High Definition Television
May 1, 1998 12:00 PM, Peter H. Putman
Unless you've been marooned on a desert island for the past few years, you've surely heard the hype about the new television standard to be phased in over the next ten years or so. Popularly known as high-definition television or HDTV, this is a system of transmitting and viewing images (and hearing audio) vastly different from the old NTSC system we've come to know and love over the past 50 years.
As usual, the general press has completely confused everyone with accounts of the various digital standards and widescreen images, not to mention the umpteen million high resolution picture formats. Some digging around on my part for another article on HDTV revealed a similar amount of confusion within the broadcast and production industries, particularly when it comes to the eventual adoption of an HDTV standard.
Well, we all know about standards-there are hundreds of them flowing through the cables in our boardroom, classroom and exhibit installations. Have you had enough fun with all the variations of VESA, XGA and workstation sync and refresh rates? Get ready for a few more, because the actual HDTV/DTV (that's digital television) standards have more than a dozen variations.
The digital difference The key to understanding HDTV in 1998 (and how it differs from NTSC) is to first grasp this point: HDTV is currently defined as a digital standard, not analog. Admittedly, the HDTV signal must at some point be converted back to an analog signal to be viewed on a monitor or projector. Otherwise, it remains a large collection of data bits.
In contrast, NTSC (and PAL) is an analog signal subject to the vagaries of analog signal artifacts and noise. You can see an NTSC signal on a conventional waveform analyzer, but not a baseband digital HDTV signal. Older versions of HDTV, such as Japan's 1125/50 system, are analog signals combining luminance, chrominance and sync burst just like an NTSC signal. They too can suffer from noise and signal degradation.
Adopting a digital television system is one way to clean up TV pictures and filter out picture artifacts. It is also a great way to move the large amounts of data around that an HDTV image requires, using MPEG video compression and expansion. Most importantly, a digital system means thatmultiple picture formats can be transmitted within each HDTV channel, a technica l feature that appeals to some broadcasters and repels others.
The current FCC standards for high-definition television are based on work done by the Advanced Television Standards Committee (ATSC). Its goal was to come up with both a higher resolution and widescreen version of television, and they've done that in several different ways that leave the ultimate standard defining to the producers and broadcasters who buy the equipment.
Just the sync rates At present, there are two pixel-by-line specifications for HDTV in the ATSC standard. The first calls for a picture with 1,920 horizontal pixels by 1,080 vertical lines, while the second calls for a picture with 1,280 horizontal pixels by 720 vertical lines. The aspect ratio of both formats works out to 16:9 (there's that magic number), and the vertical resolution of both formats exceeds the more than 480 lines now found in the NTSC system. Thus, these two formats are the only true HDTV standards on the table.
ATSC also proposed several digital standard definition television (SDTV) formats. The first measures 704 horizontal pixels by 480 lines, and the second 640 pixels by 480 lines. The 704 x 480 format is considered 16:9, although the math tells you it's more rectangular in shape. Of course, 640 x 480 is identical to the garden-variety 31.5 kHz VGA computer display.
Now, here's where things get tricky. A broadcaster can transmit one 1,920 x 1,080 interlaced signal within a channel, or several 480-line images. Or, he could combine two 640 x 480 signals with one 1,280 x 720 image. Yet another option would be just to transmit a slew of 480-line digital signals, all in 4:3 format. All of these options are possible because of the digital bitstream that characterizes HDTV.
To further complicate matters, there are several picture refresh rates and scan formats that can be selected. The 1,920 x 1,080 format can be delivered at 24 or 30 frames per second in progressive scan, or at 60 fields per second with an interlaced scan. Ditto for the 1,280 x 720 format at 24 and 30 frames per second, except that its 60 Hz rate is progressive, not interlaced. The SDTV signals can be delivered with 24, 30 or 60 frames per second progressive scanning and 60 field interlace scanning.
All of this is probably beginning to look like the computer monitor scan/sync rate tables found in Extron's Handbook of Computer/Video Interfacing. The truth is, only a few of these formats are currently supported. The 1,920 x 1,080 interlaced specification looks like it will be used by at least two major television networks (CBS and NBC), while 704 x 480 progressive appears to be the other popular choice, as it was intended to be backwards-compatible with existing 525-line interlaced signals. Dual-standard production equipment will be able to record 16:9 and 4:3 in this mode, currently favored by Fox and ABC.
Just what does all this mean to an A-V professional? In terms of signal conversion and displays, decoded 1,920 x 1,080 interlaced images fall somewhere between XGA and SXGA in terms of vertical resolution, although the picture sync rate is much lower (less than 34 kHz), thanks to the interlaced signal. This means that variable-resolution displays that use CRTs to write scan lines are HDTV-ready if they can support at least VESA 1 (35.2 kHz, 56 Hz) scan rates and have Y/Pb/Pr component inputs.
There's a catch. The vast numbers of plasma display panels coming into the market have a native resolution of 852 x 480-closer to the 480 progressive standard than true HDTV. Some sort of pixel mapping has to happen to rewrite the high-definition images to fit these displays, which are really better suited to the 704 x 480 format.
Direct-view monitors with 16:9 aspect ratios and higher resolutions are already in the professional A-V market. Panasonic introduced the DTV-2730H 27 inch (686 mm) HDTV presentation monitor, with sync rates to 70 kHz horizontal and 120 Hz vertical. It supports both the 1,080 interlaced and 720 progressive standard, although its specified maximum resolution is given as 1,024 x 768 in RGB mode. Conventional multisync displays will not be able to show a full 16:9 image, unless their picture dimensions can be scaled down both horizontally and vertically or the HD-source signal is scan-converted.
In terms of bandwidth, a decoded HDTV signal should slide quite nicely along any existing RGBS plant wiring that has been optimized at the least for XGA graphics cards. The same holds true for distribution amps and switchers (remember, we're talking the decoded analog version here) because the highest progressive-scan mode likely to be used will be 1,280 x 720. In effect, decoded HDTV just becomes another RGB signal with a picture dimension easily configured and retained in an installation-grade, large-screen display.
For production and post-production facilities, the HDTV digital packet rate of 1.5 Gbps may require a massive rewiring and conversion to fiber-optic bundles to expand system bandwidth. But hold on, there's a proposed solution making the rounds from Tektronix. It calls for mezzanine-level (light-level) digital compression of HDTV signals into the 270 Mbps to 360 Mbps range to accommodate them within existing serial digital (SD) CCIR-601 installations.
The theory here is that both digital NTSC and HDTV can share the same space, using down and up conversion where needed to add things like local station "bugs," those annoying logos in the lower right corner of a TV image that seem to get bigger every year. This could help many broadcasters make the switch over to advanced digital television without driving them into bankruptcy.
For the A-V marketplace, the Tektronix scheme means that cable infrastructures serving digital NTSC video to displays could be adapted to piggy-back lightly compressed HDTV around at the same time. Indeed, several large-screen displays and interfaces now offer direct SDI inputs.
Note that most of these products are designed to accept serial digital 525-line video, not digital HDTV. Even compressed HDTV will lose something in the translation when connected through a 4:2:2 input (it's sampled at a higher rate), so some sort of format converter will likely be needed.
Technical considerations aside, HDTV produces some dazzling pictures. That, along with the ability to show widescreen video, is really what the buzz is all about. From an infrastructure standpoint, the real technical and financial burdens will fall upon producers and broadcasters of HDTV programming. For them, this means adding digital equipment to analog facilities or upgrading existing digital plants that may not be all that old to begin with. It's a task tantamount to climbing Mt. Everest in their eyes-lots of money must be expended, and the financial risks could be extreme.
>From the professional A-V viewpoint, we're essentially already on top of that mountain, waiting for everyone else to catch up. In the meantime, it wouldn't hurt to bone up on digital signal compression and processing techniques while paying close attention to developments in the interface and display markets. Look for a few HD-compatible projectors, monitors and video scalars at INFOCOMM '98, and check out the large-venue section of the Projection Shoot-Out, where one of the test sources will be HDTV.
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