Match Me If You Can
1:1 pixel mapping is a hit-or-miss job with today’sprojectors and monitors.
Sanyo’s PLV-Z1 front projector’s unusual 964x544 pixelcount is one of many resolutions difficult to match to videocontent.
A long time ago, in a production studio far, far away, there werevideo monitors. And all of these monitors used cathode-ray tubes(CRTs). In this distant time and place, we didn't worry about how muchresolution these monitors could provide; we just turned them on andcalibrated them as best we could to display 525-line (or 625-line)interlaced, standard-definition video. Back then, if we ever discussedthe picture resolution of a CRT monitor, we spoke of “electrongun beam spot size” or “shadow masks.” Things havechanged.
The explosive growth of fixed-pixel imaging technology over the pastdecade has introduced a new phrase to the industry lexicon. Now, whenwe refer to projectors and monitors, our attention is focused more onthe projector's or monitor's “native resolution” — ameasurement derived from a specific horizontal and vertical pixelarray.
Sounds simple enough, doesn't it? As usual, the devil is in thedetails. The process of matching up video-standard pixel resolutionsand computer-standard pixel resolutions is often akin to fitting asquare peg into a round hole. Indeed; it is a rare occurrence when thepath from image source to screen is straight and true with no formatconversion whatsoever.
Every solid-state imaging system in use today, with the exception ofSony's Grating Light Valve (GLV) technology, has some sort of pixelmatrix and functions either as a light shutter (digital lightprocessing, transmissive or reflective liquid crystal) or as anemissive light source (plasma and organic light emitting diodes).
Those pixels are almost always square in shape (there are someexceptions), and the pixel array usually conforms to a PC displaystandard or variations of it. The LCD imagers used in front projectorsand rear-projection monitors typically offer XGA (1024×768 pixels)or SXGA (1280×1024 pixels) resolution, while front projectors addSVGA (800×600) pixels to that list.
There are variants on these formats. The SVGA digital micromirrordevices made by Texas Instruments have a native resolution of848×600 pixels. JVC's SXGA Direct Drive Image Light Amplifiers (afancy name for liquid crystal on silicon) sport 1365×1024 pixels.And there are now oddball LCD imagers coming to market withnon-standard pixel counts like 964×544, found in Sanyo's PLV-Z1front projector.
Want more examples? In the DMD world, we'll find 848×480 and1280×720 pixel devices, while there are now 858×488,960×520, 1280×768, and 1366×768 polysilicon LCDs.Toshiba recently introduced a rear-projection TV that is supposed touse 1920×1080 pixel LCoS devices, and JVC now offers a2048×1536 QXGA D-ILA panel in addition to its newest 4×3offering, which measures 1440×1050 pixels and is labeled“SXGA Plus.”
Confused yet? It gets worse. Plasma and LCD monitor and TVmanufacturers have adopted a crazy quilt of pixel arrays, from Wide VGA(852×480 or 853×480) and Wide XGA (1280×768,1365×768, and 1366×768) to Fujitsu's and Hitachi's ALiSdesigns (852×1024 and 1024×1024) and the new 1024×768non-square format adopted by NEC, Panasonic, and Pioneer. Note that allof these are 16×9 panels, even the last two which use non-squarepixel shapes.
And you thought the 18 different ATSC standards were confusing. This“resolution confusion” problem is further exacerbated by atrend among manufacturers of CRT-equipped, HDTV-ready direct-view andrear-projection monitors to use one and only one horizontal scan rate— 33.75kHz, the standard for 1080i/30. The reason? It keeps thecost and complexity of components down.
This means that while 1080i HDTV is shown at its native scan rate onthese TVs and monitors, all other signals are format-converted to540p/60 (which is, conveniently, one-half of 1080i). The oscillatorfrequency never changes, but progressive-scan signals go“up” from 480p or “down” from 720p beforemaking it to the screen. To accomplish this, picture information iseither sampled and pixels are increased or decimated as required.
The issue of what “native” rate to use came up recentlywhen I received an email asking whether the writer should set theoutput of his new Zenith HDV420 terrestrial DTV set-top tuner to 1080ior 720p before connecting into a 42in. plasma monitor. In essence,should he format-convert before connecting the signal, or let theplasma panel do the funky math? (Keep in mind that his panel, aPanasonic WVGA design, had only 852×480 pixel native resolution,so pixel decimation was inevitable somewhere along the line.)
So, we have video and PC signals arriving in one format that may ormay not be converted to a second format for display. And, in somecases, a de facto third format conversion comes into play! The picturetubes and projection tubes used in most HDTV-ready monitors and TVs donot have sufficient resolving power to show all of the detail in 1080iimages. At best, they are 810i or 960i devices.
There's also the issue of converting interlaced scans to progressivescan. 1080i sources are so affected, as are all NTSC or PAL videofeeds, meaning motion artifacts and possible 3:2 pulldown to correct.(The ALiS plasma panels use an alternating line presentation ofprogressive-scan picture information to reduce power consumption.)
Sometimes we get lucky with matching source to display. One examplethat jumps to mind is progressive-scan DVD playback at 704×480pixels, a very close match to the 848×480 or 852×480 WVGAstandards for some projectors and monitors. While not a true 1:1 pixelmap, it's darn close and the image quality can be exceptional.
Another good example is 720p HD content viewed on one of the newfront projectors or RPTVs using Texas Instruments' Mustang/HD2 DMDs.These have the exact same resolution (1280×720 pixels) and —assuming you have a set-top box, D-VHS, hard drive recorder, or otherplayback system with true 720p output — the picture quality canbe outstanding.
Until recently, there haven't been any practical implementations ofdevices capable of 1920×1080 pixel resolution. But Toshiba's newLCoS TV uses panels with a 1:1 map, as does LG Philips' yet-unbranded52in. LCD monitor and Samsung's new 54in. LCD display. However, unlessyou have one of these products, you'll have to settle for a pixelformat conversion to match your monitor or TV.
For 1280×720 devices, displaying 1080i content means a pixeldecimation and picture re-map to 66% of the total available pixels. Thegood news is that it's almost a linear conversion in both horizontaland vertical. Re-mapping 1080i to 1365×768 WXGA for plasma is alsoa linear down-conversion of 29% (and that's close enough for1366×768, too).
It takes a bit of nonlinear math to fit 1280×768, a commonstandard for LCD TVs and Pioneer plasma monitors. The horizontal pixeldecimation is about 33%, but the vertical pixel resizing is only 29%.For 1024×1024 ALiS panels, it gets even stranger! The verticalimage map is within 5.2% of full resolution, but the horizontal pixelsmust be sampled and compressed by 47%. This is why 1080i HDTV oftenlooks crisper on lower-resolution panels that employ a linear reductionand pixel decimation.
And of course, those 1024×768 non-square PDPs require anequally bizarre re-mapping process. Consider that a 4×31024×768 source image from a PC must be resized to 767×768non-square pixels for native display, while a 1920×1080 HD signalis decimated by 47% horizontally and 29% vertically! It's a dauntingtask, but can be pulled off with premium image scaling.
It's easy to assume that the designers of some of these projectorsand monitors were under the influence of hallucinogens. But there's abetter explanation, and that's the physical limitations of thefabrication process.
There is tremendous market pressure to bring “HD”resolution to all consumer and professional displays. 480p just doesn'tcut it anymore, and 800×600 (or 600p) imaging only survives infront projectors and some RPTVs because of attractive low prices. In aworld where the majority of computers have settled at 1024×768pixel resolution and the lowest HDTV picture display standard is1280×720p, products with lower resolution just aren't asattractive.
The unique 1024×1024 pixel matrix used by the Fujitsu-Hitachiplasma (FHP) factory on Japan's Kyushu Island came about because itsplasma fabrication line couldn't handle glass sizes larger than 42in.So, to increase pixel density and resolution, the engineers came upwith a pixel shaped like a brick standing on end. The smallest theycould make this pixel was .90mm × .51mm (which, oddly enough,works out to an aspect ratio of 1.7647:1, or close to 1.78:1).
At that size, FHP could cram 1,024 of them in both axes and call thefinished 42in. panel an “HDTV monitor”. Of course, this setoff a controversy within the Consumer Electronics Association (CEA) asto what exactly an “HD” monitor was, particularly since theCEA's own definitions call for 1:1 mapping of at least one HDTVstandard (usually 1280×720).
FHP also designed a 32in. panel that had a native pixel count of852×1024 pixels in a 16×9 aspect ratio. (These pixels measure.84mm × .39mm). This panel is also called an HDTV monitor, eventhough the highest resolution it can map close to 1:1 is 1024×768in a non-square format. The 1024×768 plasma monitors fromPanasonic, NEC, and Pioneer contain non-square pixels with an unusualsize all their own (NEC's pixels measure .897mm × .657mm).
Manufacturers of 50in. and 60in. plasma sizes haven't been asconstrained. Their pixels can be larger — often approaching oreven exceeding 1mm in pitch — so it's much easier to come up witha proportionate, true HD pixel count. Widescreen LCD monitors and LCDTVs can use smaller pixels with the result that all LCD TVs above 22in.are now at 1280×768 pixel resolution. The notable exception isSamsung's LTM245W 24in. LCD TV, which has a native resolution of1920×1200 pixels.
But there are still very few projectors and monitors that match HDTVdisplay formats exactly. The trend has always been to conform to PCdisplay standards and widescreen variations, and unfortunately none ofthe ATSC HDTV formats exactly corresponds to PC standards. They comeclose, but no cigar.
Is this a big problem? From the professional and industrialperspective, the actual pixel count matters only if you are performingimage manipulation and need to see as much detail as possible. Forcolorists, digital transfers, and animators, the goal should be tomatch the resolution of the display as closely as possible to thesource resolution. (Issues with white balance and grayscale onprojectors and monitors are best left to a future discussion.)
If this isn't possible, a display that performs a linear pixelconversion in both axes will yield the sharpest and cleanest images.(For this reason, 720p and 1080i HD content both look cleaner andcrisper on 852×480 plasma than they do on 1024×1024ALiSpanels!) If you are merely editing, mixing, dubbing, or otherwisesynching up picture and audio elements, the actual resolution of yourprojector or monitor isn't as important.
Corporate and educational users aren't as fussy, either. Becausemost current-model projectors and monitors can map XGA 1:1 and do agood approximation of SXGA, that satisfies most of their needs. Forpublic display and signs, the actual resolution of plasma and LCDmonitors is almost irrelevant because text, photos, and graphics areusually VGA or SVGA, either mapped 1:1 or scaled up to fit availablepixels.
The most finicky customers are home theater enthusiasts, many ofwhich go to great extremes to precisely match all of their videosources to plasma TVs and front projectors. Some of these displays haveDVI inputs, but there are few DVI sources available to drive them atfull resolution. Some exceptions are video scalers made by Key Digital,Focus Enhancements, Faroudja, and others.
For your purposes, you are best off mapping your source images asclosely as possible to the full pixel count of the projector or monitoryou've purchased. All of these products have internal PC and videoscalers that provide mixed results — down-rezzing usually looks alot better than up-rezzing — but there are plenty of aftermarketscaler and scan converter “solutions” to be had that canclean up these pixel re-mapping jobs.
The solution to this tangled web is to move toward all-digitalinterfaces like DVI. Granted, there will still be a fair amount ofimage scaling going on to make the tricky handshake work between TV andPC standards.
But it's possible that newer, smarter chipsets with a large numberof display standards in memory will be able to perform the necessaryscaling while minimizing artifacts.
If you are considering purchasing a video scaling product to driveyour new DVI-equipped projector or plasma/LCD monitor, I stronglysuggest you choose from models equipped with DVI-D outputs as opposedto depending on analog component signal interfaces. There is anoticeable difference in image quality by sticking with an all-digitalpath, particularly if the scaler also supports SDI or HD-SDIinputs.
For everyday viewing of video and HDTV, it's best to match theoptimum resolution or scan rate of your TV, monitor, or projector. Iuse a Toshiba 34HF81 34in. flat-screen CRT to watch HDTV in my familyroom, and the only HD rate it supports is 1080i. 720p is ignoredaltogether, and using 480p requires my changing the TV's aspect ratiosetting. So, my Samsung SIRT-165 DTV set-top receiver is set to convertall received DTV programs to 1080i output for convenience, highestresolution, and practicality.
On the other hand, my home theater uses a Sony VPH-D50HTU three-tubeCRT projector, which is a true multiscan device. In this case, I leavemy set-top tuner (Panasonic TU-DST51A) set to “native”format output, which lets me watch 480p, 720p, and 1080i programs theway they were encoded and sent out by the networks. Even 480i DTVprograms pass through and appear unadulterated, scan lines and all!
For the future, the holy grail is to employ imaging matrices withfull HD resolution of 1920×1080 pixels, or something close to it.But that's still a long way off, despite what Toshiba and JVC haveachieved. Until then, manufacturers of projectors and monitors willcontinue to use PC-based standards, while video source material iscreated in 480i, 480p, 576i/p, 720p, and 1080i/p digital formats.
Pete Putman is president of Roam Con-sulting Inc. of Doylestown, Pa.He can be reached at firstname.lastname@example.org.
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