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Technology Stocks : C-Cube -- Ignore unavailable to you. Want to Upgrade?

To: John Rieman who wrote (24291)	10/23/1997 8:33:00 PM
From: DiViT	Respond to of 50808

Welcome to DTV but beware the "cliff effect" Louis Libin ÿ 10/30/97 Broadcast Engineering Copyright (c) 1997 Intertec Publishing Corporation. All rights reserved. ÿ The most obvious quality change that DTV will bring to NTSC viewers is HDTV programming. With double the resolution of NTSC and a 16x9 aspect ratio, HD programming will certainly be obvious even to the most casual viewer. Even with multicasting programs, viewers should see better pictures without common NTSC problems like chroma crawl and noise. Also, ghosting normally associated with today's viewing will be eliminated. When DTV reaches our living rooms in the form of HDTV, the quality will be the samefor all viewers who receive the signal in the replicated Grade B contour - or so we (and the American public) have been led to believe. Reception differences The changes DTV brings to the viewer go beyond mere pictures. Until new technological advances occur, channel surfing will be slower than today. Now, it will be a case of push and wait as the receiver's MPEG decoder finds the channel and locks up on the video before changing the picture. Unfortunately, there will also be some areas that, despite being located in the Grade B area, will receive no service. It's not that the signal will be weak or noisy as in NTSC, but if the receiver cannot decode sufficient data, there will be no image displayed at all. The specifics of digital reception are complex phenomena with many varying conditions - and unfortunately for some viewers - have no simple solutions. The ongoing debate over VHF and UHF propagation will further confuse viewers. Today, it's often possible to receive an adequate VHF signal, while local UHF signals are noisy because of low signal strengths. Early work leads us to believe that UHF reception may, in fact, extend beyond the radio horizon, but both viewers and station management remain skeptical. The cliff effect The dreaded cliff effect could potentially become a station's nightmare as viewers that "should" be getting a good signal report no reception. That's likely to be particularly troublesome to early adopters, especially after they've spent thousands of dollars on that new TV set only to find they can't get all of their favorite stations. Be sure your staff and management understand how the new digital signals will travel over your coverage area. It's easy to make the mistake of thinking that reception loss will never occur inside your Grade B area and then fall off abruptly outside the boundary, but it doesn't happen that way. In fact, there is likely to be some areas inside your coverage pattern that have reception problems. And, even outside your planned coverage area, you will likely still have viewers, perhaps further out than you have now. Technical assumptions DTV transmissions, like digital video compression techniques, do not provide for graceful degradation when noise or interference is of high enough levels. As such, similar conditions will exist for many stations when they begin DTV transmissions. A reduction in the carrier-to-noise ratio of as little as 1 or 1db may increase the bit error rate (BER) of your signal to the point where receivers cannot decode the signal. This is simply a characteristic of digital systems. Therefore, the application of the NTSC Grade B coverage (90% time variability factor) does not apply. Service may be lost during the 10% of the time when noise and/or interference increase above their normal Grade B values. For location factor, the 50% value for NTSC calculations is also appropriate for DTV service. Engineers have closely determined what levels (D/U ratios) of both co-channel and taboo interference (NTSC into DTV, DTV into DTV and DTV into NTSC) are appropriate. Finally, given the characteristics of DTV signals and our requirement to provide reliable service, an even more appropriate coverage area definition would be a minimum of 99% time availability for reception. To provide this level of service, a good definition of a station's DTV signal radius for noise-limited coverage should be that which yields the highest time availability (for example, 99% of the time). This would have the effect of protecting both station management and viewers from unrealistic expectations.