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Farewell to the Great CRT - TvTechnology

Farewell to the Great CRT

I replaced my 27-inch CRT receiver with a 42-inch plasma panel HDTV display more than a year ago, and I've never regretted doing so.
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Fifty years ago, we slowly entered the world of color TV, having gone down the yellow brick road of NTSC. I was there. I built my first color TV receiver even before I joined the engineering department of Tektronix.

Aside from transmission problems, which incidentally led me to develop the Vectorscope, there was also a display problem.

The first NTSC receivers had a 15-inch round, white-faced picture tube, and inside it was a piece of plate glass, which was the actual screen. It provided a 4:3 aspect ratio picture--about the same size as a 12-inch TV receiver. The picture was simply marvelous after the sun went down, and after you adjusted the purity and convergence controls. I thought then that a different display technology would be needed to make color TV (CTV) successful. How wrong I was! It took the next 15 or more years for an army of U.S. engineers to re-invent the shadow mask tricolor CRT, but they did it.

Then along came Sony with their Trinitron and its aperture grille instead of a shadow mask--almost. In the early 1980s, it appeared that the display problem was returning again as all CRTs involve a trade-off between brightness and resolution because of the beam diameter.

So, again, it looked like another technology would have to come along for HDTV, because the public was not going to accept low-contrast pictures to get high-definition in those pictures. Again, the CRT technology was refined to meet the challenges of HDTV.

We used a 55-inch projection unit by Hitachi for testing the Grand Alliance DTV System in 1995 because of its multiscan capabilities and its high resolution. But the sheer physical bulk of a rear-screen projection display dismayed nearly everyone.

Everyone wanted a two-dimensional flat-screen display that might be hung on a wall, with the new aspect ratio, lots of contrast as well as brightness and high resolution.

I replaced my 27-inch CRT receiver with a 42-inch plasma panel HDTV display more than a year ago, and I've never regretted doing so. It looked rather large at first, but in only a few weeks, it seemed just about right. Maybe my next set will be larger still.

COLOR WHEEL RETURNS

Gone from the market are direct-view CRT TV receivers. Last summer, I saw fantastic deals on CRT PC monitors, which suggested that flat-panel displays had arrived. Now they have taken over. They are all widescreen, (16:9); the biggest being 102 inches. This made me realize that plasma-panel technology is already fast approaching the fundamental limit in screens that can pass through a doorway.

Samsung, perhaps the manufacturer with the largest floor space at CES, had their 102-inch plasma display on exhibit, and they had a wide range of large, widescreen displays. Samsung is covering all bets on display technology. By that, I mean that they have LCD displays, plasmas, and DLP projectors. Some of these non-CRT displays included something that cannot be done with direct-view CRTs. There are now a number of models that produce a 1,080 (active line) 16:9 picture progressivly scanned and refreshed 60 times per second (1,080p/60).

(click thumbnail)An aperture grill CRT

(click thumbnail)An early color wheel TV
Can you believe it--the color wheel is back! Yes, single-valve DLP projectors have a tiny color wheel. This takes me back to the CBS Field Sequential Color receivers, a few of which appeared in 1951 after the FCC adopted the CBS Field Sequential Color System.

Those sets were direct-view, with a 12-inch monochrome CRT. The color wheel was larger than the receiver, and it limited picture size to 12 inches or so. Those 1951 color sets are extremely rare collectors' items today. Of course, the NTSC compatible color system displaced the CBS system. But as with color wheels, what goes around comes around. They have returned in some of the DLP displays I saw at CES. I don't know how tiny their color wheel is, but after all, how big need it be when coupled to the light valve (actually an IC developed by Texas Instruments. I'm sure there is no audible sound coming from their micro-color wheel.)

With three DLP light valves--you guessed it--there is no need for a color wheel. Instead, there are three high-intensity lamps and the resulting displays are really, really bright!

Pioneers in television; Charles Jenkins on this side of the ocean and J.L. Baird in England, must be gloating as they look down upon us now to see mechanical television receivers in full color, and with marvelous definition and brightness, silently amazing viewers.

Until Philo Farnsworth and Vladimir Zworykin of RCA demonstrated all-electronic TV systems early in the 1930s, television scanning was mechanical. Some very ingenious, and some quite rudimentary mechanical scanners were developed. However, due to the limitations of such rotating machinery, HDTV was 100 lines, not 1,000. Why bring up ancient history? As with the color wheel, mechanical scanning has returned in the DLP. The TI light valve has more than 1 million micromirrors mounted in an IC. These mechanically tilt to modulate the light from a high-intensity lamp. These million or so micromirrors on an IC chip change the intensity of individual pixels. By separating the source of light from the light modulator, each can be optimized, and the result is something a CRT can't do--provide both high brightness and high pixel resolution. Brightness and resolution are no longer traded off, one for the other, as in a CRT.

What most impresses me is the number of major manufacturers offering DLP models. This is impressive when you consider that one firm is the sole source for the light valves. If TI is licensing other manufacturers, I don't know it, but clearly this technology has achieved a significant following.

BETTING THE FARM

Manufacturers are betting the farm on the public's enthusiastic acceptance of really large-screen HDTV. It seems unlikely that once the public sees such large-screen HDTV pictures, they will be content for long in watching plain old NTSC. Where are they going to find these large-screen HDTV pictures? Why at sports bars of course. Such places can well afford state-of-the-art HDTVs.

That is, after all, how the American public was largely introduced to TV back in 1946 when bars installed a 10- or 12-inch (large-screen) TV for their patrons to watch night baseball in New York. And the word spread fast! A few bars went for projection TV sets--"really big-screen TV," banners outside the establishment so proclaimed.

I can still recall the prophetic words of the late Julie Barnathan of ABC who said "HDTV is coming, simply because it is coming."

Julie must be gloating along with Jenkins and Baird. They were prophets before their time. Joe Flaherty of CBS also prophesized when, in the 1980s, he said that by 2000, the worst pictures in American homes will be from broadcast stations. Joe is still with us, and he is also still a prophet.

Those soft-looking NTSC pictures, up-converted for DTV, really turn me off now, and I find that we are watching DVD or true HDTV.

At CES, Sanyo displayed a DTV receiver built into a cell phone, and this is now in use in Japan. The Japanese Terrestrial Integrated Services Digital Broadcasting (ISDB-T) broadcasts in their own unique kind of DTV signals in the UHF band. The Japanese-developed DTV system is based on COFDM but it differs very much from the European DVB-T system. This is a multicarrier system and there are thousands of carriers, each digitally modulated, filling the 6-MHz UHF channels. These thousands of carriers are subdivided into 13 groups. The one in the middle can carry 180 kBps, enough to deliver a usable picture to these handheld receivers.

This group is modulated in QPSK (2 bits per symbol), so a very rugged signal results, one that can be received under very poor conditions making this suitable for mobile and pedestrian reception. When used to receive TV, the lithium ion rechargable battery lasts two hours. This receiver is not just run-of-the mill, otherwise, it couldn't work in the concrete jungles of Tokyo. It employs diversity reception technology. They have integrated this diversity TV reception capability with the same antenna that receives cellular telephony. They use MPEG-4 coding. The receiver is a direct-conversion type, which means the local oscillator is phase/frequency locked to the received UHF signal, not offset from it by the IF. So the output of the synchronous detector is at baseband and can be digitized.

This topology is ideally suited to a system on a chip (SOC)--not a new idea. The United States had zero-IF radar receivers in World War II. Since then, this receive topology has been used in many digitally tuned receivers. It is only a matter of time before direct-conversion topology takes over in DTV receivers as manufacturers seek ever-smaller and lower-cost topologies.

This would probably reduce battery drain as well. Just how well this topology would work in our crowded broadcast spectrum is not quite clear. But for handheld receivers, there is scant space for tuned circuits. There must be filtering before the A/D converter, which must handle the desired signal and the much stronger multiple undesired signals.

Well, it is coming, not because the public demanded it, but because when they see it, they will demand it. Perhaps the cutest feature is that the screen of this receiver can be scanned in portrait or landscape aspect.

Incidentally, South Korea is getting into mobile, portable and pedestrian TV too. But they are doing it differently. They will use nonbroadcast spectrum below the UHF band, so don't look for these receivers on American streets. Here, we may soon see some sort of personal, portable and pedestrian TV on former broadcast channels. There is a lot of hype about this, but there also seems to be a lot of automobile DVD players already.

IN THE BEGINNING

The shadow mask three-gun color CRT was invented in Germany in 1938. That patent was discovered by researchers at the Sarnoff Research Lab and reduced to practice within about six weeks as a result of the competition between the CBS Field Sequential TV System and NTSC. Now, after 50 years of service, the color CRT has retired. HDTV, with its life-size screens, will carry on from here. If you don't believe it, just visit any large store selling TV sets to see what they are stocking and selling.