Maintaining Video Quality

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

Thus, to consider yourself a pro (a professional who produces consistently good results), it's essential to understand the elements in this module.

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

A waveform monitor, which graphically displays and measures the brightness or luma level of the video. (In video, the more accurate term of luma is now replacing luminance.)
A vectorscope, which measures relative color (chroma) information.

Although these are generally separate instruments, in some cases you can display both on a single TV monitor or 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 (things every professional videographer should know about), and we'll stay away from their technical dimensions.

The Waveform Monitor

In critical professional video work we use waveform monitors as scenes are taped. wave-form monitor Note the waveform display on the right.

During editing, the device is 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 resulting electronic graph shows critical elements of the camera's video.

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

Let's see how that works.

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

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

Based on units established by the Institute of Radio Engineers, a scale alongside the waveform monitor starts at around -40 IRE units (at the very bottom) and goes to about +120 (at the very top).

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) -- as 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.

Waveform monitors, together with light meters (which we'll discuss 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, because the peak video level may come up to only 50 or so on the waveform monitor scale.

You can normally fix this by opening the lens iris one or more f-stops.

If you initially leave the video at a low level and then raise or boost it later in the video recording or transmission process, the resulting picture may look grainy because of video noise, as shown here (in somewhat exaggerated form).

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. (Two identical fields typically display on a waveform monitor, but to simplify things we'll show just one in these drawings.) On the gray scale below, you can also see the loss of detail in the white areas. You can fix this problem by bringing down the video level (closing down the iris of the camera).

Another problem is compressed blacks.

In this case, 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 on the right side of the scale, as shown below.

You can fix this problem by raising the black level setting of the video equipment, opening up the camera iris, or a combination of both.

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