RF CURRENT



Welcome to RF Current, a weekly electronic newsletter focusing on Broadcast technical and F.C.C. related issues. This newsletter is part of The RF Page @ www.transmitter.com, a web site devoted to TV Broadcast RF engineering. For more information see the What is... guide to the R.F. Page site.

Issues are dated each Monday, although recently I've needed an extra day or two to complete each issue. Articles may be posted earlier if time permits or if there is a major, breaking story.

NOTICE You may have noticed that near the end of the month, the RF Current page had grown rather large. Starting this month, each issue will now have its own HTML page. This should make it faster to load, easier to search and make it a little easier for me to edit. I now have access to an Excite search engine. As soon as I can figure it out, I plan to have all issues search able from the new back-issues list.

<<< Back to April 19 - Issue 162

April 26, 1999 - Issue 163 Final Edition

DTV - ATSC DTV FAQ Addresses 8VSB vs. COFDM controversy and Sinclair tests (Apr. 26)
Within the last week, ATSC posted a DTV FAQ on its web site. The FAQ (Frequently Asked Questions) is a thirteen page Adobe Acrobat PDF file. The FAQ deals with questions about mobile reception of DTV signals, comparisons with the DVB-T standard, new developments in conditional access and data broadcasting using the ATSC standard. One question is likely to generate a lot of interest and controversy. It deals specifically with indoor reception of 8VSB signals. The question is:
"What about reports that the ATSC's VSB transmission system doesn't work for indoor reception?
The short answer is:
"Recent claims by Sinclair broadcasting are based on a handful of unscientific and probably inappropriate comparisons of analog and digital reception in one city. In contrast, scientifically sound data from more than 200 indoor locations in several cities indicate that the digital VSB transmission system is performing as well or better than expected. Furthermore, laboratory and field tests from around the world have consistently shown the superiority of the VSB transmission system, and there is no published test data whatsoever that indicates that the European COFDM system offers superior indoor reception performance."

The long answer references the Sinclair Philadelphia indoor reception tests specifically and lists the conclusions of the study. (See last week's RF Current for details on the Sinclair tests.) The FAQ criticizes the Sinclair study for apparently making no assessment of the performance of the adaptive equalizers or automatic gain control circuitry in the receivers. In addition, "It is not clear whether Sinclair used the latest models of receivers, offering improved performance in these elements.) Furthermore, we understand that at least one of the DTV stations on the air in Philadelphia is not yet running at its allotted full power and antenna height."

The FAQ also questioned the test procedures, saying
"Sinclair has not released the detailed results of its tests or its test procedures, but based on the limited information it has given, it appears that reception was observed at no more than nine indoor sites. Different frequencies were used for analog and digital signals, and apparently no measurements of analog or digital signal strength or multipath (ghost) impairments were taken."
The ATSC DTV FAQ addressed the problems with indoor reception:
"Indoor reception follows the same laws of physics as outdoor reception, but indoor reception can be particularly difficult and subject to variability because signals are attenuated in passing through building materials and are subject to additional multipath impairments due to reflections. Whether or not a particular site can get good indoor reception can depend on many factors such as what kind of insulation is used in the building, whether the room is on the side of the building nearest to the transmitter, or whether the antenna is near a window. Ultimately, the key factors are the signal strength received at the antenna, the complexity of multipath impairments, and the presence of impulse noise impairments caused by engines, electrical motors, or power lines."

The FAQ also notes that the 8VSB transmission system has been tested in more 2,400 outdoor locations and more than 200 indoor locations. It acknowledged there may be locations that receive a poor analog signal that are unable to successfully receive an 8VSB signal due to the digital cliff edge effect, "However," it stated, "generally speaking, the extensive field test data collected to date confirms that within the same frequency band, viewers who can get acceptable-quality analog pictures on indoor antennas will be able to receive perfect digital pictures with the VSB transmission system."

The ATSC, in the FAQ, says the factors that make indoor reception difficult actually give 8VSB an advantage over COFDM. To understand this conclusion, it is important to follow the ATSC's argument carefully. For that reason, I'm including it in its entirety below:
"Indoor reception is not a unique condition, but an extension of outdoor reception to cases with lower signal strength, greater impulse noise, and more multipath impairments. This gives the ATSC VSB system a significant advantage, because the VSB system performs much better than COFDM in the presence of impulse noise. Impulse noise is often present in indoor reception in the VHF bands, and has been observed in many cases in the low UHF band as well.

"In indoor reception environments, wherever ghosting allows analog NTSC reception, COFDM will be at a distinct disadvantage to VSB, because data from around the world has consistently shown that COFDM requires a signal strength that is 4 dB (i.e., 2.5 times) greater than VSB to achieve the same coverage. Thus, VSB can keep working with weaker signals due to attenuation from walls long after COFDM has failed. (Proponents of COFDM have recently asserted that this advantage has been reduced to 1.5 dB, but have presented no test data to back up these claims.)

"Proponents of COFDM claim that this advantage is negated in the presence of multipath impairments, but the data shows that this is not true for practical situations. As shown in the attached chart based on comparative laboratory tests conducted in Australia, as the signal strength of the interfering ghost increases, the advantage for VSB holds steady well past the point where analog reception would fail. When the ghost gets within 3 dB of the desired signal level (i.e., approaches one-half of its strength), VSB will fail. For larger ghosts than this, it is true that COFDM will keep working, but only if the desired signal remains overpoweringly strong an unlikely event, especially for indoor reception. (COFDM will work in the presence of a 0 dB ghost (i.e., the desired signal and the ghost have the same strength), but only if the desired signal is 35 dB higher, or about 4,000 times stronger than the surrounding noise level.) Thus, as the chart shows, COFDM offers a tiny area of theoretical advantage that represents impractically high signal strength conditions, especially for indoor reception, while VSB offers a large area of advantage representing real-world signal conditions. Whatever problems may have caused a lack of DTV reception at Sinclair's Philadelphia sites, COFDM would only be worse due to the 4 dB poorer threshold.

"In the Australian field tests, real-world sites were deliberately selected for worst case multipath impairments, yet VSB showed a median signal-to-noise advantage over COFDM of 3.8 dB, showing that laboratory tests carry over reliably into real world statistics. In absolute terms, VSB had a better signal-to-noise ratio than COFDM at 108 out of 110 sites. With respect to impulse noise, laboratory tests showed a 10 dB advantage for VSB. Field test results were affected by impulse noise and the advantage of VSB over COFDM would have been even greater if the sites had been pre-selected for worst case impulse noise impairments typical of VHF transmission.

"Carefully planned and conducted indoor tests in Charlotte NC, Chicago IL, Raleigh NC, Washington DC, and Dallas, TX, covering more than 200 indoor test sites, demonstrate that the VSB transmission system generally provides good indoor reception, with results being better than expected in a number of cities. In addition, VSB outperformed COFDM in the limited number of multipath-rich outdoor and indoor reception comparison tests that were conducted in Singapore in 1998.
The FAQ was blunt about revisiting the selection of the 8VSB transmission standard: "Sinclair's proposal to "revisit the transmission standard . . . before any final conclusion [is] reached," is completely unwarranted and irresponsible." It argued more than fifty stations are already on the air and manufacturers have "already developed and introduced dozens of models of DTV receivers, set-top boxes and related products." In conclusion, the FAQ answer says, "Furthermore, even if there were a serious problem with indoor reception, existing test data already makes clear that adopting COFDM would not be an effective solution."

Indoor testing is not easy. There are many variables and subtle differences in building construction and location can have a signficant impact on the results. The ATSC DTV FAQ makes a good technical case for 8VSB and backs it up with over 200 measurements in different locations, although the results do vary with location. To obtain a statistically significant sample, tests in a large number of homes of different construction at either specific radial or, preferrably, grid points will be needed. Given the difficulty in obtaining permission to enter peoples' home and conduct these tests, it is unlikely this will ever be possible. An alternative may be a pseudo indoor test arrangement with an antenna inside a large trailer simulating a room with specific reflectors and lossy surroundings that can be driven from point to point. This is a topic of crucial interest to all TV broadcasters. Take time to read the ATSC DTV FAQ and the Sinclair Broadcast Group Philadelphia ATSC Reception web page. RF Current will continue to report any on new test information or analysis available to the public.

INDUSTRY - California Microwave Becomes Adaptive Broadband (Apr. 26)
California Microwave said it had completed the sale of its Government business to Northrop Grumman. Effective April 29, 1999, the company will change its name to Adaptive Broadband. At NAB I was told the company will keep the "MRC" (Microwave Radio) designation for its broadcast auxiliary product line. The name Adaptive Broadband reflects the company's emphasis on data transmission on the ground and via satellite.

In addition to changing its name, the company will change its stock symbol from CMIC to ADAP on the NASDAQ National Market and its web address to www.adaptivebroadband.com. Attempts to connect to pages within the old site will result in error messages. The links for the NAB news relating to California Microwave in the April 19, 1999 RF Current have been updated.

DTV - Next CEMA DTV Summit '99 to Focus on Content and Programming (Apr. 18)
The Consumer Electronics Manufacturers Association (CEMA) will holds its fifth DTV Summit in Los Angeles, CA, on September 28, 1999. A CEMA News Release said the "Summit program will include a review of the latest consumer research, plans for broadcast, cable and satellite delivery of DTV, manufacturers' product plans, and plans for DTV programming and content availability."

Gary Shapiro, CEMA President, said "Digital Television is the future! We want to work with the content community to ensure that consumers have compelling DTV programming. The Summit will provide an opportunity for our industries to discuss our DTV plans and work together to bring consumers the products and programming that will make DTV a success," The event will be co-located with and sponsored by Digital Hollywood.

Manufacturers, broadcasters, cable and satellite executives, retailers, content providers and media are invited to attend the Summit at the Beverly Hills Hilton Hotel in Los Angeles. More information about Summit registration can be found on the CEMA web site.

DTV - Additional NAB 1999 DTV News (Apr.)
The following are news releases that I either missed or were not available on the Internet until after April 19.

ADC Telecommunications
General Instrument
OTHER Items of Interest

Next May 3 - Issue 164


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Last modified May 2, 1999 by Doug Lung dlung@transmitter.com
Copyright 1999 H. Douglas Lung