MATCH ME IF YOU CAN

Is 1:1 pixel mapping possible with today's projectors and monitors?

by Peter H. Putman, CTS

The explosion in flat-panel imaging has introduced a new phrase to the industry lexicon. It used to be that we talked about electron gun beam spot size or shadow masks. But things have changed! Now, when we refer to projectors and monitors, we usually mention the 'native resolution' in terms of a specific horizontal and vertical pixel array.

Every solid-state imaging system in use today with the exception of Sony's Grating Light Valve (GLV) technology has some sort of pixel matrix and functions either as a light shutter (digital light processing, transmissive or reflective liquid crystal) or as an emissive light source (plasma and organic light emitting diodes).

Those pixels are almost always square in shape (there are some exceptions) and the pixel array usually conforms to a PC display standard or variations of it. The LCD imagers used in front projectors and rear-projection monitors typically offer XGA (1024x768 pixels) or SXGA (1280x1024 pixels) resolution, while front projectors add SVGA (800x600) pixels to that list.

There are variants on these formats. The SVGA digital micromirror devices made by Texas Instruments have a native resolution of 848x600 pixels. JVC's SXGA Direct Drive Image Light Amplifiers (a fancy name for liquid crystal on silicon) sport 1365x1024 pixels. And there are now oddball LCD imagers coming to market with non-standard pixel counts like 964x544, found in Sanyo's PLV-Z1 front projector.

Want more? In the DMD world, we'll find 848x480 and 1280x720 pixel devices, while there are now 858x488, 960x520, 1280x768, and 1366x768 polysilicon LCDs. Toshiba recently introduced a rear-projection TV that is supposed to use 1920x1080 pixel LCoS devices, and JVC now offers a 2048x1536 QXGA D-ILA panel in addition to their newest 4x3 offering, which measures 1440x1050 pixels and is labeled 'SXGA Plus'.

It gets worse. Plasma and LCD monitor and TV manufacturers have adopted a crazy quilt of pixel arrays, from Wide VGA (852x480 or 853x480) and Wide XGA (1280x768, 1365x768, and 1366x768) to Fujitsu's and Hitachi's ALiS designs (852x1024 and 1024x1024) and the new 1024x768 non-square format adopted by NEC, Panasonic, and Pioneer. Note that all of these are 16x9 panels, even the last two which use non-square pixel shapes.

STOP MAKING SENSE

And you thought the 18 different ATSC standards were confusing. This 'resolution confusion' problem is further exacerbated by a trend among manufacturers of CRT-equipped HDTV-ready direct-view and rear-projection monitors to use one and only one horizontal scan rate - 33.75 kHz, or the standard for 1080i/30. The reason? It keeps the cost and complexity of components down.

This means that while 1080i HDTV is shown in its native format on these TVs and monitors, all other signals are format-converted to 540p/60 (which is, conveniently, one-half of 1080i). The oscillator frequency never changes, but progressive-scan signals go "up" from 480p or "down" from 720p. To accomplish this, picture information is either sampled and increased or decimated as required.

The issue of what 'native' rate to use came up recently when I received an email asking whether the writer should set the output of his new Zenith HDV420 terrestrial DTV set-top tuner to 1080i or 720p before connecting into a 42" plasma monitor. In essence, should he format-convert before connecting the signal, or let the plasma panel do the funky math? (Keep in mind that his panel, a Panasonic WVGA design, had only 852x480 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 or may not be converted to a second format for display. And, in some cases, a de facto third format conversion comes into play! The picture tubes and projection tubes used in most HDTV-ready monitors and TVs do not have sufficient resolving power to show all of the detail in 1080i images. At best, they are 810i or 960i devices.

There's also the issue of converting interlaced scans to progressive scan. 1080i sources are so affected, as are all NTSC or PAL video feeds. This means there will be motion artifacts and possible 3:2 pulldown to correct. (The ALiS plasma panels use an alternating line presentation of progressive-scan picture information to cut down on power consumption).

TO WYSIWYG OR NOT?

Indeed, it is a rare occurrence when the path from image source to screen is straight and true with no format conversion whatsoever. One example that jumps to mind immediately is progressive-scan DVD playback at 704x480 pixels, a very close match to the 848x480 or 852x480 WVGA standards for some projectors and monitors. While not a true 1:1 map, it's darn close and the image quality can be exceptional.

Another good example is 720p HD content viewed on one of the new front projectors or RPTVs using Texas Instruments' Mustang/HD2 DMDs. These have the exact same resolution (1280x720 pixels) and - assuming you have a set-top box, D-VHS, hard drive recorder, or other playback system with true 720p output - the picture quality can be outstanding.

Until recently, there haven't been any practical implementations of devices capable of 1920x1080 pixel resolution. But Toshiba's new LCoS TV uses panels with a 1:1 map, as does LG Philips' yet-unbranded 52" AM LCD monitor. Unless you have one of these products, you'll have to settle for a format conversion to match your monitor or TV.

For 1280x720 devices, this means a pixel decimation and picture re-map to 66% of the total available pixels. The good news is, it's almost a linear conversion in both horizontal and vertical axes to get to 1280x720 from 1920x1080. Getting to 1365x768 WXGA for plasma is also a linear down-conversion of 29% (and that's close enough for 1366x768, too.)

1280x768, a common standard for LCD TVs and Pioneer plasma monitors, takes a bit of non-linear math to fit. The horizontal pixel decimation is about 33%, but the vertical pixel re-sizing is only 29%. For 1024x1024 ALiS panels, it gets even stranger! The vertical image map is within 5.2% of full resolution, but the horizontal pixels must be sampled and compressed by 47%. This is why 1080i HDTV often looks crisper on lower4-resolution panels that employ a linear reduction and pixel decimation.

And of course, those 1024x768 non-square panels require an equally bizarre re-mapping process. Consider that a 4x3 1024x768 source image from a PC must be resized to 767x768 non-square pixels for display, while a 1920x1080 HD signal is decimated by 47% horizontally and 29% vertically! It's a daunting task, but can be pulled off with premium image scaling.

WHO DESIGNED THIS MESS?

For the end-user, it's understandable to assume that the designers of some of these displays were under the influence of hallucinogens. But there's a better explanation, and that's the physical limitations of the fabrication process.

There is tremendous market pressure to bring "HD" resolution to all consumer and professional displays. 480p just doesn't cut it anymore, and 800x600 (or 600p) imaging only survives in front projectors and some RPTVs because of the attractive low prices. In a world where the majority of computers have settled at 1024x768 and the lowest HDTV picture display standard is 1280x720p, products with lower resolution just aren't as attractive.

The unique 1024x1024 pixel matrix used by the Fujitsu-Hitachi plasma (FHP) factory on Kyushu Island in Japan came about because their plasma fabrication line couldn't handle glass sizes over 42". So, to increase pixel density and resolution, the engineers came up with a pixel shaped like a brick standing on end. The smallest they could make this pixel was .90mm x .51mm (which, oddly enough, works out to an aspect ratio of 1.7647:1 or close to 1.78:1).

At that size, they could cram 1024 of them in both axes and call the finished 42" panel an "HDTV monitor". Of course, this set off a controversy within the Consumer Electronics Association (CEA) as to what exactly an "HD" monitor was, particularly since the CEA's own definitions call for 1:1 mapping of at least one HDTV standard (usually 1280x720).

FHP also designed a 32" panel that had a native pixel count of 852x1024 pixels in a 16x9 aspect ratio. (These pixels measure .84mm x .39mm). This panel is also called an HDTV monitor even though the highest resolution it can map close to 1:1 is 1024x768 in a non-square format. The 1024x768 non-square plasma monitors offered by Panasonic, NEC, and Pioneer have an unusual size all their own (NEC's measure 897mm x .657mm).

CAN'T ANYBODY HERE MATCH MY DISPLAY?

Manufacturers of 50" and 60" plasma sizes haven't been as constrained. Their pixels can be larger - often approaching or even exceeding 1 mm in pitch - so it's much easier to come up with a proportionate, true HD pixel count. Widescreen LCD monitors and LCD TVs can use smaller pixels with the result that all LCD TVs above 22 inches are now at 1280x768 pixel resolution. The notable exception is Samsung's LTM245W 24" LCD TV, which has a native resolution of 1920x1200 pixels.

But there are still very few projectors and monitors that exactly match HDTV display formats. The trend has always been to conform to PC display standards and widescreen variations, and unfortunately none of the ATSC HDTV formats exactly correspond to PC standards. They come close, but no cigar.

Is this a big problem? From the professional and industrial perspective, the actual pixel count only matters if you are performing image manipulation and need to see as much detail as possible. For colorists, digital transfers, and animators, the goal should be to match the resolution of the display as closely as possible to the source resolution. (Issues with white balance and grayscale on projectors and monitors are best left to a future discussion.)

If this isn't possible, a display that performs a linear pixel conversion in both axes will yield the sharpest and cleanest images. (Believe it or not, 720p and 1080i HD content both look cleaner and crisper on 852x480 plasma than they do on 1024x1024ALiS panels!) If you are merely editing, mixing, dubbing or otherwise synching up picture and audio elements, the actual resolution isn't as important.

Corporate and educational users aren't as fussy, either. Since most of the projectors and monitors can map XGA 1:1 and do a good approximation of SXGA, that satisfies most of their needs. For public display and signs, the actual resolution of plasma and LCD monitors is almost irrelevant as the text, photos, and graphics seen on these displays is usually VGA or SVGA, either mapped 1:1 or scaled up to fit the available pixels.

The most finicky crowd are home theater enthusiasts, many of which go to great extremes to precisely match all of their video sources to plasma TVs and front projectors. Some of these displays have DVI inputs, but there are few DVI sources available to drive them at full resolution. Some exceptions are video scalars made by Key Digital, Focus Enhancements, Faroudja, and others.

For your purposes, you are best off mapping your source images as closely as possible to the full pixel count of the projector or monitor you've purchased. All of these products have internal PC and video scalars that provide mixed results - down-rezzing usually looks a lot better than up-rezzing - but there are plenty of aftermarket scalar and scan converter 'solutions' to be had that can clean up these pixel re-mapping jobs.

HELP IS ON THE WAY (SORT OF...)

The solution to this tangled web is to move towards all-digital interfaces like DVI. Granted, there will still be a fair amount of image scaling going on to make the tricky handshake work between TV and PC standards. But it's possible that newer, smarter chip sets with a large number of display standards in memory will be able to perform the necessary scaling while minimizing artifacts.

If you are considering the purchase of a video scaling product to drive your new DVI-equipped projector or plasma/LCD monitor, I strongly suggest you choose from models with equipped DVI-D outputs as opposed to depending on analog component signal interfaces. There is a noticeable difference in image quality by sticking with an all-digital path, particularly if the scaler also supports SDI or HD-SDI inputs.

For everyday viewing of video and HDTV, it's best to match the optimum resolution or scan rate of your TV, monitor, or projector. I use a Toshiba 34HF81 34" flat-screen CRT to watch HDTV in my family room and the only HD rate it supports is 1080i. 720p is ignored altogether, and using 480p requires me to change the TV's aspect ratio setting. So, my Samsung SIRT-165 DTV set-top receiver is set to convert all received DTV programs to 1080i output for convenience, highest resolution, and practicality.

On the other hand, my home theater uses a Sony VPH-D50HTU three-tube CRT projector which is a true multiscan device. In this case, I leave my set-top tuner (Panasonic TU-DST51A) set to 'native' format output, which lets me watch 480p, 720p, and 1080i programs the way they were encoded and sent out by the networks. Even 480i DTV programs pass-through and appear unadulterated, scan lines and all!

For the future, the 'holy grail' is to employ imaging matrices with full HD resolution of 1920x1080 pixels, or something close to it. But that's still a long way off, despite what Toshiba and JVC have achieved. Until then, manufacturers of projectors and monitors will continue to use PC-based standards, while video source material is created in 480i, 480p, 576i/p, 720p, and 1080i/p digital formats.

Copyright ©2003 Peter H. Putman / Primedia Business Media