by exploiting quirks in the NTSC color system called artifact color which TVs were attempting to suppress. The Apple II computer are unique in that not only was it the first home computer ever released to the mass market, it was the first computer released to support color graphics, all the way back in 1977.