Evaluation of proposed ATSC enhancements to the 8-VSB modulation system have included testing of “walkabout portability,” using the test configuration pictured in the images above; the separate receiver is not shown. Note the wireless phone and headset used for communications between test personnel.
In a rare break with tradition, this month's episode of “Download” continues down the path we began exploring in the February column, “DeVolution.” When we conceived the editorial calendar for this year, we anticipated the need to talk about enhancements to the ATSC modulation standard. At that time, the ATSC expected a decision by the end of 2002.
But, as is often the case, things did not work out as expected. After the T3/S9 ATSC Specialist Group on RF Transmission makes its recommendations to the parent committee, the committee could make its final decisions about which, if any, enhancements it will standardize in time for NAB in April, or in May.
Fortunately, there is plenty to write about regarding potential enhancements to the ATSC standard. Some of the proposed changes dovetail nicely with the subject of February's column, which examined the prospects for replacing the aging MPEG-2 video-compression standard.
In December, the ATSC issued a request for information on advanced video and audio coding systems for use in a robust ATSC DTV channel. Also in December, the ATSC published a candidate standard: Synchronization Standard for Distributed Transmission. The standard paves the way for single-frequency networks (SFNs) that are largely compatible with existing receiver designs, but optimized for next-generation receivers with improved adaptive equalizers.
With this in mind, let's examine three issues: the potential for changes to the ATSC standard, the enhanced video-compression technologies that are vying to co-exist with or to replace MPEG-2, and the prospects for these new compression techniques to displace MPEG-2 in other digital-distribution infrastructures, including cable and DBS.
Give and take
The ATSC Specialist Group on RF Transmission has been evaluating three techniques to improve reception through backward-compatible modifications to the 8-VSB modulation standard. Each of these proposals reduces the 19.3Mb/s payload of the current 8-VSB standard. Two of the proposals involve significant reductions in the payload to support a smaller robust channel that legacy receivers would ignore. Let's look briefly at what each proposal involves.
Broadcom has proposed adding an extended training signal. This would make it easier for new receivers to lock onto the 8-VSB signal, improving reception under most circumstances. The training signal would have minimal impact on the 19.39Mb/s payload — only a fraction of a percent.
Zenith and ATI Technologies have proposed a robust channel that would reduce the 8-VSB payload by 3Mb/s to provide a robust channel with 1.5Mb/s throughput. ATI acquired receiver-chip manufacturer NxtWave Communications in June of 2002; NxtWave developed this proposal jointly with Zenith.
Philips has proposed a robust channel that would reduce the 8-VSB payload by 9Mb/s to provide a robust channel with 4.5Mb/s throughput.
The Specialist Group has not released the results of testing these enhanced-modulation proposals to the public. Members of the group are bound by a non-disclosure agreement regarding committee work and these tests. In anticipation of the final decision, however, a public debate has already begun.
On one side of the debate are broadcasters who believe that adding a robust transmission mode will provide new video and data broadcast services for portable and mobile receivers. The testing process for the enhancement proposals has included a test of “walkabout portability.” In essence, this would allow the use of handheld receivers comparable to NTSC WatchMan receivers, and the delivery of data to digital media devices such as PDAs.
On the other side of the debate are broadcasters who believe that it is unnecessary to make any changes to the ATSC modulation standard. This camp believes that improved adaptive equalizers and active-antenna designs are addressing reception problems adequately.
A December demonstration of improved adaptive-equalizer technology developed by Linx Electronics showed significant reception improvements in the demanding multipath environment of downtown Chicago. The complexity of the implementation, however, raises questions about the costs for the chip(s) being developed by Linx.
On Jan. 1, NewsCorp's Fox Unit and Philips Research announced a new technology they say will deliver perfect digital TV signals to more homes through indoor antennas. The technology, developed through laboratory simulations using RF signals recorded in major cities, is based on the use of multiple antennas, multiple equalizers and a summing decision network to derive a reliable signal. No one has announced plans to commercialize this technology.
The proposed enhancements being developed by the ATSC Specialist Group on RF Transmission may encounter resistance as they work their way through the ATSC standardization process. Further indications that the ATSC will choose not to change the modulation standard have emerged from off-the-record discussions about the testing of the three proposals. According to these reports, there is no way to show from the test results that any of these enhancements will improve reception in a quantifiable way, and none of these proposals will support mobile reception.
While U.S. broadcasters would have much to gain from a “hard reset” (changing both the modulation and compression standards), most currently view this as being highly unlikely because it would disenfranchise the approximately 500,000 ATSC receivers already deployed. Meanwhile, broadcast competitors (including cable and DBS, which operate in less hostile channel environments) are evaluating more efficient modulation and video-compression techniques — despite the fact that they have deployed tens of millions of set-top boxes that they would have to replace.
The DBS operators have already begun to migrate to 8PSK modulation on new transponders. To facilitate a phased-migration strategy, they are using these transponders to deliver services to next-generation set-top boxes that support 8PSK. When EchoStar proposed a merger with DirecTV, the company stated that it was allocating a sizable amount of money to replace all of the existing set-top boxes for both systems. Now that the proposed merger has been blocked, improved modulation and improved compression may be even more important. To serve more local-into-local broadcast markets, throughput will have to increase significantly.
The cable industry is less affected by the need for more bandwidth because each cable system need only provide non-duplicated broadcast signals from surrounding markets. And recent digital upgrades to 750- to 900MHz systems provide cable with sufficient bandwidth to meet current needs. But future needs may dictate radical changes for cable as well.
Cable uses a hybrid fiber/coax network infrastructure. Fiber runs to neighborhood nodes where it terminates at the traditional coax cable plant. These nodes typically serve between 250 and 600 homes. A significant portion of the forward bandwidth serves legacy analog cable service. The remaining bandwidth is segmented between forward channels and a return channel. The forward channels serve digital cable and cable modems; the return channel is for signaling and for the cable-modem upstream path.
Currently, digital cable systems are using many of the digital channels for near video on demand (NVOD). But cable is now beginning to migrate to true video on demand (VOD), which allows the user to pause, rewind and fast-forward through the content.
Each home using a cable modem or watching a VOD event is receiving private, rather than broadcast, bits. And, as more homes take advantage of these services, the demand can exceed the capacity of the neighborhood loop.
The traditional solution is to reduce the number of subscribers on a neighborhood loop. This requires lighting up another fiber to the neighborhood and adding additional switching gear at the headend(s). The alternative is to use the existing plant more efficiently; more efficientmodulation, more efficient video compression, and more efficient use of the available bandwidth by eliminating service segmentation.
The cable industry is currently evaluating a shift to 1024 QAM, and to an all-IP-based digital infrastructure to eliminate the segmentation of cable modem and digital TV services. The analog tier is likely to endure because it is a major competitive advantage over DBS.
Let's make a deal
The question of whether the ATSC standard will be enhanced may soon become irrelevant. On Dec. 19, the consumer-electronics and cable industries announced an agreement that sets the stage for a national “plug-and-play” standard for digital television products and digital cable systems. The FCC, which issued a mandate last fall to include ATSC receivers in virtually all digital television products by 2007, is reviewing the agreement, and has opened a proceeding to gather industry comments.
The consumer-electronics industry has challenged the ATSC-receiver mandate in court. There are strong indications, however, that they would drop this challenge if the FCC adopts the recent agreement with the cable industry and codifies it into rules that would require digital television products to include both ATSC and cable tuners.
Meanwhile, the Sinclair Broadcast Group has asked the FCC to impose minimum-performance specifications for the mandated ATSC receivers, a requirement that the consumer-electronics industry strongly opposes because it would increase the cost of the front-end and adaptive equalizers in ATSC receivers.
Since most consumers will never use the ATSC receiver in new digital televisions, it is likely that there will be little incentive for the consumer-electronics industry to adopt evolutionary designs that will improve performance. This may, however, create a niche market for higher-performance receivers that can be added to TVs with integrated receivers when the internal ATSC receiver does not work reliably — that is, if over-the-air broadcasting survives the DTV transition.
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In February, we asked the question: “Given the historic longevity of broadcast standards, why are some people, including this author, suggesting that MPEG-2 is growing old?” We pointed out that there are more than 200 million MPEG-2 decoders deployed worldwide. This would seem to create an insurmountable barrier to “DeVolution,” the term created to describe the evolution of digital video-compression technology.
We also pointed out that the MPEG-2 standard was constrained by the processing power available in 1995. In seven years, the capabilities of MPEG-2 have been fully exploited, thanks in part to a 400 to 500 percent increase in processing power.
Figure 1. MPEG-4 part 10, or Advanced Video Coding, uses reconstruction filtering to minimize the perception of the blocking artifacts common with earlier MPEG standards. The image sequence above includes the original, the result after AVC encoding/decoding, and the final result after reconstruction filtering. Images courtesy of Iain Richardson.
SMPTE fully anticipated and understood this evolution in capabilities as early as 1992, when it published its Task Force Report on Digital Image Architecture. The report was an input document to the Working Party 4 (WP4) interoperability review of the systems proposed for the U.S. Advanced Television standard. The Advisory Committee on Advanced Television Services (ACATS) commissioned the WP4 review at the behest of FCC Chairman Al Sikes, after the committee pointed out that the shift to digital HDTV would facilitate convergence with technologies being developed by the rapidly growing U.S. computer industry.
The Task Force Report identified critical issues for an interoperable, scalable and extensible digital-imaging architecture. According to the report, extensibility “implies designing evolution into the system.” A hierarchical, modular architecture is used to allow higher levels of performance as technology advances but, at the same time, provides backward compatibility to existing products.
The rapid evolution of video codecs used for streaming video over the Internet is a classic example of extensibility. The downside is that, periodically, it is desirable to replace the underlying hardware to take advantage of the improved technology.
In this light, it is desirable to separate the components of a digital television system. For example, expensive components that do not evolve rapidly, like displays, can be used with several generations of receivers that evolve rapidly with new capabilities. It is worth noting that the consumer-electronics industry apparently got the message. Only a small percentage of the more than five million HDTV-capable displays shipped to date have integrated digital receivers.
The DBS industry has been built almost entirely on the large installed base of NTSC receivers. Now it is driving the development of premium HDTV services for the growing base of HDTV monitors. In turn, this is causing the cable industry to roll out premium HDTV services. Both of these industries are driving DeVolution through modular set-top receivers. Both are now looking to next-generation digital-video-compression technologies to make room for HDTV.
A variety of next-generation video-compression algorithms are vying for the opportunity to replace MPEG-2. Microsoft and Real Video have been pushing the envelope in the PC-based streaming-video markets. Microsoft recently announced a licensing program for its Windows Media 9 technology, for use in applications other than Windows PCs.
Work began on MPEG-4 as soon as the MPEG-2 standard was finished. The improvements in video-compression efficiency offered by the current MPEG-4 standard were incremental — about 10 percent at most. Noting the rapid advance in video-compression technology, MPEG evaluated next-generation compression algorithms two years ago. As a result, they formed the Joint Video Team with the video-compression experts from the International Telecommunications Union (ITU), which has traditionally worked on video-compression technology for videoconferencing and video telephony. The result is a new video-compression standard that will be known as MPEG-4 Part 10 as an ISO standard, and H.264 as an ITU standard. The informal name for the new codec is Advanced Video Coding (AVC). (See Figure 1.)
Each of the would-be contenders vying to dethrone MPEG-2 offers at least a 200 percent improvement in compression efficiency. These new codecs also include image-reconstruction filters that minimize the perception of compression artifacts. Thus, for highly compressed applications, they may offer as much as a 300 percent improvement in bandwidth efficiency relative to MPEG-2.
Obviously, there is a cost. The complexity of these algorithms is about four times that of MPEG-2. As noted earlier, however, the 400 percent improvement in processing power is already here.
The new compression algorithms do a much better job of adapting to changing image information, especially to critical edge details for which the MPEG-2 motion-compensated prediction and quantization tools are relatively crude. MPEG-2 tends to distort edge detail, replacing it with quantization errors that look like noise.
Major improvements in the way that differences from predictions are quantized contribute to improved picture quality, and reconstruction filtering largely eliminates the blocking artifacts MPEG-2 reveals when it is severely stressed.
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Craig Birkmaier is a technology consultant at Pcube Labs, and hosts and moderates the OpenDTV Forum.
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