Life After CRT
The venerable cathode ray tube monitor, our long-standing reference, has become difficult to maintain and now even more difficult to replace. Flat panel displays are fast taking over from CRTs in various professional applications. However, these progressive imaging displays need to be evaluated differently than CRTs.
DTV format conversion is a very complex process; it’s more than just scaling. The goal is to make all sources, including SD, approach the look of HD while minimizing artifacts. To understand how we need to evaluate the performance of FPD HD monitors, we need to understand the various display technologies.
EMISSIVE VS. TRANSMISSIVE
CRT is an emissive technology, with impressive contrast ratios and black levels. It handles interlaced or progressive formats seamlessly. CRT also offers a small viewing surface for the footprint it occupies—and as an analog technology, it requires substantial amounts of maintenance to keep convergence, purity and color balance correct and reliable.
The property of phosphor to glow when exposed to radiation is put to use in CRT technology. CRT monitors are made of phosphors that release red, green and blue light. Phosphor absorbs ultraviolet light and emits visible light of fluorescent color. The negatively charges electrons are in the form of three beams, which strike the phosphors to create the desired colors.
Plasma display panels also use an emissive technology. In a controlled light environment it does quite well, with contrast ratio and overall image performance approaching CRT performance. Plasma’s viewing angle is quite good, with no color shift or loss when viewed at angle.
While plasmas offer large viewing areas with small footprints, they are quite heavy. They also consume substantial amounts of power. Heat dissipation can become an issue, especially in small work areas like OB vans or studio control rooms.
Much like a CRT, it is the phosphors themselves that create the light you see on a screen. PDPs are basically a network of red, green and blue phosphors (each triad makes up a single pixel) mounted between two thin layers of glass. Plasma screens use a small electric pulse for each pixel to excite the rare natural gases argon, neon and xenon, used to produce the color information and light. As electrons excite the phosphors, oxygen atoms dissipate and create plasma, emitting UV light. Brightness will fade over time, and the intense light-emitting process can cause image burn-in, especially if a static picture is left to display for long periods of time.
Liquid crystal display has proven its reliability, low power consumption and compact size in laptops and mobile display applications over the years. The technology is quite efficient and reliable; it can substantially improve mean time to failures, while reducing overall maintenance costs in any professional facility.
Unlike CRT or PDP, LCD is a transmissive technology. A light source is projected through an LCD membrane to make the image viewable. LCD uses a fixed array of pixels suspended in a liquid that is sandwiched between transparent panels. Behind this liquid crystal sandwich is bright florescent light, though a white LED array will replace these in the near future.
Each pixel is electrically polarized to either let the light pass through or not. Color filters are used to determine red, green and blue colors. By applying voltage to the pixels, they can be darkened to prevent the backlight from showing.
In well-lit environments it does quite well, but in low light and at low video levels the light source does remain somewhat visible. LCDs also have some image retention (or image lag), which is constantly being improved and reduced.
LCD displays are relatively light, consume small amounts of power and dissipate less heat than CRTs or PDPs, so they’re a great choice for OB van applications. Like PDPs, LCDs offer large viewing areas with small footprints.
When it comes to evaluating CRT monitors we use traditional methods; our criteria is based on performance on static test signals, like SMPTE color bars or modulated and unmodulated stair steps. We look for proper black levels, color temperature, peak brightness, dynamic range and display contrast. We also look for gamma and color gamut performance and luminance linearity. The latest generation of FPDs performs quite well on all these static tests as well.
One important distinction, however, is that CRTs can handle interlaced signals easily, but PDPs and LCDs cannot. They’re progressive format displays—the biggest difference in display quality is seen when observing images in motion. Noticeable display artifacts can occur with poor video processing.
Some key tests needed to evaluate an FPD’s dynamic display performance are horizontal and vertical resolution, motion adaptive de-interlacing, directional interpolation and film mode detection (inverse 3:2).
Horizontal and vertical resolution determines detail reproduction capability. We use a test pattern with alternating black/white horizontal and vertical lines. The pass criteria is to clearly see visible horizontal and vertical lines. Blurry lines imply poor scaling performance, while missing lines or flickering implies lack of appropriate de-interlacing capability within the display’s processing.
Motion adaptive de-interlacing requires sophisticated motion detection to be able to alternate between “bob” and “weave” based on image motion. Decisions should be made at the pixel level. To pass, a motion adaptive de-interlacing algorithm must show clean text.
Directional interpolation makes up for loss of resolution in moving objects and interpolates along the diagonal edges to recover lost resolution. Finally, film mode detection (inverse 3:2) format converters need to recognize 3:2 inserted at the telecine. When film mode is not properly detected, half-resolution images result. When a detailed film scene is processed, the pass criteria is to produce an image free of Moiré patterns.
When combined with the right processing, LCDs and PDPs can replace CRTs for professional applications. Make sure the de-interlacing processor supports full resolution. If the wrong processing decisions are made, half of the image resolution will be displayed.
Thoroughly test all static and dynamic criteria before making a final decision. There are many criteria in evaluating fixed pixel displays; both static and dynamic performance is driven by the capability of the video processor. Seriously consider an external display processor with any FPD, such as Teranex’s ClearVue. You can also evaluate DTV format converter performance with the “HQV” DVD from Silicon Optix.
Mike Poirier is director of product marketing and communications for Teranex. Contact him at firstname.lastname@example.org.
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