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Using Asymmetric Digital Sidebands with FM IBOC Since the FCC’s authorization earlier this year of a voluntary digital power increase for FM radio stations, many FM broadcasters using the HD Radio IBOC digital radio system have been considering taking advantage of this new capability (see the February 1, 2010 issue of Radio TechCheck for information on the FCC’s authorization, and the August 9, 2010 issue for information on stations that have already increased their power). One “tool” for managing this power increase that is currently being experimented with is the use of asymmetric digital sidebands, whereby one sideband is kept lower than the other in order to protect a radio station on a first adjacent channel frequency from undue interference. With the asymmetric sideband approach, a broadcaster can maximize the digital power in each sideband and not be limited on both sidebands due to a potential interference situation that exists on only one. Currently, FCC rules do not allow for asymmetric sideband operation, so any station interested in doing this must apply for an experimental license. At the recently held Radio Show (Washington, D.C., www.radioshowweb.com), FCC staffers indicated that once sufficient experimental information on this mode of operation has been submitted, the Media Bureau will be able to consider modifying the rules to allow asymmetric sideband operation. At this point, manufacturers have just begun to offer an asymmetric digital sideband option, so it is not likely that any existing broadcast equipment in use will support an asymmetric mode of operation.
Last month, Washington,
D.C. station WAMU (88.5 MHz, channel 203B) was granted an experimental
license for asymmetric sideband operation and this was discussed recently
both at the Public Engineering Radio Conference (PERC) and the Radio
Show (see the October
4, 2010 issue of Radio TechCheck for a brief write-up as
well as a spectrum plot). The application for experimental license
filed by WAMU is available on the FCC Media Bureau’s CDBS Public
Access Web page – to obtain a copy: 
Included in this application was a propagation study done for WAMU by NPR Labs, an excerpt from which is shown in the figure at right. The two WAMU contours in the figure correspond to limits placed upon stations by the FCC on the amount of digital power increase allowed, which depends upon the proximity of nearby upper and lower first-adjacent signals. Under the new FCC rules, nearly all stations are allowed to increase their digital power from the original -20 dBc to -14 dBc. For a station to increase its digital power to the maximum allowable -10 dBc, all first-adjacent channel 60 dBu contours must fall outside of the digital station’s 49.5 dBu contour (the 51.2 dBu contour represents the point at which a station may increase digital power from -14 to -13 dBc). The NPR Labs study determined that the 60 dBu contours for two first-adjacent channel signals, WHMM (lower) and WKNZ (upper), fall within WAMU’s 49.5 dBu contour, hence prohibiting an increase to -10 dBc on both upper and lower digital sidebands. However, the engineering study points out that for WKNZ, the overlap of WAMU’s 49.5 dBu contour occurs in a rural area with low population, and further, based on the Longley-Rice study, there are virtually no locations within the overlap area that would be subject to interference from WAMU. Consequently, WAMU requested (and obtained) permission to operate the upper digital sideband at the maximum allowed -10 dBc power level. Also noted in the study is the fact that neither WKNZ nor WHMM are currently operational (WKNZ is expected to become operational mid-October, and WHMM has not yet been constructed), further ensuring that granting of the experimental license should not result in any undue interference. Under the terms of the license, WAMU must submit a report to the FCC detailing the methodology employed and the results obtained within 90 days following the conclusion of the experimental operation. This information will likely be helpful to the FCC in determining whether to authorize asymmetric sideband operation. Additional testing is scheduled for later this month at Greater Media station WKLB (Waltham, Mass., 102.5 MHz, channel 273B). This testing is being conducted by iBiquity Digital Corporation and is partly funded by the NAB FASTROAD technology advocacy program (www.nabfastroad.org). The results from these tests will be submitted to the FCC, as well. |
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Plan
to Attend Keynote speakers
for this year's symposium include James Martin, Director, ISR Programs
for the U.S. Department of Defense and James O'Neal, Technology
Editor, TV Technology, USA. Additional details on the technical
program and how to register
are availale on the IEEE Broadcast Technology Symposium website.
Submit
Your Proposal Now In order to be considered, proposals must explain what attendees can expect to learn from the paper, must not be a sales pitch, and should be no more than 200 words in length. Papers accepted for presentation at the 2011 NAB Broadcast Engineering Conference will be eligible for the NAB Best Paper Award. Established in 2010, the Best Paper Award honors the author(s) of a paper of exceptional merit published in the NAB Broadcast Engineering Conference Proceedings. The yearly proceedings, published as both a book and a CD-ROM is a compendium of these technical papers, and an important archive of the leading edge of broadcast engineering issues. Technical paper proposals submitted for the 65th annual Broadcast Engineering Conference will be accepted until the October 22 deadline. If you have any questions, contact John Marino, VP NAB Science and Technology at (202) 429-5346. The October 4, 2010 Radio TechCheck is also available in an Adobe Acrobat file. Please click here to read the Adobe Acrobat version of Radio TechCheck |
