In recent months a controversy has arisen over 8-VSB, the modulation method chosen by the Advanced Television Standards Committee (ATSC) for sending digital and high-definition television (DTV/HDTV) through the air.
Some members of the broadcast industry have questioned the viability of 8-VSB for terrestrial broadcasting based on reception issues in specific dense urban areas.
Recent field testing using first generation DTV receivers has led some parties to question the robustness of the FCCís DTV broadcasting standard, which specifies 8-VSB modulation/demodulation techniques. Receiver technology developed by Motorola and Sarnoff, however, demonstrates that the FCC's 8-VSB standard is sufficiently robust and , thus, that no changes to the standard are warranted/needed. In summury, this is a correctable DTV receiver issue, not a problem with the FCC standard itself.
A Reconsideration of the FCC-approved ATSC standard would delay the rollout of DTV and HDTV in the United States by slowing broadcastersí implementation of the standard and confusing consumers. especially those who have already purchased 8-VSB-defined DTV sets.
Motorola's MCT2100 demodulation chip, designed in a joint effort with Sarnoff Corporation, makes changing the ATSC standard unnecessary. The MCT2100 meets the challenges of terrestrial broadcasting for the U.S. market by providing outstanding signal reception in severe multipath environments, even in dense urban areas.
The Challenge of 8-VSB
Motorola is the world's largest supplier of COFDM demodulation chips and is therefore uniquely qualified to comment on COFDM and 8-VSB demodulation. Extensive Comparisons of the 8-VSB modulation scheme with other technologies, including the COFDM method adopted by the Digital Video Broadcast (DVB) committee in Europe, were carried out by the ATSC, the FCC's advisory committee. The primary Key reasons for selecting 8-VSB for DTV/HDTV transmission in the United States include the ability to readily fit a full ATSC packet stream of 19.4 Mbps into a single 6 MHz channel, the channel bandwidth standard for terrestrial broadcasting in the United States; power efficiency, allowing broadcasters to achieve coverage equivalent to current analog signals with less transmitter power; reduced peak-to-average power ratio, which further reduces the required power rating of transmitter power amplifiers; and robust immunity to impulse noise interference.
While it was generally known that 8-VSB could be susceptible to multipath interference (interference from multiple signals arriving at an antenna at different times due to obstacles such as buildings or automobiles) in extreme circumstances, this was not judged to be an impeding factor. Recently, however, tests with off-the-air DTV signals in highly populated urban locations demonstrated reception problems with first generation receivers , whichthat were significant enough for some broadcasters to declare believe the FCCís 8-VSB standard to be unworkable. It has been suggested that the FCC standard be modified to include COFDM in order should be used to solve address these reception issues.
Unlike the ATSC system U.S. DTV broadcasting system based on the FCC standard, The European terrestrial DVB system (DVB-T), which uses COFDM modulation, is inherently resistant to multipath. But COFDM systems requires more than twice the transmitter output power for equivalent coverage and have less data capacity than 8-VSB. The higher peak-to-average power ratio of COFDM requires transmitter power amplifiers with a higher output rating, operating with more ëback off.í The DVB-T system is also less tolerant of impulse noise interference, which is commonly produced by home electrical appliances, automotive ignition systems and high-voltage power transmission lines.
As a result In sum, 8-VSB and COFDM serve different purposeseach have advantages. For DTV in the Americas, 8-VSB is the system of choice hasoffers more advantagesbenefits, including due to its strengths in increased bandwidth for video, audio and auxiliary data as well as lower power transmission power. For mobile datacasting applications, there is value in COFDM. In short, there is no need to change the ATSC FCC standard to achieve excellent DTV reception; an improved 8-VSB demodulator is all that is required.
How the MCT2100 Performs
Motorola and Sarnoff developed the MCT2100 demodulator chip because they recognized that no changes to the FCC standard were necessary, but; thathowever, improvements to first- generation receivers were needed. The key to the superior performance of the MCT2100 is an improved architecture and advanced proprietary algorithms in its adaptive equalizer. While some manufacturers sacrifice performance to save cost by using ësparseí equalizer techniques, the equalizer filter in the MCT2100 is full span, meaning that all possible echo delays within its range are covered at all times. The multistage ëblindí adaptation algorithms in the MCT2100 equalizer do not rely upon the training sequences embedded in the ATSC DTV broadcast signal and are able to rapidly converge and track fast-changing channel dynamics.
Key features and measured performance of the MCT2100 include:
Pilot/carrier tracking comfortably tolerates standard NTSC tuners
Symbol clock timing PLL supports ordinary free-running crystal clock oscillators
Digital downconverter supports bandpass sub-sampling of tuner IF (43.75 MHz) with 25 MHz sampling clock and industry-standard analog-to-digital converters (ADCs)
Low power: 1.8W maximum
Extremely fast signal acquisition: Less than 100 microseconds under worst-case multipath ensembles
Dynamic echo cancellation with phase changes of over 30 Hz and ñ4dB amplitude
Excess signal-to-noise ratio (SNR) above threshold of visibility of at least 3 dB at equalizer output for worst-case multipath ensembles, 6dB typical excess SNR.
Robust pilot tracking loop acquires and maintains phase lock under extreme dynamic multipath conditions: 15 dB pilot amplitude variation with 10 Hz multipath phase changes.
Laboratory test measurements were conducted to represent some typical and severe terrestrial scenarios. The tests demonstrate that the MCT2100 compensates for a broad range of static and dynamic multipath ensembles with echo amplitudes approaching the level of the desired signal and with delays in the range of -2.9 to 41 microseconds.
Field measurements to date were performed at 48 sites in the Philadelphia and Washington, D.C. areas. These sites exhibited strong signal and severe multipath conditions. The MCT2100 performed as expected throughout these tests. Motorola and Sarnoff anticipate being able to release results from all of the field testing locations upon completion in November, along with comprehensive laboratory test results.
Copyright 1999
Sarnoff and Motorola Confidential. All rights reserved.
