Plasma competes with LCD to dethrone CRT.
Some say we've gotten to the point where the venerable CRT is about to become obsolete. This month, we'll take a look at the existing and emerging display technologies and see what they bring to the professional and consumer viewing experience.
It's useful to look at consumer technology because it affects the ultimate presentation, as well as influences the design of professional monitors. In addition, as new technologies find their way into professional studios, it's important to understand their pros and cons. Before looking at different displays, it's useful to consider some of their measured and perceived characteristics.
At last year's NAB conference, several video producers and cinematographers lamented that once their products get into distribution, they have no artistic control over the display device. Displays are also getting to the point where their quality may be second to the breadth of adjustment the user can apply. Compounding this is the vast array of devices that can affect the quality of the video — both for and against good pictures.
For faithful video reproduction, the most important characteristics of a display are resolution, peak brightness and black level (dynamic range), brightness linearity (gray scale), and color gamut (saturation). Unfortunately, manufacturers do not uniformly define the terms brightness and contrast.
In fact, many displays are mislabeled when it comes to these controls. The brightness control on most sets, for example, essentially sets the point of the black level. (The exception are LCD panels with a brightness control that varies the intensity of the back panel illuminator.) Similarly, the contrast control often sets the peak brightness of the set.
It is well known that CRTs do not produce luminance in direct proportion to the applied signal, but have a nonlinear transfer characteristic. This nonlinearity is quantified by the gamma (λ) of the display. (A true contrast control would change the gamma of the display, not the brightness.) From the signal standpoint, Vout = Vλin, with λ= 2.5, usually for CRTs.
When capturing video, the inverse of this characteristic is applied at the source, so the transmission medium will behave linearly. However, because of the phenomenon of simultaneous contrast, where the level of ambient lighting can affect the perceived screen contrast, NTSC engineers have standardized gamma precorrection to 2.2, or 2.8 for PAL systems.
Because gamma is caused by the CRT electron gun, plasma display panels (PDPs) and Digital Micromirror Devices (DMDs) have a linear characteristic (λ = 1), and a correction must be applied through signal processing in the set. (LCDs have an S-shaped transfer characteristic.)
Often, professional monitors have a control to change the gamma. Some consumer sets have this, too, but it is usually lumped into some preset label. (See Figure 1.)
Long live the CRT?
Many analysts have already proclaimed the demise of the CRT, driven in part by a growing interest in larger screens. However, although it is impractical to build a CRT larger than about 40in (primarily because of weight considerations), it is still believed by many to present the best quality display over many parameters, including color saturation, peak brightness and deep black level.
Of course, the perceived black level will depend on ambient lighting. And considering most viewers at home rarely view their TVs in a completely dark room, the CRT may lose its advantage, practically speaking, in many situations. In a broadcast studio, however, the lighting is usually subdued, so this characteristic will be more significant.
Another advantage of CRTs is scan agility. With multiple scanning formats now typical in the broadcast plant, a CRT is the only display device that can inherently change its scanning format to display the video in its native format. However, because of the increased cost of a more complex deflection system, this technique has been used almost exclusively in computer CRT monitors.
Another shortcoming of CRTs is the moiré pattern (aliasing) that can result when shadow-mask and dot-pitch frequencies are mismatched. The highest quality monitors therefore have a very fine dot pitch.
LCD, the future king?
LCDs have proliferated in the past five years. This is due in part to advances in manufacturing technologies. By using electrically variable twisted nematic cells sandwiched between polarizers and a flat back-panel illuminator, high-performance video displays are now possible.
Because the panel is essentially a large, single piece of silicon, a few defective pixels — while acceptable for computer displays — would result in inadequate video display. For this reason, displays larger than 40in are prohibitively expensive.
Also, the response time of the cells was initially a problem, causing smearing with moving images. Better panels have greatly reduced this artifact through a combination of active cells and clever signal processing.
LCDs provide the highest peak full-screen brightness of the display technologies, so this may be a factor in brightly lit situations. Viewing angle is still somewhat of an issue on LCDs, however.
And lack of deep blacks still limits the performance of some units. Nonetheless, the reproduction of good colors — and flat form factor — keep the units in favor with many consumers. By lighting the back panel with LEDs instead of a fluorescent lamp, a color gamut exceeding that of CRTs is now possible, and these units have begun to appear in professional video monitoring applications.
PDP's bid for the throne
LCD's biggest competitor is the PDP. Plasma panels offer a combination of flat form factor, cost-effective large sizes and excellent color saturation, making them appealing to consumers. Wide viewing angles also make the displays practical in ordinary viewing situations.
The critical issue of display burn-in has largely been eliminated. Black-level performance, however, can vary widely in these units, and larger displays require a fan for cooling.
Another factor not realized by many owners is that plasma sets, because of power and heat dissipation, reduce the peak brightness depending on the average picture level. Thus, a PDP that can produce a certain peak white level over 25 percent of the screen may in fact be limited to less than half that when attempting to produce peak white over the entire screen. CRT and LCD technologies do not suffer this limitation.
PDPs and LCDs also carry another burden. Higher pixel densities carry a hefty price tag. And some manufacturers tout “HD” performance when the actual resolution is often less than that. These sets must then scale the HD inputs to the physical display — a process that can involve not only downconversion, but also deinterlacing and film mode conversion. Add to this the potential for transmitting 24p material, and the issue of scan conversion becomes a significant differentiator.
Tech on the horizon
Front- and rear-projection technologies, while not in the mainstream of television viewing, are becoming popular for those wanting a home theater experience. By using a DMD or LCD as the imaging device, full 1080p performance is now available to the consumer. These imaging technologies are capable of the deepest black levels and the highest dynamic range of any of the displays. And thanks to advances in compact light-engine technologies, the bulkiness of original sets is also giving way to nearly flat displays that can hang on a wall. Color reproduction is usually achieved by using a single DMD device with color wheel or by three registered LCD (or LCOS) devices.
A new development in video technology is the use of lasers or LEDs as the light source for front- and rear-projection TVs. By eliminating the filter wheel, these technologies hold promise for displays with breathtaking color gamut. Still to be achieved, however, is the manufacture of devices at the necessary cost and performance points. But it may happen soon enough.
Armed with the knowledge of what's important in choosing a display, the variety is still daunting. Just remember that, in growing numbers, your viewers may have better displays than you do!
Aldo Cugnini is a consultant in the digital television industry.
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