Last year, at NAB2000, the hot topic among TV broadcast engineers was whether or not COFDM would be allowed to join 8-VSB as one of the authorized DTV modulation formats. This year, I got the impression many engineers would rather focus on building out their DTV plants than continue to debate 8-VSB versus COFDM. I continue to believe, however, that if over-the-air TV broadcasting is to succeed in an increasingly digital world, we must have a reliable, robust modulation method.
Those who took the time this past April to attend the NAB Engineering sessions and visit demonstrations in private suites at the Hilton found that progress was being made in improving the robustness of 8-VSB. Because most of the RF transmission products introduced at NAB2001 will be of little use if a modulation standard doesn't work in today's over-the-air reception environments, I'll look at the 8-VSB improvements first and examine interesting new RF products next month.
HIERARCHICAL MODULATION
In my October 4, 2000 TV Technology article on hierarchical modulation I discussed the possibility of using the existing 8-VSB system to create pseudo 2-VSB or 4-VSB signals, but indicated this would not be easy and it would be even more difficult to make compatible with existing ATSC receivers. I was pleased to find that at least two companies have succeeded in creating a more robust 8-VSB signal solely through coding.
New receivers will be required to take advantage of the robust stream, but both methods allow the simultaneous transmission of a "normal" ATSC signal along with the robust signal. Because the robust packets are easier to receive, receivers can be designed to use the robust packets to improve reception of the "normal" signal.
The 8-VSB enhancements are not as flexible as the hierarchical modulation available with COFDM, as the DVB-T standard allows a wider range of modulation methods and data rates. The enhancements will, however, provide a way for U.S. broadcasters to provide a lower-resolution picture to portable TV sets in difficult reception environments while maintaining a high-resolution picture for viewers in better reception areas or with outdoor antennas.
Of course, these enhancements would be of little value if they were not incorporated into the ATSC standard. Before I describe the enhancements, it is worth pointing out they were submitted to the ATSC as part of an 8-VSB improvement effort undertaken by the ATSC. The RF Task Force report and the request for proposals are available on the ATSC Web site at dlung@transmitter.com.
