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Camera shading basics

Dec 1, 2008 12:00 PM, By Tony Manard

Making your cameras match is both art and science.

Chances are that somewhere this very moment an engineer has been called into the control room five minutes before a live broadcast because the director noticed that one of the cameras “just doesn't quite look like the other ones.” Perhaps it is simply an issue of black level or iris. Maybe somebody has been fiddling with the knobs, and the gamma settings have been changed. Regardless, the person responsible for painting or shading the cameras has to respond quickly.

Figure 1. Typical engineering setup for camera shading
Click to enlarge

Camera shading is the task of making multiple cameras match, providing a picture that is pleasing to the viewer. Camera shading situations range from the highly controlled environment of the broadcast studio to the sometimes less-than-ideal remote broadcast. Other variables that conspire to make our jobs more interesting are time constraints and the availability of test equipment.

Even though modern cameras have made vast improvements over earlier models, they have also introduced a host of additional features and settings. For our purposes, we will look at what it takes to match cameras and shade them for a pleasant picture with a minimum of test equipment and in a timely manner. Beyond extensive setups that are typically performed when new cameras are commissioned and left alone, there are a few parameters that should be checked and adjusted to fit the given lighting situation. The focus here is to get up and running quickly.

Tools of the trade

Effective camera shading requires a few basic tools. (See Figure 1.) A camera shading rig should consist of a calibrated reference monitor, a waveform/vectorscope and a switcher to go between the camera control unit (CCU) outputs. It is also a good idea to put the production switcher's preview and program outputs on the switcher to see if the signal is being altered through the rest of the camera chain.

A reference monitor

There must be one monitor that the cameras are referenced on. In an ideal situation, the color, brightness and contrast values of all the monitors in a facility are calibrated to produce exact and consistent picture reproduction. In reality, this is rarely the case.

The engineering reference monitor has traditionally been a high-quality CRT in the 13in to 21in range, although LCD monitors are starting to make significant inroads. By choosing one monitor as the reference and calibrating it to a known good set of color bars, you can save yourself a lot of second guessing as you look at the picture on different monitors.

Reference monitor calibration

A tried and true method for calibrating a monitor involves using 75 percent SMPTE color bars and the blue-only feature, if the monitor supports it. Begin by sending color bars to the monitor, typically from one of the CCUs.

Examine the three black bars toward the lower right-hand corner of the screen. This is known as the picture lineup generation equipment (PLUGE). The first bar is 3.5 IRE or about -4 percent video, the second bar is 7.5 percent video or 0 percent video (sometimes called pedestal or black setup), and the third bar is at 11.5 IRE or 4 percent video. Turn the brightness up until all three bars are distinctly visible. Now turn the brightness down until the 3.5 IRE bar and the 7.5 IRE bar are no longer distinguishable from one another. This will give you the proper brightness level.

If the monitor has a blue-only feature, activate it now. This feature deactivates the red and green signals, leaving you with a monochrome reproduction of the color bar signal. The bars are set up in such a way that the long bar on the bottom and the short bar just below it will appear at the same luminance level when the color circuitry is set up properly.

Figure 2. Example of SMPTE bars on waveform monitor. The first horizontal line is filtered for luminance only. The second includes chroma.
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Manufacturers use different terms, such as phase or hue, chroma or saturation, but these are the handles you will adjust to get the top and bottom bars as close to one another as possible. The chroma or saturation controls the outer bars, and the phase or tint controls the inner bars. Adjust the contrast for even steps of luminance change between the alternating gray bars at the top of the screen. In the case of monitors without the blue-only function, you may look at the monitor through a blue filter or gel. Check out SMPTE Engineering Guideline EG 1-1990 for a more thorough explanation of the color bar signal.

Waveform/vectorscope

Using the same color bars from the CCU, check the calibration of your waveform monitor and vectorscope. Set the waveform monitor to one or two horizontal lines with the line select turned off. The second white bar at the bottom of the screen should show at 100 percent video with the picture lineup group showing up at 3.5 IRE, 7.5 IRE and 11.5 IRE, respectively. (See Figure 2.) On the vectorscope, the dots should land within each of the color boxes.