If you ever attended summer camp, you might remember that besides being fun and often teeming with insects, in retrospect it was a learning experience. Maybe you learned how to tie knots, canoe or swim. Skills learned in summer camp are often taught because they are useful throughout life. This edition of the Transition to Digital tutorial is a digital version of a first day at video summer camp, complete with a lesson or two that everyone who doesn't already know, needs to know to be a the most successful videographer, operator or technician they can be.
Using only a picture display to monitor the quality of a baseband video signal as it is digitized or recorded is like driving a car without a dashboard. Sooner or later, you're going to run out of gas, get a speeding ticket or worse. In the video world, the equivalent of a dashboard is a waveform monitor. However, a waveform monitor is of little value if the user doesn't understand how to interpret the waveform display.
At best, the quality of a digital signal will only equal the quality of the baseband signal that was originally recorded or ingested. Other than computer-generated images, baseband video signals start in the analog physical world as they pass through a camera's optic systems and sensors. The quality of that baseband signal as it is digitized depends on all the measurable values of the baseband signal being closely monitored and adjusted to conform to industry standards and practices. This is the primary purpose of waveform monitors.
Waveform monitoring explained
Waveform monitors are valuable test and measurement tools for maintenance and engineering functions, as nearly every technical detail of a baseband video signal can be measured with fine precision. For the purpose of this tutorial, we will limit the discussion to the use of a waveform monitor to accurately view and adjust the levels of video image signals during the production, post or ingest process. A waveform monitor can be stand-alone device, a rasterizer for display on a video or VGA screen, or built into a product such as Final Cut Pro. All monitor baseband video.
A baseband video waveform monitor consists of a display with a graticule superimposed over the actual waveform. The horizontal lines on the graticule are used to measure signal amplitude. Typically, the markers on the right of a standalone waveform monitor graticule are for RF measurements made at a transmitter and are generally ignored by everyone but transmitter engineers. For studio and production work, the markers on the left side of the display are what are important.
A video waveform stretching the distance from the -40 to the +100 markers on the left side of the display is 1V peak-to-peak, sometimes referred to as 140 IRE (Institute of Radio Engineers) units. While the 1V baseband video signal standard around most stations and facilities has been replaced with SDI, what it represents remains quite significant.
The continuous signal from the zero base line to -40 is the video sync, which is not necessarily a concern for operators, other than to know it is necessary. The area from the zero base line to 100 is the area of most concern for operators and technicians because that's where the actual video image is represented. The space from zero to 7.5 represents black. The 7.5 IRE unit baseline is also known as pedestal or set-up. While some digital formats use zero pedestal, many SD digitization systems are looking for a 7.5 IRE unit pedestal, and values under 7.5 will usually be recognized by most viewers as pure black. The brightest white is represented by 100 IRE units. Using the dashboard analogy, 100 is the universal speed limit.
Most waveform monitors and displays also have a few switches. For the purposes of this tutorial, we'll discuss only the most important ones for operator monitoring. One set of switches allow users to switch between field and line modes, one or two fields, which make up a single interlaced frame. For typical monitoring, operators use the 2-line mode, as shown in Figure 1.
Another important switch is the filter switch, which for monitoring purposes is typically set in the luminance position. In the flat position, all information, including chroma, is displayed and is often harder to see. The luminance position filters out the chroma information, revealing only the luminance information in the signal. For most typical quality control scenarios, luminance monitoring in the 2-line mode is the most useful. Can you use the 1-line mode? Sure, but most operators find the 2-line mode more convenient.
A typical composite video signal stretches from 7.5 IRE units to 100 IRE units. Therefore, the useable video dynamic range is 92.5 units from the darkest to the brightest. This is also the range most digitizers work within, so any excess will likely be distorted by the digitization process.
Not every picture needs to stretch from 7.5 to 100. Like audio, the black to white range within the image depends on its content, so not every field in a darker picture needs to peak at 100. On the other hand, boosting the gain so perhaps the brightest shade of gray in a scene is set to 100 could reveal image detail that might be otherwise lost within the available dynamic range of the baseband signal.
Other useful features
As you look at the waveform displayed in the 2-line mode, you will see that each of the two waveforms from left to right represents the picture from left to right. However, to represent the picture from top to bottom, the waveform must be displayed in the field mode, as shown in Figure 2. In the field mode, the top of the picture is on the left and the bottom is on the right of each waveform. Most operators find the line mode more useful. Not only can it be used for electronically setting signal levels, it is also useful for determining the best iris and ND filter adjustments at the camera.
Some operators and engineers find the line and field modes useful to ensure lighting is uniform across a set. If, for example, you expect a set to be uniformly lit from left to right and the waveform amplitude falls off on the right, there may be too much light on the left or not enough light on the right. Similarly, the field mode can be useful for lighting a set from the top of the picture to the bottom. Figure 3 shows a 2-line display of an evenly lit set. This waveform monitoring technique can be particularly useful when lighting a green screen background and foreground for chroma keying.
An experienced lighting director might argue this point. They may have a golden eye that can identify tiny lighting anomalies in his or her mind, but from an engineering perspective, the waveform monitor always tells the truth, is never distracted and never has a bad day.
This tutorial is dedicated to my recently departed stagehand and lighting director friend Mike Hennessey who would have stood his ground arguing this point until the coffee ran out.
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