Use a Waveform Monitor
A waveform monitor is an oscilloscope calibrated to measure a video signal. It graphs the voltage vertically and traces the scan lines horizontally. A waveform monitor has an essential role in video production. Here’s use it to adjust exposure of cameras or monitor the luminance output of graphics.
1. Make sure that you are matching your waveform monitor to the signal you are generating. Video today comes in 4 basic flavors, NTSC, National Television Standards Committee, the broadcast video signal used in the United States; PAL, Phase Alternation Line, used in most of the world; SECAM, used for broadcasting in France, and multiple formats of high definition. To further confuse the issue, there are waveform monitors that also look at component and YC signals. For the purposes of this article we’ll look at composite NTSC.
2. Hook your video source into the waveform monitor making sure that it is terminated, which is the process of putting a 75 ohm resistor at the terminus of the line, either a terminator that locks on with a BNC connector, or a simple switch, usually found on a monitor with the position marked 75 or on. The standard NTSC signal is 1 volt peak to peak, from the bottom of the signal to the top across 75 ohms. Make sure that your calibration switch is set to default. If you have signal that seems so big that it wants to leap off the bottom and top of your box, chances are your signal isn’t terminated. If the signal appears shrunken, it is double terminated. Make sure that you haven’t flipped 2 termination switches on, or used both a switch and twist on terminator.
3. Check these switches: Make sure that your input switch is set appropriately to either the A or B side. Switch to 2H mode, which stands for 2 horizontal lines. Remember, an NTSC television picture consists of 525 horizontal lines, split into 2 fields. You are now looking at a display of half of all the lines laid on top of each other with one field on the left side of the screen, and the other on the right. The center displays horizontal blanking. This waveform consists of a brightness signal, called luminance and a color signal, called chrominance, which is a sine wave piggybacking on the brightness signal. The IRE switch displays the luminance portion alone of the video signal. Flat displays the chrominance plus luminance. Some waveform monitors have a chroma only switch as well. Use flat to get the full picture.
4. Before measuring your video inputs, it’s essential to calibrate your waveform monitor. Most have a REF button that will generate a 1 volt peak to peak square wave. Switch it on. Use your vertical positioning knob to put the bottom of the signal, horizontal synch at -40 IRE. Your top should be 100 IRE. If it isn’t you can click your vertical display knob into adjustable mode to expand or contract the signal, until it covers the full range of 140 IRE.
5. Take the waveform monitor off REF. Look at your video source. The color bars should match up. If they don’t it’s time to call in a tech to adjust your source. If they do, then take your source off bars and start making video to monitor its output on your waveform monitor.
6. By adjusting different video sources whether they be cameras or graphics generators so that their output matches as seen on this scale, relative brightness and color saturation will also match. Keep in mind that the NTSC color bars signal is designed to cover the entire range of video from darkest black to peak white. Not all video signals will have span this spread. If you’re using a camera, open its iris, until the waveform peaks at 100 IRE. It’s okay if spikes go past. They’ll be clipped off in broadcast going to white. Most faces should be at about 80 IRE. If your scene has vast areas that are too dark when you’re already at 100 IRE, add more light rather than opening iris up further. If a large portion of your signal is over 100 IRE, iris down. Experiment with different looks as you shoot. That upper end is not absolute, but too much brightness can leak into your audio, causing noise there when your video is broadcast. A waveform monitor is invaluable and gives you far more exposure control than either auto iris or zebra stripes in your viewfinder.