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HDTV

Fri, 07/31/1998 - 8:00pm
Dana Cervenka, Managing Editor

Ready or not, here it comes. Broadcasters will begin transmitting some high definition television programming late this fall, and have announced their chosen HD formats: NBC and CBS have committed to 1080i, while ABC and Fox have opted to use 720p.

Given that cable operators reach about 67 percent of all TV households in the U.S., the industry's participation in the HDTV process is arguably critical to its success.

And now, the issue of digital "must-carry" is looming large on the horizon. Early last month, the FCC issued a Notice of Proposed Rulemaking seeking comment on the carriage of digital broadcast television signals by cable systems. Among other items, the notice is seeking input on several possible must-carry alternatives for digital TV signals, ranging from full must-carry requirements to no must-carry requirements, during the transition period. The notice also seeks comment on what cable operator actions affecting the picture quality of DTV signals would be considered material degradation, which is prohibited for analog signals under the Communications Act.

For obvious reasons, the cable industry favors marketplace — not legislative — solutions to the digital carriage dilemma. First and foremost, many operators fear that if digital must-carry becomes a reality, they will be forced to drop many existing programming services in order to make room for broadcasters' digital transmissions, a move sure to be unpopular with subscribers.

One of the areas of the HDTV debate which has churned up the most muddy water is the question of format preference — which of the 18 ATSC* formats constitute the most desirable ones for cable operators' carriage, given the technology, business and subscriber demands they must balance (*see sidebar)?

Format and modulation

Tele-Communications Inc. has voiced a strong preference for the 720 progressive high definition format, which TCI says is more spectrally efficient than 1080 interlace. In fact, the MSO says that with the 720p format, it can pack four high definition channels into one, 6-MHz channel, while the 1080i format would, at best, allow the MSO to transmit two channels in 6 MHz. TCI also favors the progressive transmission format because it is computer-friendly: today's computers already use progressive scanning in their display technologies.

Regardless of which format a given broadcaster chooses, TCI executives say that the General Instrument DCT-5000 advanced digital set-tops it will deploy have the ability to pass through 480p, 720p or 1080i transmissions to HD receivers, and further, that "no cable customer with an HDTV receiver will be disenfranchised from receiving an HDTV broadcast signal through the cable system."

At the other end of the spectrum, Cox Communications' Vice President of Technology Development, Chris Bowick, says that the MSO is "pretty much format-agnostic, but that doesn't mean that our bandwidth is free. We wouldn't like it if the FCC mandated technical requirements for carriage on cable systems. We would like to have the option to choose our own modulation format, and that would of course require a translation from 8 VSB, which is being broadcast over-the-air, to some QAM format, yet to be determined."

TCI President Leo Hindery, Jr. also voiced a similar concern during testimony last spring to the House Subcommittee on Telecommunications, Trade and Consumer Protection. ". . . A government attempt to impose the same digital transmission standard on cable operators that was adopted for broadcasters — that is to carry digital content delivered over cable systems in VSB (format), as some have suggested — would make no sense at all, and, in fact, would severely undermine the efficiencies of the cable network by forcing cable operators to utilize double the amount of bandwidth to carry digital broadcast content," said Hindery.

Meanwhile, the SCTE's digital video subcommittee has been working to address the cable industry's specific technical concerns about high definition television by extending the standard to the cable environment.

"Our objective has been to recognize the ATSC standard, and any other existing standard that is likely to be brought into practice in the North American setting, when that is possible and appropriate. And supplement those standards in areas that are specific or of primary importance to cable," says Paul Hearty, VP, Canadian sales, General Instrument and chair of the committee. "Some examples of that are encryption and access control — that is one area where we have supplemented the ATSC standard. Other areas include closed captioning extensions, which we are working on now, and interactive and network architectures. There are very different issues and opportunities for cable than there are for broadcasters."

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Essentially, the subcommittee has adopted all 18 of the ATSC's formats, while creating a few of its own to reflect "existing cable industry practices" on the standard definition side. The new formats take three-quarters and one-half the horizontal resolution of the 704×480 standard definition format, to come up with 528 pixels by 480 lines, and 352×480, respectively, which allows the operator some bandwidth savings. Hearty is careful to note, though, that the SCTE is "format agnostic" and does not advocate any of the ATSC formats over the others.

However, Hearty does have some personal opinions on the subject. The reality of how many high definition signals cable operators can pack into a 6 MHz channel will greatly depend on the type of content being transmitted, as well as the type of modulation being used, he says.

"There is no question that two of any of the high definition formats will fit into a 6 MHz channel, even 1080i, if you are using 256 QAM," he notes. "Let's look at 1080i and 720p. The nominal bandwidth allocation for each of those in a broadcast multiplex is 19.3 megabits total, of which approximately 18 would be assigned to the video. The actual bitrate utilization varies tremendously with content. Measurements that I have made show that the momentary bitrate requirement for either 1080 or 720 is anywhere from nine megabits to a full 18.5. The grossest difference is between live action sports, on one side of the spectrum; and on the other, you have more or less still pictures, with 24-frame film somewhere in the middle. So the actual bitrate utilization varies over a wide range, depending on the actual medium of origination, and on the content of the material itself."

Hearty theorizes that up to three HD signals could probably occupy one 256 QAM cable channel, but adds that "those won't be three ESPNs." He also thinks that the 1080i "bandwidth hog" label may be a bit of a misnomer.

"My personal experience is that 1080i may take five to 10 percent more bits than 720p. My experience is that the difference between the two formats is very modest, and you should make your decision based on the actual performance, and that will depend on the type of content that you are trying to compress."

While cable operators are concerned about the HD video formats broadcasters will be using, the broadcasters are equally concerned that those transmissions make their way to consumers relatively unscathed.

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"Our official position is that it should not be downgraded; what we send out should be carried by cable," says John Earnhardt, spokesperson for the National Association of Broadcasters. "So if it is sent in 1080i, they (cable) will send it to their subscribers so that they receive it in 1080i."

The broadcasters are probably worrying needlessly, says consumer electronics expert Walter Ciciora, Ph.D. "To downgrade it means you'd have to spend money on hardware. You'd have to receive the signal, take it apart, and put it back together again in a manner that is useful. I would bet that equipment would be horribly expensive."

Given the uncertainty swirling around HDTV, what can cable operators do now to prepare for the great unknown? "Operators need to be preparing their plant utilization plan," says GI's Hearty. "They need to get their overall headend and network architecture in place to accommodate digital, and many have already done that. The next step of course is to plan to accommodate broadcast and locally-originated SD/HDTV signals in terms of bandwidth utilization. Some operators are moving to upgrade their plants to extend their bandwidth. That is very expensive. Others are looking into optimizing their channel lineups, and making transition plans."

But capacity issues are just one element swirling in the HD whirlwind. Part two of this article will discuss headend and consumer electronics equipment issues.

 

The ATSC standard

The Advanced Television Systems Committee (ATSC) standard consists of a method for compressing video, compressing audio, multiplexing the two resulting bitstreams together into a single bitstream, adding auxiliary data to go with the television picture and sound, and a method for transmitting it over-the-air, says Craig Tanner, executive director, ATSC.

While there is only one standard, there are 18 different video formats. The first split is between high definition and standard definition TV. Six of the video formats in the ATSC DTV standard are high definition TV: these are the 1080-line by 1920-pixel formats at 24 and 30 frames per second, and at 60 fields per second for interlaced HDTV, and the 720-line by 1280-pixel formats at 24, 30 and 60 fps. The HDTV formats have a 16:9 aspect ratio.

The 12 video formats which compose the remainder are standard definition television — not high definition. These consist of the 480-line by 704-pixel formats in 16:9 widescreen and 4:3 aspect ratios (at the 24, 30 and 60 pictures per second rates); and the 480-line by 640-pixel format at a 4:3 aspect ratio at the same picture rates.

The "i" and the "p" in the format names refer to interlaced and progressive scanning. "In interlaced scanning, half of the lines in a full frame are scanned onto the screen in a sixtieth of a second, followed by the remaining half of the scan lines in the next sixtieth. The odd lines are scanned first, then filled in by the even lines.

"In progressive scanning, each line of the frame is scanned in sequence from top to bottom, in a single pass, in a sixtieth of a second, without interlacing . . .This requires twice the bandwidth, but provides better quality vertical and motion resolution. Because of progressive scanning's bandwidth consumption, the ATSC standard's 720-progressive scanning has its horizontal resolution reduced to 1280 pixels, as compared to 1080-interlace's 1920 pixels," explains Tanner.

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