Scaling New Heights
Apr 1, 1998 12:00 PM, Peter H. Putman
The 1998 trade show season is here, and the explosive growth inlarge-screen display technology shows no evidence of abating. It seems thatthere's always a new projector or monitor around every corner, a clevertwist to existing technology or a new marketing alliance. Almost lost inthis crossfire of electron beams, photons, twisting crystals and jigglingmirrors is a major change in the process used to match up incompatiblevideo signals and flat-screen displays. It's known as image scaling, and ithas come a long way from those pixelated, dithered video images so commonto early LCD and DLP projectors.
Video scalars evolved from the same school as line doublers and linequadruplers, both created to take advantage of higher resolution,multiscanning CRT projectors and monitors. In the line-doubling process,all 525 scan lines in a video signal are scanned progressively, like astandard RGB computer signal. It doesn't improve the resolution of theimage, but eliminates flicker. Line quadruplers go one step further andfill in the blanks by interpolating an extra scan line for each existingline. This results in twice the amount of scan lines, increasing theapparent resolution of the image and its brightness. It also kicks thehorizontal scan rate up by a factor of two to around 63 kHz.
A high-quality line doubler/quadrupler can be a worthwhile enhancement to avideo projector or monitor, so long as the display can show all of theresulting picture information. For faster scanning displays like CRTprojectors, this is a piece of cake. But that's not the case withflat-screen displays, whose native resolutions rarely match the output ofthe doubler or quadrupler.
Here's an example. A line-doubled NTSC video image will occupy less than60% of the available pixelson an 848 x 600 DLP projector. But aline-quadrupled NTSC image will far exceed the available pixels on thatsame projector. Going to a 1,024 x 768 projector won't help if theprojector overscans the image. What to do?
Enter the video scalar, a collection of microprocessors and frame buffersthat could care less about pixel counts and scan lines. Instead, a scalarlooks at the optimum settings for the projector or monitor to be used,performs some calculations, and enlarges the incoming video signal to aprecise fit. If the projector is using 832 x 624 LCD panels and is happiestwith 37.9 kHz/60 Hz signals, so be it. If 1,024 x 768 panels are employedand the projector's maximum sync rate is 57 kHz/70 Hz, then that's what thescalar will feed it.
A scalar's usefulness doesn't stop with flat-panel displays. CRT projectorsand monitors all have a sweet spot, a point where maximum resolution andmaximum brightness coincide, and the display works at optimum efficiency.For 7 inch (178 mm) CRTs, this is somewhere around SVGA resolution, or a 40kHz/60 Hz to 70 Hz maximum scan rate. For 8 inch (203 mm) CRTs, the sweetspot usually lies somewhere between XGA (1,024 x 768) and SXGA (1,280 x1,024) resolution, and 9 inch (229 mm) CRTs peak out slightly higher.Multiscanning CRT monitors in the 27 inch (686 mm) to 37 inch (940 mm)range are limited by their coarse dot pitch (0.75 mm to 0.85 mm) andtypically work best around 640 x 480 or 800 x 600.
Once the optimum scanning rates are determined for any of these displays,the scalar sends progressive-scan video at that optimum rate to theprojector. It may actually turn out that the sweet spot for a givenprojector/monitor is some crazy combination of scan lines and syncfrequencies, like 900 x 675 at 42 kHz/72 Hz. Again, the scalar simplyreprocesses the incoming signal and sends it.
Sounds easy, but designing a professional-quality video scalar is not easy.The best scalars I've seen to date are Snell & Wilcox' Supervisor andInterpolator products, both of which cost a pretty penny. The Interpolatorseries was developed as a product for high-end home theater installationsand can scale composite, component and 4:2:2 video to maximum resolutionsof 1,024 x 768 (standard Interpolator, $30,000) and 1,280 x 1,024(Interpolator Gold, $35,000). Both scalars look at each video frame andconvert video signals by looking at nine adjacent pixels for each pixel tobe converted, plus the next ten and preceding ten.
Faroudja has also been active with video scaling and has demonstrated acomparable product (PV400), which also scales composite and component videoup to 1,024 x 768. Both the Snell & Wilcox and Faroudja scalars can combinethe active video channel with a Windows overlay or underlay by keying acomputer source as a second video channel. This process lets you resize thescaled video to any size or aspect ratio, crop it or stretch/compress it.
There's interest in scalars at lower price points, too. CommunicationsSpecialties recently rolled out their Deuce scalar, which has six presetscalable resolutions-640 x 480, 852 x 480 (for plasma 16:9 monitors), 800 x600, 832 x 624, 1,024 x 768, and 1,280 x 1,024. It's not the full-rangesolution that Snell & Wilcox offers, but it is aimed at the flat-paneldisplay market and costs only $2,195. You'll no doubt see competitiveproducts introduced at INFOCOMM '98.
Like any other silicon-based technology, video scalars can only get betterand cheaper. Genesis Microchip supplies many of the video scaling enginesused in portable and desktop flat-screen projectors and introduced the gmZ1Advanced Image Magnification processor at INFOCOMM '97. It scales,de-interlaces and offers RGB overlay features similar to the processes usedin the Snell & Wilcox and Faroudja scalars.
Presently, scaling engines are limited to interlaced video sources,although many manufacturers use similar processes to scale RGBnon-interlaced signals up and down with mixed results. Expect moremanufacturers to jump on the video scaling bandwagon in the coming year,motivated by the growing demand for high-quality, lightvalve projectors using LCD and DLP technologies.