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The Power Of Pixel Response

Analyzing the impact of LCD pixel response on images in motion.

Can blur be eliminated on an LCD?

The simple answer is no, not totally. Images captured in interlace format will always have some “blur” as a result of the de-interlace and interpolation process. Progressive images like 480p-60/50fps or 720p can have intra-frame blur of fast-moving images until technology produces shorter than frame rate exposure times for the acquisition device. Image retention during frame transition can also become blurred by the way your eyes perceive the motion. And in some rare cases, slow pixel response times may produce the blur.

At the current sample rates for interlaced video, pixel response is often the mistaken diagnosis for blur. Factors such as image processing, de-interlacing, scaling to fit the screen, and even the physical size of the LCD pixel cluster all add together to create motion artifacts that may appear differently on an LCD/TFT panel than on your CRT.

COMMON MYTHS ABOUT LCD MONITORS

Myth #1: LCD monitors are “instant-on” devices. Temperature stability is a major factor in LCD performance. All LCD monitors should be turned on with images displayed for 30 to 60 minutes before any critical viewing should occur.

Myth #2: LCDs don't burn in static images. While impossible in theory, some LCDs do experience image retention problems. In some cases, the valve function of sub-pixels can get “lazy” from lack of “exercise” and can be corrected over time by displaying moving images. Image retention can also be attributed to problematic de-interlace and interpolation techniques, where the “burn in” is retained in a portion of the memory ICs used to hold data for processing and display.

Myth #3: LCDs can't reproduce blacks. In some cases this is true, especially on LCD screens that have 6-bit color depth. With 262,000 possible color and luminance values per pixel, these pixels often have bigger steps at the lowest and brightest values. In the case of low luminance content, this can result in muddy or washed out images. To counter this, many LCD monitors are limited in range and generate hard black for images below a certain intensity (for example, 15 IRE). Eight-bit displays have more than 16 million possible values, but can experience the same lack of quantized resolution with lower luminance values and are often clipped to give the appearance of a “crisper” image. The best way to know what the LCD is really doing is to ask the monitor manufacturer to provide a Gamma curve, which will instantly show the luminance limits programmed into the monitor.

Myth #4: Higher resolution LCD screens generate the best video images. In the case of LCDs, resolution is determined by the image you're providing. If you're mainly viewing standard definition images, lower resolution LCD screens will actually generate better images. The key is to match the resolution of the LCD screen to the image format you generate.

Myth #5: LCD manufacturers have conspired to eliminate CRT displays. A major component of CRTs is the lead used in the mask of a CRT. Many countries and states in the U.S. (California, for example) have legislated bans or recycling fees for products with lead, identifying these items as possible environmental hazards. CRT technology isn't “green,” while the newer LCD screens are much more environment-friendly, consume much less power, and may even be designed for refurbishment as a form of recycling.

Robert Foster is national sales manager at Marshall Electronics, El Segundo, CA. He can be reached at rob@marshallelectronics.net.



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