An Introduction to Video Scopes

There are a number of different types of video scopes, and various editing and compositing software packages will offer different diagnostic tools for analyzing images, but the underlying concepts are the same for measuring brightness and color. Below are examples of how standard NTSC color bars look when displayed on each scope.

The Waveform Monitor displays the luminance, or the black and white levels in your picture. Each white dot in the scope represents the luminance, or gray-value, of a pixel in your video image.

The display directly corresponds to the image from left to right. That is, looking left to right on the scope corresponds to looking left to right in the image. This means you can look at a scope and tell immediately where the dark and bright images are and roughly where they are located in the image itself.

Traditional waveform monitors are oscilloscopes configured for television monitoring, which measured the raw voltage of the video signal to check that all the pulses and scans of the signal are occurring at the proper times.

The main purpose of measuring the voltage was to make sure that the white levels didn’t exceed 100% or fall below 7.5%, as that would cause problems for the analog video signal. With digital video, top levels should not exceed 110%; signals that are too high will clip and blow out, and details that fall below black will disappear.

A well-exposed picture will be spread across all different levels, taking advantage of the entire range of the scope. A poorly exposed picture will tend to have the entire image squashed into a much smaller space. In post-production, the waveform monitor should be used like a histogram; keeping whites white and blacks black while the picture is adjusted.

The Vectorscope is another specialized oscilloscope, which measures color information, displaying saturation and hue. The closer a color is to the wheel’s center, the less saturated it is, and where it falls around the circle indicates which color it is. Brightness levels do not show up on the vectorscope.

In the same way that whites can be too bright, some colors, particularly reds and greens, can be too saturated and cause smearing and “bleeding” when displayed on television monitors. Being able to see exactly how saturated these colors are is helpful. Colors that fall outside the circle are not “broadcast legal” and colors outside the hexagon defined by the squares are not recommended.

The cross in the center of the circle represents zero saturation. Black and white values will appear here. Spaced around the circle are six (or on some scopes twelve) labeled squares representing the pure component colors of RGB and YU component colors of RGB and YUV video signals. When color bars are displayed, there should be six spots indicating the six main hues of the color bars, and these should end up inside the squares.

Of special interest is the diagonal line almost 45 degrees from the top, between red and yellow. This indicates flesh tone, which is a handy thing to remember when color-correcting footage and trying to keep skin tones properly balanced.