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MillimeterSee All that You Can See: Video Monitors for the HDTV Age
Aug 1, 1999 12:00 PM, Peter Putman
It's 1953. The National Television Standards Committee (NTSC) has released a comprehensive set of technical parameters for color television broadcasting and display. Everything from the specific mathematical values for color and luminance components to phosphor color coordinates is contained in this document. The only problem is that no monitors exist with enough resolution, correct phosphors, and sufficiently accurate color decoding to show an NTSC signal-a problem that will not be overcome for nearly two decades.
It's 1999. Digital television is coming, and broadcasts have commenced in major markets. Studios are purchasing equipment for HDTV acquisition, editing, and postproduction. A comprehensive standard is in place, thanks to the efforts of the Grand Alliance and the Advanced Television Standards Committee. Once again, the parameters of color encoding and decoding, compression, bandwidth, and color phosphor coordinates are clearly spelled out.
And once again, very few broadcast, studio, postproduction, and consumer video monitors exist that can accurately show a 1920 x 1080i HDTV signal. Yes, they can get the colors decoded correctly. Yes, they use the specified phosphors. And all of them have the proper 16:9 image aspect ratio. What most of these monitors do not have is a fine enough CRT dot pitch to resolve 1920 pixels along 1080 vertically scanned picture lines-if they can even resolve the 1080 scan lines.
A Question of Pitch There are many technical hurdles to designing and manufacturing high-resolution, direct-view CRT monitors. Fortunately, phosphor accuracy, color decoders, and scan-rate compatibility are not big issues anymore. The HDTV color gamut essentially follows the SMPTE C and CCIR-601 color space. Microprocessor technology and retention of a component color system (Y, Pb, Pr) will ensure accurate color decoding, and multi-scan monitor technology is "old hat."
The big trade-off comes when you must choose between image brightness and dot size, or "pitch," of the CRT's electron gun. Computer monitors can have dot pitches as low as .22mm, making it relatively easy to map out 1024 x 768, 1280 x 1024, 1600 x 1200, 1920 x 1080 and other high-resolution display formats. The price that we pay for this added resolution? Small screen sizes.
Studio and group viewing calls for a larger-screen broadcast monitor. Generally, broadcast-quality monitors with picture sizes larger than 21" employ CRTs with coarse dot pitch aperture grills, which range from .6mm all the way to 1mm. While these monitors have picture scan rates compatible with a wide variety of video and computer signals, they do not display images with the same degree of resolution as computer monitors.
Do the Math Here's some simple math to better illustrate the problem. Let's say we are using a 20" 4:3 computer monitor with a dot pitch of .25mm to work at 1024 x 768 (XGA) resolution. Assuming the images we view on that monitor have all the detail we want, we'd have to find a 27" 4:3 monitor with a dot pitch of .31mm or a 37" 4:3 monitor with .54mm dot pitch to view the same images with the equivalent resolution.
One problem, though: No such monitors exist! With .75mm dot pitch (a common aperture grill size), we would need a 65" diagonal picture tube to see all the detail displayed on the 20" monitor. But the largest picture tubes currently manufactured measure only 37" diagonally. The problem worsens when viewing image details near the edge of the picture tube. Here, the deflection angle of the electron gun is quite severe, causing the dot shape to elongate and distort. This results in a larger spot size and further degrades picture resolution.
Confused about dot pitch? Think of an artist attempting to produce some very fine calligraphy with only crayons or felt-tip markers instead of a fine-point pen. The end result certainly looks like script with plenty of elaborate curves and filigrees, but none of it is legible-the "dot pitch" is just too coarse to capture all the subtleties of script lettering.
Newer picture tubes with .6mm dot pitch are specified as having SVGA (800 x 600) resolution and are now beginning to find their way into broadcast monitor applications. However, even with a .6mm dot pitch, our broadcast monitor's optimal picture tube size would have to measure about 52" diagonally, which is still too large to manufacture. So, we compromise by working with what is available and toss away some picture resolution in the process.
The truth is, 4:3 broadcast monitors in the 25" to 37" range with .7mm to .8mm dot pitch have just enough resolution to handle 525-line NTSC (analog or digital), plus 640 pixel x 480 pixel computer images. (At least one manufacturer of monitors-Sony-recognized this fact some time ago and started building scan converters into 27" 4:3 presentation monitors to convert higher resolutions down to VGA.)
Wider is Better? It should come as no surprise that many of the new 16:9 HDTV video monitors have the same resolution limitations as conventional 4:3 monitors. Flip through data sheets, and you will see that many evaluation and viewing monitors have a published specification of 700 (4:3) to 1000 (16:9) TV lines horizontally. This is about one-third to one-half the resolution needed to correctly show a 1920 x 1080 signal and one-half to three-fourths the required resolution for a 1280 x 720 signal.
Things are not that much better on the consumer side. Even the newest direct-view consumer HDTV sets are using picture tubes with only .6mm dot pitch and yield a maximum resolution of 800 pixels/lines. These sets do a fine job with 480p signals, but they display less than one-half of the total resolution in a 1080i picture. Older sets are still hamstrung with .7mm to .8mm dot sizes.
Even rear-projection TVs and monitors are handicapped by using 7" CRTs, which are primarily used for projecting video and VGA (640 x 480) pictures. The industry consensus is that it takes at least 9" tubes in a CRT video projector to do justice to a 1080i image, but such tubes add considerable size, weight, and cost to a projection system. The cheapest consumer-model, rear-projection HDTV set using 9" tubes (Philips' 64PP9901) costs about $10,000.
What all of this means is that we are rolling out a high-resolution TV system that exceeds the capacity of most devices used to display it! An average viewer will not see a tremendous difference between 480p and 720p material as a result-most monitors are incapable of showing all the extra detail. The use of set-top boxes that work at only two resolutions (1080i or 480i/p) and down-convert 720p decoded signals to 480p will further compound the problem.
This situation is not lost on many in the broadcast industry. The leap in image quality just from analog 525-line video to digital 480p is substantial for most viewers. There is even a school of thought that says 720p images are effectively as good as 1080i, given the loss of resolution due to interlacing and the fact that most displays cannot do 1080i justice. This is part of the reason for ABC's decision to transmit in the 720p format (a signal that many set-top HDTV decoders will down-convert to 480p anyway).
As a result of this resolution conundrum, there is a considerable "buzz" about plasma display panels (PDPs), large-screen monitors that can hang on a wall or sit discretely on small tabletops. Newer 50" models from Pioneer (PDP-v502x) and NEC (PlasmaSync 5000W) have defined pixel resolutions of 1280 x 768 and 1365 x 768, respectively. This makes both models immediately compatible with 720p signals as well as 480p and allows 1080i broadcasts to be shown with about 33-percent compression. However, the pixel sizes on many plasma displays are quite large-just under 1mm for the largest PDPs-and can result in a noticeable pixel structure.
A different problem faces manufacturers of rear-projection LCD and DLP monitors. Current models are only capable of XGA (1024 x 768) resolution, with 1280 x 1024 (720p compatible) not expected until next year. At present, neither Sony nor Epson have plans to produce 1920 x 1080 liquid-crystal panels, while Texas Instruments has only made a handful of prototype 1920 x 1080 digital micro-mirror devices. However, TI has announced partnerships with Hitachi and Mitsubishi to build all-digital HDTV sets for home use, presumably using 1280 x 1024 (16:9) DMDs.
Current Offerings Where is still a need for HDTV monitors, even if they cannot handle all the picture resolution. Several evaluation and presentation-quality models are now available. Many of the smaller picture sizes (20" and under) are using a fine-pitch picture tube similar to that found in a desktop computer monitor, but they are really intended for single-user viewing. The larger models (22" and up) have coarser resolution, but they can be located some distance from the viewer.
While the original NTSC specification called for a viewing distance of seven-times the screen height, 1080 signals are optimally viewed at three-times the screen height. This means that facilities installing HD monitors into existing editing, post, and graphics suites must either move the monitors closer to viewers to get the full benefit of the added resolution or upgrade to larger monitors.
For now, the majority of presentation-quality HDTV monitors measure between 22" and 30" diagonally and deliver optimum viewing distances of about five to seven feet. If you are in the market for HDTV monitors, be careful-some models use 4:3 tubes with masking to yield a 16:9 image. The process is like viewing letterboxed DVDs on a standard 4:3 television. Look for resolution specifications-if it goes up in 16:9 mode, chances are it uses a widescreen picture tube. If they go down, it's a 4:3 tube.
Sony's new BVM-D series digital studio/field monitors are available in sizes from 8" to 22.5" and are digital-ready with no external decoder cards required. The BVM-D9H1U/H5U AC/battery 4:3 monitors have 8" viewable areas and are compatible with 480i/p, 720p, and 1035i/1080i signals, although their resolutions are specified at only 340 TV lines in 16:9 mode (450 lines in 4:3). Sony's BVW-D14H1U/H5U 13" monitors claim 600 TV lines of resolution (800 lines in 4:3) while showing everything from 480i to 1080i.
The BVW-D20F1U evaluation monitor has a claimed resolution of 700 TV lines in 16:9 mode (900 in 4:3), while the BVM-D24E1WU evaluation monitor boasts of 1000 widescreen TV lines. You should note that all but the last monitor use conventional 4:3 tubes with a hard bezel mask to get a 16:9 image-widescreen images are letterboxed to fit the screen width, while 4:3 images fill the frame top to bottom. Only the BVM-D24E1WU uses a true 16:9 picture tube, which accounts for the increase in picture resolution when in widescreen mode.
For HDTV production, Sony offers the HDM-series evaluation and PHM-series viewing monitors. The PHM-14M8U, HDM-14E5U, and HDM-14E1U all feature 12" screens rated at 700, 700, and 600 TV lines, respectively. Unlike the D-series, these monitors can only accept 480p or 1035i/1080i signals. The 17" HDM-20M8U viewing monitor and HDM-20E1U evaluation monitor sport 700 and 850 lines of resolution with 480p and 1035/1080i compatibility, while the HDM-2830/4 evaluation monitor specs at 1000 TV lines (again 1035/1080i and 480p compatible).
Want plasma? Sony's PFM-500A1WU 42" PDP has a native pixel resolution of 852 x 480 and can show both 720p and 1080i as well as 480p and VGA through SXGA computer graphics. (Ditto the improved-contrast PFM-500A2WU 42" panel, which is also 852 x 480 native.) For increased resolution, Sony now offers the 42" PFM-510A1WU with 1024 x 1024 non-square pixels and full 1035/1080i, 720p, and 480p plus computer compatibility.
Panasonic has rolled out a few new models for digital and HD production. The DT-M3050W 30" DTV color monitor uses a super-flat picture tube with a dot pitch of .7mm and a claimed resolution of 640 TV lines in HD mode. It's an analog monitor that accepts decoded 480i/p, 720p, and 1080i signals. For those with a few more dollars to spend, Panasonic offers the AT-H3015W 30" HDTV master monitor. It sports a .36mm dot pitch and claims 1000 lines of resolution. Digital convergence and full 480-to-1080 compatibility are standard.
Other Panasonic monitors are good for less-critical viewing of DTV and HD signals. The DT-2750MS presentation monitor has a 27", 4:3 screen and 600 TV lines (800 x 600) of resolution, adequate for 480p applications. For portable use, the BT-S915DA 4:3 9" monitor will accept analog Y/Pb/Pr signals or SDI input via an optional ET-SD06U interface and will letterbox 1080i and 720p material. It claims 300 TV lines of resolution with a .5mm dot pitch CRT.
Panasonic has also gone "flat" with the PT-42P1 42" plasma panel. It has a native pixel count of 852 x 480 and is compatible with both DTV and HDTV signals, although the maximum native vertical image resolution is only 480 lines (33-percent reduction from a 1920 x 1080 signal). At this year's NAB and INFOCOMM, Panasonic showed a prototype 60" plasma monitor with native resolution of 1366 x 768 pixels-adequate for scaled displays of 1080 material and a one-to-one match for viewing 1280 x 720p programs. Contrast and brightness are considerably improved over older PDPs.
Princeton Graphics, long known for high-quality computer monitors, introduced a monitor specifically designed for HDTV viewing. The company first aimed the AF 3.0 HD 32" monitor (30" viewable) at the consumer market, but it is also perfectly at home in studio and broadcast HD applications. It sports a.6mm dot pitch and 600 TV lines (800 x 600) of resolution and is compatible with decoded 1080i, 720p, and 480i/p signals as well as VGA, SVGA, and XGA computer graphics. A built-in line doubler converts 525/625 inputs and 480i to 480p.
Barco-a name long associated with high-quality monitors-has one HD and a few digital offerings as well. The ADVM-28, -20, and -14 models are all 4:3 hybrid digital/analog monitors and are capable of accepting direct SD inputs. While not compatible with DTV signals, they are useful for monitoring the down-converting/up-converting of HD material to 480i or direct viewing of multiple 480i feeds. The ADVM-28 uses a .8mm dot pitch CRT, while the ADVM-20 and ADVM-14 are rated at .26mm dot pitch/1000 TV lines and .28mm dot pitch/750 TV lines, respectively.
Barco also offers the HDM-5049 17" 16:9 monitor, a true HDTV-compatible display with analog signal inputs. It uses a .26mm dot pitch CRT and claims 1200-plus lines of resolution. In addition to compatibility with 480i/p, 720p, and 1080i signals, Barco claims that the internal synch (30kHz to 82kHz) and bandwidth (135mHz) are sufficient to handle 1080p inputs, although the calculated bandwidth of a 1080p signal is somewhere around 180-plus mHz.
Finally, JVC has gone high-def with the DT-V2000SU 4:3 broadcast/edit monitor, a 20" display that is compatible with analog 480i/p, 720p, and 1080i formats. Resolution is specified at 900 TV lines in 4:3 mode and 700 TV lines in 16:9. It can also display computer signals from VGA to XGA. For true 16:9 imaging, JVC also offers the HV-M300VSU production monitor with a 32" screen. It has a dot pitch of about .68mm and claims 550 TV lines of resolution (800 x 600 pixels) in 16:9 mode. Two separate sets of Y/Pb/Pr or RGB inputs are standard for viewing 480, 720, or 1080 content.
JVC has also introduced a plasma product, the GD-V425PZW. Like most 42" PDPs, it supports wide VGA resolution (852 x 480 pixels) and can show 720p and 1080 with some compression in detail. It also supports both RGB and Y/Pb/Pr inputs. Although JVC has based the GD-V425PZW on a Fujitsu Electronics glass panel array, the company has done some work on the driving electronics to improve video quality, contrast, and color saturation.
Reach Peter Putman at phputman@pil.net or visit his Web site at: www.phpcom.com.
