COFDM vs. VSB?
May 4, 2004
My short NAB review in last week's RF Report included a note that "It was clear that recent developments in receiver technology have made VSB equal to or better than COFDM for most terrestrial DTV applications." I have to admit I hesitated before writing that, but after sitting through the papers showing the advances 8-VSB had made and hearing encouraging comments about VSB from others on both sides of the COFDM versus 8-VSB debate that raged not too long ago, I felt the statement was justified especially with the qualifier "most" rather than "all."
Shortly after RF Report was posted, Barry Tew in England sent me a note challenging that conclusion. He pointed out that DVB-T (COFDM) technology is much better than it was when the results of the Brazil tests were released. Captured data from the Brazil test has long been used to test and compare DTV receiver performance. He said that while 8-VSB technology may be able to approach the static performance of the original DVB-T receiver chips, DVB-T performance has also improved.
Barry pointed out that the DVB-H specification allows reliable DTV reception in vehicles and on cell phones. I've reported on new cell phones with DVB DTV reception capability in previous RF Reports. More information on the DVB-H standard for hand-held devices is available in the April 2004 DVB whitepaper, DVB H Handheld - IP broadcasting to handheld devices based on DVB-T. The paper notes that DVB-H can co-exist with DVB-T in the same multiplex.
He also said that new technology from French company DiBcom allows substantial improvement in reception of DVB-T broadcasts. Investigating DiBcom, I found a paper by Yannick Levy, CEO of DiBcom and Gerald Pousset, VP of Marketing at DiBcom - A Revolution for Indoor DVB-T portable reception and Outdoor DVB-T mobile reception -- that described how DiBcom uses a diversity receiver to give 3 to 9 dB more margin and twice the Doppler frequency capability while reducing problems with ingress noise and short echoes. Refer to the paper for details on the testing. Many of the tests used 64-QAM 8K COFDM. While adding the second front end necessary for diversity reception increases costs, that cost is offset by the need for fewer transmitters and the ability to use higher data rates. Barry added, "Recent improvements in chip algorithms are said to be able to meet similar results with single antennas."
Barry Tew noted that while he has been involved with numerous DVB-T demonstrations at NAB in past years, he has been amazed that 8-VSB reception "is never demonstrated on simple antennas." I was disappointed that there was no demonstration of E-VSB at NAB this year. As the standard is getting close to being finalized, it would have been a boost for the ATSC DTV system if broadcasters attending NAB 2004 could see how it would work.
Linx Electronics demonstrated mobile reception of a VSB signal (although not 8-VSB) at NAB last year. See the TV Technology News Bytes article Microsoft Teams with LINX, Acrodyne to Demo Mobile HDTV at NAB and the Microsoft press release First Mobile Delivery of High-Definition DTV Demonstrated at NAB2003. Richard Citta presented a paper at NAB this year that he and Drs. Jun Sun and Wenjun Zhang from Shanghai Jiatong University wrote about Single Carrier Mobile DTV Transmission. The paper showed how the VSB based system outperformed the COFDM based system during mobile tests on Shanghai buses.
DTV modulation methods are improving and enhancements are being rolled out for both the DVB and ATSC standards. Only field-testing in a variety of real-world situations will show which is best. In any case, significant improvements in both standards will benefit consumers and broadcasters.