Broadcast monitors

Broadcast monitors are in the middle of a transition, not unlike the digital transition. CRT monitors, used for decades as the ultimate standard for judging picture quality and manipulating video processing, are on the decline. In Europe, a restriction on the use of certain hazardous substances (RoHS) since 2006 means that new electrical and electronic equipment cannot contain certain materials, including lead, mercury and cadmium. This means that broadcast monitor technology is changing — and much of it is for the better.

At last year's NAB show, several new LCD monitors were introduced, with performance rivaling that of the gold standard CRT. One manufacturer even had the bravado to put its new LCD reference monitor side-by-side with a CRT monitor. With the bezels masked off, the images looked impressively similar, the tip-off to this viewer being the lack of vertical blanking.

The real daring, though, was that the LCD monitor was listed at $25,000 — well above that of its CRT-equipped neighbor. Expect that price difference to eventually vanish.

Specs and features paint a new picture

Digital processing now makes numerous features available in high-performance monitors. In order to present a dependable and repeatable reference, a calibration feature must be available and should offer numerous presets to the user. Various operating modes also ensure that the monitor will always accurately represent the incoming video.

In 2007, the EBU formally defined three grades of broadcast monitors used in a professional TV production environment in EBU Tech 3320. (The different grades are also called “classes” by some.) The most rigorous requirements are for Grade 1 monitors, which are used as reference devices for high-grade technical quality evaluation of picture capturing, post production, transmission and storage. Typical applications of these include camera control, color grading and content evaluation.

In the official document, EBU recommends that in a Grade 1 monitor, the black level should be adjustable to be below 0.1cd/m2 (nits), the full-screen (sequential) contrast ratio should be above 1000:1, and the simultaneous contrast ratio should be above 200:1. Grade 1 monitors should present pictures at a D65 reference white, and Grade 1 and Grade 2 monitors should have no visible pixel defects. Annoyingly, most nonprofessional monitors — and some professional ones — mislabel the brightness and contrast controls. On these sets, the controls actually set the black level and peak brightness, respectively.

Usually, professional LCD monitors will convert the spatial sampling of the incoming video to appropriately render on the display. However, this may result in new or masked artifacts. To address this, many displays offer a pixel-to-pixel mode, wherein each input sample is mapped to a specific pixel. Of course, this will result in cropping, when the input resolution exceeds that of the display, or windowboxing, when the reverse is true. Another available option is a black frame insertion mode that can reduce motion blur by alternating video and black frames at a 120Hz frame rate. Of course, such a mode will decrease the brightness of the image, so it cannot be used at all times. For camera monitors, a focus-in-red function displays object edges in red when sharp focus is achieved.

LCD monitors are always progressive scan devices. Hence, when reproducing an interlaced source, the monitor must provide scan conversion in order to display the video properly. This conversion must be done with a very high quality; otherwise, it will introduce artifacts that were not originally present in the video. Usually, some combination of motion-compensated interpolation, together with variable spatial filtering, will be needed. And often several modes will be offered, depending on the characteristics of the video.

In addition, some monitors now include an interlace simulator, which can produce alternating black lines on the display. By switching the black lines between odd and even fields, a raster is created that can reproduce many of the original line-twitter and spatial-temporal-alias interlace artifacts.

Color gamut

Gamma is the nonlinear input-voltage vs. brightness transfer characteristic of a display. Originally conceived as a display mechanism to compensate for camera nonlinearity, the concept is essentially a holdover from the characteristics of electron-gun image capture. Today, with CCDs in widespread camera use, the gamma correction is used to provide compatibility.

LCD displays, however, have an S-shaped transfer characteristic, so gamma matching is done through digital signal processing. This also means that the monitor must internally use more bits than the video source, so that contouring does not occur when the compensation is large. Most Grade 1 monitors will allow the user to select from different gamma settings.

Color gamut (or chromaticity) is the term used to describe the range of colors that can be reproduced in a color system. Various gamuts have been proposed or used over the years, as seen in Figure 1, together with the original CIE 1931 XYZ gamut describing human color vision. While an NTSC gamut was proposed many years ago, it has never actually been used, and has simply become a reference with which to compare other systems. Instead, SMPTE C (SMPTE RP 145) is used to encode analog NTSC video in the United States; ITU-R BT.601 (formerly called CCIR 601 and derived from SMPTE RP 125 and EBU 3246E) is used in Europe; and ITU-R BT.709 is used worldwide for HDTV.

The other gamut shown in Figure 1 is the Digital Cinema Initiatives' Digital Cinema System Specification. Note that this gamut (indicated in green in Figure 1) far exceeds most of the others, especially toward the top of the chart. This means that video shot in this gamut will have to be color-space converted to display properly in the other gamuts. Interestingly, the algorithm for this conversion is not yet specified, owing to disagreement on its subjective effects.

Other functions that should be present in Grade 1 monitors are overscan, H/V delay, mono mode, blue-only mode and image markers. Many of these displays also support bridging inputs and external sync, as well as composite, S-video, RGB, Y-Pb-Pr, SDI and HD-SDI input signals; multiple image formats; tally lights; remote control; and rack mounting.

Grade 2 and 3 specs

Grade 2 monitors have specifications more relaxed than those of Grade 1, and are used for control, switching, editing, camera preview and composition, lighting control, camera viewfinders, editing and graphics generation, and similar applications where critical picture quality manipulation is not generally carried out.

Grade 3 monitors are used for continuity, observation, audio production, signal presence monitoring, video switching and the like. Multiple Grade 3 monitors are often used in large arrays to assist live production. With large-screen displays and specialized signal processors, it is also now possible to replace multiple physical monitors with a single large-screen display. Grade 3 monitors are also used in low-priority situations for confidence checking, and are used as on-camera props.

Daylight viewing rounds out the possibilities

A new feature that has appeared in recent years is daylight viewability. Although this had existed in the past, it was usually limited to small-form monitors used on handheld camera stabilization platforms. With the high brightness achieved by using a very high beam voltage on a CRT, this led to limited device life. Today, high brightness on an LCD monitor can be achieved by a number of technologies, such as very high output backlights, highly efficient illuminator optics and low-reflection front surfaces.

As with most technologies, we are seeing an evolution of video displays. Amazingly, it has been just over a century since the first CRT devices were invented. Technological progress, however, seems to be accelerating, and we shouldn't be surprised if other developments are just around the corner.

Aldo Cugnini is a consultant in the digital television industry.

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