HDTV: boon or boondoggle?
Sep 1, 1998 12:00 PM,
Peter H. Putman
At INFOCOMM ’98, I participated with several industry notables in a paneldiscussion on the present and future of electronic display systems. One ofthe more interesting questions that came up involved high-definitiontelevision-specifically, the challenge of accommodating it in an existinginstall or incorporating HDTV capabilities into future projects.
There are a few considerations here, not the least of which is the scarcityof HDTV decoders and the lack of HDTV programming. Another problem is theaspect ratio: 16:9 versus the standard 4:3 we commonly use for screenspecifications. Finally, the HDTV signal in its purest form is a digitalbitstream that requires about 1.5 Gigabits per second data rates, somethingthat coaxial cable is hard pressed to handle and is better suited to fiberoptics.
The most likely scenario for conference rooms, classrooms and boardrooms isthat a set-top decoder box will be used to convert the digital data intoone of the many HD formats, which will be ported around as a conventionalanalog signal. Unless your company is involved with the design andconstruction of broadcast HDTV facilities, you’ll probably deal withfamiliar cable, distribution amps and switchers.
As I mentioned in a column earlier this year, there are several advancedtelevision standards proposed by the ATSC, only six of which are truehigh-definition television. The others are simply digital versions of ourexisting 525-line system (known as standard definition television or STV)or an anamorphic signal specification (704 X 480 non-square pixel, 16:9image).
The highest resolution HDTV standard provides for 1,920 pixels by 1,080picture lines progressively scanned at 24 or 30 frames per second orinterlaced at 60 fields per second. At present, CBS and NBC have announcedsupport for the 1,080-interlaced (1080i) standard, which means that thedecoded analog signal will have characteristics similar to SXGA (1,280pixels by 1,024 lines) computer displays.
The other true HD standard uses a picture measuring 1,280 pixels by 720picture lines progressively scanned at 24, 30 or 60 frames per second. ABCand Fox have announced support for this standard, which uses the samecompressed digital data rate as 1080i but has around 33% less verticalpicture lines. The decoded analog signal has a scan and refresh rate thatfalls somewhere between XGA (1,024 pixels by 768 lines) and SXGA.
If an existing installation isn’t set up to handle the required bandwidth,you’ll most likely see loss of fine-line detail in the resulting images,negating some of the advantages of an HDTV signal. Screen issues aside,this will be the biggest hurdle to overcome when integrating HDTV withconventional RGB computer displays and video.
How much bandwidth is enough for HDTV? Somewhere on the order of 35 MHz to40 MHz will be called for; if you’ve designed that much capacity in thesystem, it should be able to handle 1,280 X 720p signals, and 1,920 X1,080i images won’t require much more. Although the total pixel count hasincreased by 33%, the picture lines are interlaced, calling for a systembandwidth in the 40 MHz to 50 MHz area. If you are able to procure aconverter to show 1080 progressive HDTV, you’ll need to increase systembandwidth to around 70 MHz to 80 MHz.
Wired for YUV signal distribution? HDTV uses a similar component (Y, Cr,Cb) color system with one luminance and two color difference signalscompatible with standard RGB cabling. Unlike NTSC, this is the only way adecoded HDTV signal can be moved; there’s no composite or S-Video when withHDTV. Cable lengths won’t be any more critical than in a YUV system, butphasing errors in distribution and signal processing amps will play havocwith proper color rendering.
For those looking to retrofit an older single-coax or S-Video installation:Nobody (yet) makes a reasonably priced, professional-grade scan converterto knock down 1080i and 720p analog signals to baseband NTSC video. Firstof all, why bother? It negates the advantage of HDTV in the first place.Second, current models of scan converters aren’t designed to handle 16:9aspect ratio material.
As you can see, sending HDTV component signals around a system isn’tparticularly difficult if the cabling and bandwidth requirements are met.Decoded HDTV just looks like another computer display format to a switcheror distribution amp, but viewing HDTV is another matter altogether thatwill likely result in all kinds of compromises. It’s a problem that extendswell beyond the boardroom; there are millions of 4:3 standard-definitiontelevision sets in use that will require external conversion of HDTVsignals for viewing.
If you are locked into a 4:3, 480 scan line image format, it may be worthpurchasing a set-top box with consumer-level scan conversion to view the HDprogram content. This lowest-common-denominator approach probably won’tmake your customer happy, but he will be able to see what’s on the HDchannels.
If your system can handle higher-resolution images, you may be able to showone or more of the HD signals in all its glory. The catch is that you’llneed a variable resolution imaging system to do that, such as a CRT-basedprojector or retro monitor. XGA display systems may be able to lock on tothe horizontal sync rate, but they won’t be able to show the entire widthof the image without scaling down the HDTV signal, negating some of thehi-def benefits.
Additionally, 1,280 X 720p signals may work with a lot of signal crunchingon an XGA LCD or DLP projector, but you’ll be a lot happier viewing 720p ona 1,280 X 1,024 (or 1,365 X 1,024) system where all the horizontal pixelsand vertical lines will appear 1:1. Coincidentally, there is a lot ofinterest in SXGA displays-from reflective and transmissive LCD panels toDMDs.
If your client is making noises about HDTV capability, you’ll be safe ifyour project includes 8 inch (203 mm) and 9 inch (229 mm) CRT projectors(with their adjustable picture geometry) or an SXGA fixed-resolutionLCD/DLP/ILA projector. This way, you can say that the installation isHDTV-ready even if that’s only 50% true.
The 1080i standard presents the real problem. A 1,920 X 1,080 signal isabout as high as a high-resolution signal can be, and the only displayengines that can map every one of the pixels and scan lines with any degreeof brightness would be single-pixel liquid crystal light valves or CRTprojectors. Only the Hughes-JVC Model 200 currently uses a 16:9 aspectratio imaging device. Currently, no other manufacturers are making 16:9flat-panel imaging systems with more than 480 pixels of vertical imageresolution, nor are there great plans to do so. Even Texas Instrumentsisn’t entirely enamored with the idea and cost of producing 16:9 DMDs,especially when the 1,280 X 1,024 version is just coming to market.
As a result, you may want to use a set-top HDTV decoder that transcodes1080i to 720p or 720i, should they become available. As things stand now,the only transcoding done by set-top HDTV receivers will be down to 480p,or up to 1080i. With ABC and Fox originating 720p material, chances are aset-top box with 1080i transcoding will be available within a year.
There’s another way to integrate a sort-of HDTV signal. RCA has announcedthe DTC100, a set-top receiver/converter, which supports the 704 X 480non-square (anamorphic) format mentioned earlier. With a CRT projectionengine, the picture geometry can be adjusted to 16:9, which then matchesthe 42 inch (1,067 mm) 852 X 480, 16:9 plasma format.
The last part of the puzzle is the projection screen. Here again, it’s aquestion of “either/or.” Do you install a 16:9 screen and leave the sidesblank when viewing 4:3 material or specify a motorized screen with positionpresets? This question drove manufacturers crazy when trying to designdual-aspect sets for the consumer market.
Be careful if you are using a zoom lens on your projection system to fillthe available screen area when alternating between 16:9 and 4:3 sourcematerial. If the projector has a built-in image offset, the image will moveup the screen as you zoom out from a front/rear, right-side-up projectorand move down the screen with a front/rear inverted projector. For jumpingback and forth between these formats, zero offset is preferred. Failingthat, a motorized lens shift feature will be very handy.
In terms of infrastructure (and we haven’t even discussed audio), you mayfind your existing or planned installations are already HDTV-compliant, atleast at the lower (1,280 X 720) resolution. That may be all you need fornow. Both formats have been demonstrated at NAB ’98, and it is verydifficult to see the improvement from 720p to 1080i, particularly on ascreen measuring 27 inches (686 mm) to 42 inches (1,067 mm) diagonally.
If you think there’s any chance that HDTV support might be needed, checkwith your customer. Costs are still steep for source material, and set-topreceivers are still waiting in the wings, but it couldn’t hurt to build inthe capacity needed for decoded HDTV, regardless of which formats prove tobe popular.
Consider this as well. Broadcasters have a given television channel thatthey can fill up any way they choose with digital TV signals. That couldmean one 1080i signal, one 720p with two 480i channels or a whole bunch of480i and 480p signals. To some extent, the availability of 16:9 HDTVsignals may be governed by simple economics.