Tip of the Week - NTSC

If you'd like to know more about conventional TV and why we use RGB as our primary colors, head on over and read this Univeristy of Washington EE article about it. I'm going to keep this fairly simple, and won't go into detail unless you comment that you'd like to know more.

The NTSC standard calls for a image the size of 720x486 running at 29.97 frames per second. Since NTSC is interlaced, those 29.97 frames per second becomes about 59.94 fields per second, rounded to 60. What is the reasoning for such an odd number? Power. Specifically AC power coming out of your wall socket. This just dictates how the image plays back, and nothing about what realm of colors we can use.

Color Gamut
This range of available color shades is known as a color gamut and will also include colors we may not even be able to see, such as infrared (long wavelength) and ultraviolet (short wavelength) colors.

Think of a color gamut as being a triangle - one corner marked “red,” one marked “blue,” and one marked “green.” The area inside the triangle represents all of the possible color combinations that result from mixing red, green, and blue primary colors in an additive system.

The widest color gamut possible is the visible color gamut. This measurement of visible color was first developed in 1931 by the Commission Internationale de l'Eclairage (C.I.E), and takes into account our sensitivity to different colors. It was subsequently updated in 1976 to more accurately show the additional component of luminance where these colors mix.

The NTSC color television system was designed and adopted in 1953 with human color sensitivity in mind; it encodes, transmits, and decodes red, green, and blue in the proportions of 30%, 59%, and 11%, respectively. These numbers are derived from the original CIE human color sensitivity studies. Even our new digital TV system preserves essentially the same color space.

Another color gamut is the RGB (red-green-blue) gamut, which defines those colors that can be generated by artificial means, such as with the color phosphors used in direct-view and plasma TVs, and the color filters employed in LCD TVs. This RGB color gamut is reduced somewhat from the visible color gamut, but still includes a wide range of reds, greens, and blues and their mixed color shades.

For analog television and DTV/HDTV broadcasts, the RGB color gamut defines the boundaries of the visible color world. The reason is that color phosphors and filters are not as versatile as sunlight when it comes to mixing and creating color shades.

Here is an example of a CIE chart. Because the world of TV is so myriad, I'm not going into detail about why certain monitors are better at displaying the CIE color gamut more accurately than others, or why we can't display more colors on a monitor.

So now you know a little about the framerate and color gamut of NTSC. How do we use this knowledge? If you don't have a vectorscope or broadcast monitor handy (and how many of us really do at home?), here are some things to keep in mind when creating your images for broadcast on an NTSC monitor.

There are limits to what a monitor can display, in terms of brightness and color saturation (This is called IRE). A rule of thumb for creating images for the TV is to not have extremely contrasting areas in your frame. Pure red, green, or blue imagery on a black background will usually make your TV cry out in pain from these illegal colors. Anything really bright will also make your TV throw up.

Strictly speaking, an illegal NTSC broadcast color is any pixel or area of the picture that has over 75% saturation of the chroma (color). For example, an RGB graphic that had a yellow area of Red 191, Green 191, Blue 0, would have 75% saturated color & would be legal. 75% of a 0 - 255 range is 191, so as long as there isn't a spread of more than 191 between each of your RGB channels for any pixel, it's not illegal. An RGB graphic that had a yellow area of Red 255, Green 255, Blue 64, would also have 75% saturated color & would also be legal. But an RGB graphic that had a yellow area of Red 255, Green 255, Blue 0, would be 100% color saturated & would be illegal.

Here comes the part that muddies the definition of illegal color a bit. There are many combinations of colors over 75% color saturation that do not cause any kind of problem for NTSC broadcasters or receiving television sets, so some video producers take a chance & sneak up the color saturation beyond 75% for certain colors (we all want our video to look pretty).

Unfortunately the world of NTSC is very huge and contains a lot of information. After all, television has been around for close to 60 years, and there have been a bevy of changes in technology. I wish I could go more into detail about NTSC than in this single, off-kilter post, and I'm also sorry for alienating you poor PAL folks still reading this.