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<<< Back to April 19 - Issue 162
"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."
"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."
"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.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."
"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.
Next May 3 - Issue 164
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