According to NAB, as of Oct. 10, there were 210 DTV stations in operation in 71 markets, reaching over 70 percent of U.S. households. This figure sounds good, but are there any sets out there to receive the new digital signals?
It's difficult to get a reliable count of digital sets that have actually been sold. The best figures seem to be between 150,000 and 200,000.
Little more than thirteen percent of all high-power television stations have made the transition and the deadline is May 2002 for commercial stations and May 2003 for non-commercial stations. The FCC has candidly said they'd grant one waiver per station, but after that, all bets are off.
Early on in the life of real-world digital over-the-air television, the most debilitating problem encountered was multipath. Attempts to address this issue have resulted in many generations of receiver/detectors. Most visible of the chip makers addressing this issue have been Motorola and NxtWave.
Because Zenith holds a lion's share of patents on 8-VSB, they obviously have a vested interest in seeing it succeed. Receiver/decoder chip developers have come up with a myriad of patches. The Advanced Television Systems Committee (ATSC) recently issued a Request for Proposal (RFP) to improve 8-VSB performance that essentially had only three requirements: that it perform measurably better than the current ATSC standard; that it be backward compatible with at least some current receivers; and that it not use any modulation method other than VSB.
With millions of dollars being spent attempting to make a decoder chip that could cope with the multipath issue, it was only logical that NxtWave would seek an alliance with the parent of 8-VSB — Zenith Electronics Corp. Before collaborating, NxtWave and Zenith had independently developed enhancements to the VSB spec and submitted them to the ATSC. The result of their alliance is the merger of their competing modulation technologies into a single multi-rate, dual-stream system.
They have developed a compromise that combines NxtWave's error-correction coding and precoder technology with Zenith's data frame mapping, interleaving and packing algorithm. The combined system provides better signal-to-noise ratio than either company's individual approach. Under the Zenith/NxtWave multi-rate, dual-stream architecture, a mix of regular 8-VSB and enhanced, robust streams are sent to receivers.
Broadcasters will have control over the dual streams; lending flexibility to the new system. They will be able to change the mix and bit rates of the two streams, as needed and on the fly, sending separate data, audio or video programs via the robust stream while the main video program is sent in the 8-VSB stream.
The backward compatibility requirements are met in that those few legacy receivers/decoders currently in use will be able to incorporate the “conventional” 8-VSB signal stream in their respective demodulator chips, while ignoring data sent via the robust stream.
The key to the new system's success lies in the fact that a DTV receiver with the new dual-stream demodulation capabilities will improve the pictures sent via 8-VSB due to the “training signals�� sent via the robust stream. The bottom line is that the new system trades data rate for reliability.
If the Zenith/NxtWave alliance can demonstrate that their new “enhanced modulation scheme” improves indoor DTV reception, it may open the door to portable and mobile applications. That would be the capability COFDM proponents have been asking for all along.
According to NxtWave, if the Zenith/NxtWave proposal stands up under the ATSC magnifying glass, the combined modulation approach would be ready for integration into DTV receivers 12 to 18 months after its adoption.
