The issue of 8-VSB vs. COFDM modulation for DTV transmission is officially settled, but that doesn't mean the industry is standing pat with 8-VSB in its current form. In fact, it can be argued that the only reason 8-VSB was able to win the battle against COFDM was the promise that it would steadily improve.
To fulfill the promise, the Advanced Television Systems Committee (ATSC), an industry standards-setting body, is evaluating two ways of improving the transmission of the 8-VSB signals: the robust-stream method and the "modified training signal."
"In both cases, the enhancements are backwards-compatible, and broadcasters can choose whether or not to use them," says ATSC President Mark Richer. "It's also important to remember that the ATSC work on VSB complements the rapid advancement of receiver technology in terms of multipath performance."
The robust-stream method involves encoding content in such a way that it lowers the minimum carrier-to-noise threshold, he says. "In layman's terms, this means that it can be received where the signal strength is lower."
But the method involves a trade-off. It reduces the bits available for services. In fact, Richer says, "it is unlikely broadcasters would use the robust stream for HDTV because it would take a large number of bits."
However, he adds, the robust stream could be used for broadcasting TV audio or data services to mobile or portable devices—laptops, PDAs and such. The data services could include compressed video and audio.
TV audio could be sent in the robust mode so that it would continue to be received even if the TV signal fails, Richer says. This would make the DTV signal akin to current analog signals.
Some believe the robust stream could also be used to overcome multipath interference and improve signal acquisition, he says.
A consortium of the National Association of Broadcasters and the Association of Maximum Service Television (MSTV) is testing the robust stream to see how well it works and how it impacts the rest of the DTV signal, so it's still too early to draw conclusions, Richer says.
The modified training signal is designed to provide additional immunity to multipath interference that has marred VSB use in areas with tall buildings and other structures. The training signal is a code sent with the DTV signal that helps the receiver optimize itself for reception of the digital signal and better deal with multipath interference. Unlike the robust-stream method, the training signal does not come at the expense of content. An NAB-MSTV consortium of companies is currently testing it.
The success of the modified training signal could be an important milestone for 8-VSB, one that could place it on steady ground. The inability of DTV tuners to easily receive VSB signals in city environments has garnered most of the criticism of the standard.
Richer says the two-pronged approach by the DTV-receiver manufacturers and the ATSC will drive the necessary improvements. "There's a lot going on in receiver technology that has nothing to do with the standard. That's a natural progress of technology, and it's especially true in terms of multipath performance."
