Video Signal Analysis

Professional video equipment may need to be calibrated.

Professional standard and high-definition television productions require reproducing video that conforms to specific broadcast technical standards. These standards ensure the picture quality is within the technical limits of cameras and recording equipment. Digital video editing software often includes analysis tools, but may not adequately explain the meaning of the displayed information.

Analysis Tools

The two primary tools used for video analysis is the waveform monitor and vectorscope. These tools complement one another in providing a complete range of information that includes video brightness, the amount of color information contained in the video and whether the color information is being reproduced accurately.

Luminance

A video waveform monitor displays brightness levels referred to as luminance. The brightness of the incoming video signal is measured on a scale of 0 to 100 units. Brightness levels that go past 100 units may exceed the ability of camera and television equipment to accurately reproduce the image without causing an effect known as "clipping." Clipping can cause image distortion or a loss of detail because the video signal is too bright.

Chrominance

While luminance refers to overall video brightness, chrominance refers only to color information. Reducing chrominance levels will turn the color image into black and white. Too much color information, as with brightness, can lead to a distortion of the original image. While chrominance is measured on the same 0 to 100 scale, video equipment is more forgiving of chrominance levels that exceed 100 units.

Chroma Phase

Beyond evaluating merely how much color exists in a video image is the need to determine if the color is being accurately reproduced. The vectorscope displays color information in a two-dimensional plane, much like a simple graph. Points on the graph represent where certain colors should be located, with each of the television primary colors having a designated location on the vectorscope display. When a color test pattern is fed into the vectorscope, each of the primary colors appears as small dots in their designated locations. If the dots land outside of their targets, the color is not being correctly reproduced by the equipment. This condition is referred to as "being out of phase."

Pedestal (Black Level)

While the brightest white yields luminance values of 100 or greater, the darkest video levels are revealed at the bottom end of the scale. An important factor in video reproduction is the use of black levels. Video uses black levels below 7.5 units for information other than what is visible on the television screen. As a result, a "pedestal" level of 7.5 units describes the darkest visible part of an image. Levels below 7.5 units may be used in the creation of special effects that allow video equipment to replace darker video information with another video source through a process known as a "luminance key." For this purpose, video equipment may produce black levels at 0 or below (measured in negative numbers). This type of black video information is referred to as "super" or "negative" black.

References

• Tektronix: Basic Video Testing--Waveform Monitor Techniques