Making Computer Graphics NTSC Compliant

by Tom Buehler

When making titles and graphics for video, one must make sure that they conform to the standards of NTSC video. NTSC is much more restrictive than what computers can show, so graphics that may look fine on your computer monitor may flicker or appear washed out on TV. On the other side, graphics that conform to NTSC standards may look dark and dull on a computer monitor. By working within these guidelines, it may seem like you are making your graphics much worse, but in fact you are ensuring that they will look better when viewed on a television.

Luma Values

The luma range for NTSC video is 16-235, while in computer graphics the RGB range is 0-255. Before you make titles or graphics, adjust the output levels of your file to 16 and 235.

Color Values

As with the luma values, all color values should be within 16-235, with black being 16,16,16 and white 235,235,235. However, you should stay away from using saturated colors, which tend to bleed even if they are within the legal range. A rule of thumb is to not use values higher than 180 for colors. Also, be very careful when using red. Even with values a bit lower than 180 you still might see some bleeding with red. As stated above, these colors will look darker than the colors you are usually accustomed to working with in graphics, but on video they will look better.

Graphics programs often have filters that allow only NTSC-safe colors, but these vary in quality. Some might permit some illegal colors to sneak through, while others, like the one in most Adobe products, can be too aggressive and reduce your color too much. For the most control and best results, it's best to take note of the colors you're using yourself.

Lines and Edges

Avoid using thin horizontal lines, which can flicker or "twitter" due to video's interlacing. Lines should be at least two pixels high, although preferably larger than that. Also avoid using serifed fonts, unless they are large or bold (in fact, any fonts you use should be large and bold, since small fonts may become unreadable on a television.)

Overly sharp edges can cause a problem for video as well. Since a scan line in video is represented by an analog waveform, it cannot instantaneously change from white to black; it needs some room to "ramp." When video attempts this instant change, it often overshoots, creating a ringing effect around the edge or causing the image to pulsate. Overly sharp edges can also create mosquito noise artifacts when compressed to DV or MPEG-2 or, in extreme cases when they are combined with illegal colors, the overmodulated picture signal leaks into the sound subcarrier and causes a buzzing noise.

The solution to the issue of overly sharp edges is to add a gaussian blur of between 0.2-1.0 pixels. The will have the effect of antialiasing the image and eliminating the hard edges. I know it seems counter-intuitive to be blurring your images to make them look better, but pretty much nothing about NTSC is intuitive.

Safe Zones

Early TV picture tubes distorted the image near the edges, so it was decided that the picture would be overscanned with the edge of the picture hidden by a plastic bezel. However, when working with video on a computer you see the whole image, so you must always be aware that the edges of your image will not be visible on a TV. The non-visible portion of the image corresponds roughly to the area inset 5% from each edge (about 36 pixels from each side and 24 pixels from the top and bottom). The area inside is often called the "action-safe zone." Make sure that anything that has to be seen is inside this zone.

Additionally, there is another region that is inset 5% from the edges of the action-safe zone known as the "title-safe zone." Older picture tubes still produced some distortion outside this zone, making text harder to read, so titles were always including inside this zone. Most TVs nowadays don't have a problem with text outside of this zone, but it's still common practice to use it. You may want to play it safe, but if it's not possible I wouldn't worry about it, since the likelihood of someone viewing the video on an old TV that has this problem is low.

Video editing software such as Premiere often have an option where you can lay a diagram of the safe zones over your video for reference.