Digital broadcasting has now replaced high-powered analog television broadcasting in the United States. But is reliable over-the-air reception a given? The FCC planning factors for DTV were developed to replace analog broadcast with a similar digital footprint. Successful digital reception, however, is subject to a different set of conditions.
This month, we'll look at a number of methods being used to maximize digital reception quality. While some of these issues relate directly to the transmission side of broadcasting, some are a function of receiver technology. Nonetheless, broadcasters can do their part to improve the performance of receivers by participating in discussions when relevant standards or rules are revised, and as products are specified and developed.
Circular polarization
Recently, SPX Dielectric Corp. conducted a study into the reception characteristics of receivers, with the aim of investigating the use of circular polarization for mobile handheld televisions. Using simulation software to evaluate a sample device's RF reception characteristics at VHF and UHF frequencies, its conclusion was that the small antennas in a handheld phone essentially behave with the characteristics of the long dimension of the phone; the phone itself acts like a dipole, and the antenna's polarization is along the axis of the phone.
The company then conducted a field study of the impact of signal polarization on reception by a handheld device. The noise margin of the receivers was measured to determine any possible improvement of vertical and circular polarization over that of horizontal polarization. According to the company, circular polarization offered a 5dB average improvement over horizontal polarization and a 7.5dB average improvement over vertical polarization. These results suggest that coverage area and/or reception reliability could be enhanced by the use of circular polarization. Of course, further studies into different sample cases should be carried out before embarking on the expense of a new antenna.
Continuing receiver improvement
Last October, the FCC Office of Engineering and Technology (OET) published a report on its testing program, conducted on behalf of the NTIA DTV converter box coupon program. While the report concentrated on lessons learned in testing converter boxes, the information is useful for assessing the state of the art of DTV reception technology (i.e., tuners and demodulators), which is independent of the final display device.
Historically, the spacing of UHF TV stations has been regulated by the FCC as a consequence of the technical limitations of TV receivers. These so-called UHF taboos are a function of various tuner mechanisms, including local oscillator (LO) radiation, intermediate frequency (IF) beat, IF image rejection, nonlinear distortions and out-of-band rejection. Receiver performance in the presence of interfering signals from adjacent channels (N±1) and taboos (N±2, 3…15) is therefore critical to good reception in a crowded market.
The good news is that receiver performance continues to improve with time. According to the FCC report, taboo-channel rejection performance of the converter boxes showed significant improvements over the 2005 and 2006 DTV receivers that had previously been tested, with median performance of the converter boxes exceeding that of the earlier DTV receivers at every channel offset that produced a measurable result.
There have been concerns by some that the FCC DTV channel allocation process — which started more than 10 years ago — did not fully take into account the effect of multiple interferers in a coverage area, and this could challenge a receiver's intermodulation susceptibility. According to the FCC report, the converter boxes tested exhibited better rejection performance against paired interferers than the group of eight 2005 and 2006 DTV receivers that had previously been tested. In addition, the measured VHF and UHF sensitivities of converter boxes were found to be about 2dB better than the minimum performance requirements recommended by the ATSC for all DTVs and required by the NTIA for coupon-eligible converter boxes.
During the FCC channel allocation process, the commission used various propagation models to determine DTV broadcast coverage. The report also shows a receiver improvement in this area, with the median sensitivities of receivers about 4dB better than the sensitivity assumed by the coverage model for VHF and 1dB better at UHF. The 10th percentile (near worst) performance on all tested channels was about 1dB better than the ATSC guidelines and NTIA requirements. Median sensitivities of the converter boxes were from 1dB to 3dB better than those of the 28 DTV receivers that the FCC tested in 2005, and 10th percentile performance in the high and low VHF bands was 4dB and 7dB, respectively — better than that of the DTV receivers from the 2005 tests.
Guidance on DTV antennas
The Consumer Electronics Association (CEA) and MSTV have issued various documents to help consumers maximize their DTV reception. CEA has published several standards, such as CEA-774-B, CEA-2032-A and CEA-2028-A, which help to match the correct antenna performance to a viewer's particular reception situation. These three documents specify general performance criteria, indoor receiving antenna performance and product color codes, respectively, and form the basis for a consistent marketplace consumer education program.
MSTV has published a set of consumer tips for VHF reception, as well as an advisory on indoor antennas. Both are intended to be used as tools for broadcasters to aid consumers experiencing difficult reception conditions. The consumer tips address some interesting factors that will not be obvious to many consumers, such as the fact that an antenna should not be placed near the TV itself (due to radiated electronic interference), and that amplified VHF antennas may increase the interference from other electrical devices in the home.
NAB sponsored a program last year to help develop smart antennas and a single-wire interface for them. (Smart antenna technology allows a DTV receiver to dynamically reconfigure an electronically adjustable antenna to optimize the reception performance across multiple stations. A new compatible receiver and antenna are needed; the two require a CEA-909A interface.) The NAB FASTROAD project had a number of interesting findings. Apparently, many currently available CEA-909A enabled DTV converter boxes do not rigorously implement or conform to the CEA-909A specification, leading to implementation and crosscompatibility issues.
Also, while the firmware on most converter boxes will support a simple antenna (e.g., a switched dipole), the support for advanced smart antennas varies widely. Since firmware functionality and compatibility are not defined in the current CEA-909A standard, success of the smart antenna technology in the marketplace may hinge on establishing compatibility standards. Sources tell us that a revision to the standard is in the works to address these issues.
DTV gap fillers
Good DTV reception at fixed locations is sometimes an issue due to geography (terrain) and a coverage pattern different from that of NTSC, potentially leading to holes in coverage. DTV transmission can get a boost in this respect with gap fillers or repeaters that operate on-channel to extend or fill in coverage. Last year, bills were floated in both houses of Congress, called the DTV Cliff Effect Assistance Act of 2009, which could help broadcasters deploy such equipment.
The bills call for establishing an assistance program for the construction of digital TV translators, with the Department of Commerce making payments of up to $125 million in the aggregate, during fiscal years 2009 through 2012, from the Digital Television Transition and Public Safety Fund. The program would target local civil government bodies, which would receive funding for the construction and equipment of digital TV translators. At press time, the bills were referred to committees in both houses, and await debate.
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to:aldo.cugnini@penton.com
