LCD displays have rapidly displaced CRT monitors, first for computer users and then for consumer TV viewing. The same evolution is now taking place for the most challenging application of video displays — in critical viewing environments, such as for television and film production and distribution. How close are we to the ideal flat-panel display replacing the CRT?
LCD state of the art
Two technologies define the flat-panel industry: LCDs and plasma display panels (PDPs). Manufacturing and yield issues generally segregate displays by size; while LCDs run up to 82in and higher, PDPs are comparably less expensive for the larger sizes and more expensive at the smaller end. Because most critical video evaluation is done on monitors of less than 24in, essentially the entire application has gone to LCD displays. In addition, PDPs rarely come with signal monitoring features like waveform displays, now common in LCD production monitors.
Economy of scale, driven by the consumer electronics business, has caused a widespread shutdown of CRT production lines. Professional CRT monitors can still be found, but no new monitors have hit the market in quite some time. This means the industries formerly served by CRTs are increasingly moving to flat-panel technology. Also, California's Legislature has adopted a RoHS (Restriction on the use of certain Hazardous Substances) Law, limiting the use of lead in displays. While it is not expected that federal or widespread state laws will soon follow, the writing is on the wall, and manufacturers are fleeing from CRT production.
In 2008, the EBU revised its Tech 3320 document, “User requirements for Video Monitors in Television Production,” which provides an excellent set of recommendations on the technical characteristics of video broadcast monitors. While this document specifies numerical values for many parameters, we'll concentrate here on important qualitative characteristics for monitors.
Most monitors have no trouble achieving a satisfactory peak luminance level. What is more important is how other parameters such as gray scale, color temperature (white point) and color saturation track at different levels of luminance. Surprisingly, some professional LCD monitors come shipped with “contrast” and “brightness” presets that greatly compromise the gamma and gray scale tracking, as seen in Figure 1 on page 22.
In fact, LCD display panels inherently have this S-shaped transfer characteristic, and some monitors arrive from the factory with the peak luminance set rather high, resulting in this undesired transfer characteristic. Thankfully, most monitors operate in an environment with subdued lighting, allowing operation at lower peak luminance levels, where the gray scale performance of the display is usually more linear; black level and gamma are consequently improved.
Contrast is a term that manufacturers and dealers have abused. By definition, it represents the ratio between two luminance levels on a display, usually at minimum and maximum video input levels. However, contrast can vary depending on how it is measured. Simultaneous contrast (also called static contrast) describes the ratio of the brightest white to the darkest black that a display can produce within an image. ANSI defines this as the ratio between the averages of multiple 100 percent-white level boxes in a checkerboard and the averages of 0 percent-black level boxes in that same checkerboard.
Some manufacturers, however, will instead publish the sequential (or on/off, or dynamic) contrast, which is determined by measuring the output difference between a 100 percent-white level signal and a 0 percent-black level signal. This measurement will usually provide a contrast figure that is orders of magnitude greater than the more-meaningful simultaneous contrast.
LCD monitors have come up short of CRT performance for black level, due to the combination of the liquid crystal attenuation characteristic and the constant illumination of the cold-cathode fluorescent lamp (CCFL) light source. One solution to this is in the use of variable CCFL illumination, but this usually improves only the sequential contrast (and black level); LED backlight units can similarly improve this. Through some proprietary techniques, a simultaneous contrast ratio of 15,000:1 is now possible, though at a price premium. Nonetheless, a 1000:1 contrast ratio can be provided at a reasonable cost, and can be satisfactory for many applications.
Other shortcomings of LCD displays include a variation of performance with viewing angle, and motion blurring due to lag in the liquid crystals. The former — which can affect off-axis contrast, black level, color balance and saturation, and can be worse with wide-color-gamut displays — remains an area of needed improvement. Motion blurring, however, has been greatly reduced due to evolving design improvements.
LCD monitors can provide interesting new features. Many pro LCD displays now support numerous color spaces (gamuts), including emulation of handheld devices (such as the iPod), often by the use of LED backlight units. However, standards for the mapping of wide-color-gamut signals in the broadcast environment still don't exist, as well as a standard for the handling of out-of-gamut signals. Some monitors, however, provide a mode that will indicate out-of-gamut colors. Because all LCD displays are progressively scanned, some form of deinterlacing and scan conversion must take place to display interlaced video. LCD monitors should thus have a mode that emulates interlace artifacts. In addition, displays should present images at the frame rate of the source, or optionally, at some integer multiple thereof, such as the 72fps display of 24fps material.
Users expecting the highest performance — and taking steps to maintain it — will want to test and set up their own monitors. The EBU Technical specification Tech 3325 “Methods for the Measurement of the Performance of Studio Monitors” provides insight into how to do this. The document also describes new test patterns to support the methodology. Similarly, SMPTE began a study in 2004 with the intent of identifying new standards for new reference monitor technologies. A subsequent working group convened in June 2008 to define the specifications needed to achieve interoperability between content and reference or quality control monitors. Scheduled to complete its mission by the end of 2009, the group will also define relevant measurement and calibration procedures.
For those wishing to test and maintain monitor performance using off-the-shelf tools, tristimulus colorimeters are available, together with automatic software for performing monitor measurements and calibration adjustments. These include low-cost devices that are temporarily mounted onto a monitor display, as well as contactless devices that use optics to measure display output.
Other display technologies remain elusive. The much-heralded field emission display (FED) is a flat-panel display that uses individual electron emitters (like the electron guns used in a CRT) to generate each RGB group of pixels. The surface-conduction electron-emitter display (SED) is similar to the FED, and shares its positive traits, but uses a single emitter for each column of dots instead of the individual dot emitters used in the FED. These displays share many qualities of the CRT, especially high contrast and low black level. While the technologies originally looked very promising, sharp falls in LCD prices — together with technical, economic and intellectual property issues — have brought development of FED and SED devices to a near standstill earlier this year. The optimistic news is that at least one panel manufacturer has announced plans to continue development of SED technology.
LCD monitors are now available that enable critical monitoring in many applications; the catch, however, is that high performance still comes at a premium price. Nonetheless, displays can be acquired at competitive prices, which provide excellent performance — provided the user takes the time to properly set them up. Having the requisite knowledge can turn an otherwise weak performer into a useful quality assessment tool.
Aldo Cugnini is a consultant in the digital television industry.
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