Module 16-1

Updated: 07/05/2005

Part I

Although today's video equipment includes automatic circuitry to adjust audio and video levels, these automatic controls are programmed to maintain only rudimentary technical parameters, which may or may not be in line with your desired results.

Thus, if you are to consider yourself "a pro," (i.e., a professional who can produce consistently good results) it's essential that you understand the elements in this module.

In monitoring and controlling picture quality two pieces of equipment are essential. 

• A waveform monitor, which graphically displays and measures the brightness or luminance level of the video. 
  
• A vectorscope, which measures relative color (chroma) information.

Although these are generally separate instruments, in some cases both can be displayed on a single TV monitor or on the screen of a computer-based editing system.

In this module we'll cover some of the most basic elements of the waveform monitor and vectorscope—those that any professional videographer should know—and we'll stay away from their technical dimensions. 

The Waveform Monitor  

In critical professional video work waveform monitors are used as scenes are being taped.

During editing, they are used to monitor and maintain video quality and scene-to-scene consistency.

By looping the video signal from a camera through a waveform monitor, the output of the camera can be electronically graphed as shown on the right.

What you see will tell you a lot about video quality and provide information needed to fix many problems.

The photograph on the left contains tonal values from full black to bright white. It will give you a normal waveform pattern such as the one shown above.

The dark areas of the video picture are represented near the bottom of the waveform scale (marked "black level" above), and white areas appear at the top (marked "white level").

A scale along the side of the waveform monitor starts at about -40 (at the very bottom) and goes to about +120 (at the very top). The numbers on the waveform monitor are based on IRE units, units first established by the Institute of Radio Engineers. 

Ideally, video levels (for an average picture) should be somewhat evenly distributed between 7.5 (where "black" should start) and 100 (where "white" should end) — just as it's illustrated in the waveform above.  

A grayscale pattern picked up by a camera should distinctly reproduce the various divisions across the scale.

Ideally, with a properly adjusted computer monitor (set to 256 colors or more), you should see 16 divisions in this gray scale. Good computer monitors can reproduce colors and gray scale divisions far beyond what's shown above, and with greater accuracy than standard TV sets.

Waveform monitors, together with light meters (to be discussed in the modules on lighting), are your primary tools in ensuring proper camera exposure and good video quality. In this regard, it's helpful to know that one f-stop in a light meter translates into 20 IRE units on a waveform monitor. 

In TV, as in life, things are not always the way they are supposed to be, so now let's look at some problem areas.

Camera underexposure (insufficient light on the target) results in low video levels. On a waveform monitor this is immediately obvious, since the peak video level may only come up to 50 or so on the waveform monitor scale. This can normally be fixed by opening up the lens iris one or more f-stops.

If video is initially left at a low level and then raised or boosted later in the video recording or transmission process, the resulting picture may look grainy because of video noise, as shown (in somewhat exaggerated form) here.

If the target of the camera is significantly overexposed (too much light), the waveform monitor will show a video signal significantly above 100. Left uncorrected, this will cause significant distortion in the video picture.

Under these conditions some camera circuits clip off the white level as shown above. Note that detail has been lost in the white areas.

On a waveform monitor the result would be similar to what you see on the right. (Although two identical fields are typically displayed on a waveform monitor, to simplify things we'll show just one in the remaining waveform drawings.) On the gray scale below you can also see the loss of detail in the white areas. This problem can be fixed by bringing down the video level (closing down the iris of the camera).

Another problem is compressed blacks. The resulting video will be dark, without any detail in the dark areas. A gray scale would show a loss of separation between the divisions at the lower end (right side) of the scale, as shown below.

This problem can generally be fixed by raising the black level setting of the video equipment, or opening up the camera iris, or possibly a combination of both.

In Part II of this module we'll look at the issue that causes the most problems for video quality.

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