http://www.rbr.com/features/ideas-working-now/26693.html?print The WJFK HD Power increase and quadcast
13 August, 2010 03:00:00 CBS Radio has been a long term supporter of digital radio as it is one of the three original companies who funded the development of the HD Radio system and has supported improvements to the system that improve reception and functionality. Dan Mason, President and CEO, in the fall of 2010, had a vision for a sports multicast where multiple sports programming options would be available to HD radio listeners. In addition the new rules allowing a 6 dB power increase was imminent. CBS chose WJFK as a candidate station to implement a power increase and to provide 4 channels of sports programming. CBS Radio is home to some of the best sporting brands in radio and it was decided to make some of these brands available to the listeners of WJFK. When the In Band On Channel (IBOC) -- now HD Radio -- system was being developed little was known about the extent of existing interference and noise levels in the FM band. iBiquity’s predecessor, USA Digital Radio worked with the best consulting engineers and the FCC to characterize the existing band conditions. Even though it had never been done before, the existing levels of co and adjacent channel interference were readily determined, however, the FCC nor the community of consulting engineers had little information on the noise level in the FM band. The iBiquity system was modeled in software using assumed noise levels, known multipath scenarios and the levels of co and adjacent channel interference in the FM band prior to the development of any hardware. Test hardware and field testing followed with results showing good results with mobile field testing. The tests showed that the system better endured the effects of multipath than its analog component and mobile digital coverage extended to the point where the analog signal beginning to show impairment, but not to the full extent of the analog coverage. iBiquity, acting out of an abundance of caution, held the total power at 1% (-20 dBc) of the analog signal to insure that no interference to adjacent channels would occur, which meant stations like WCBS FM, in NYC, limited their power to an ERP of 82 watts, which provided a less than acceptable mobile service and woefully inadequate indoor reception. Early assumptions on noise levels were off by a factor of 10 or more and indoor noise proved to be even worse than the assumptions for iBiquity’s consultants. With the advent of the portable HD receiver, HD power levels needed a boost in order to provide reasonable service and interference from HD radio had proved to be far less than anticipated. CBS, Clear Channel and Greater Media began a program to test elevated power HD broadcasts on stations that were known to have multiple egregiously short spaced adjacent first adjacent channels. The first test was with WMGK, 102.9, Philadelphia and WNEW, 102.7 (Now WWFS), New York. These Class B first adjacent channel stations were not egregiously short spaced at 90 KM, however, they served as a “first look” at potential first adjacent channel interference increases that may result from a 10 dB power increase. Listening was conducted on 102.7 in an area outside of its protected 54 dBμ contour where Longley Rice had predicted should be expected. Those early tests demonstrated that even though 102.9 had increased its power by 10 dB the noise only rose by 1 – 2 dB. The spectrum analyzer showed in those tests that when operating at -20 dBc the HD carriers on 102.9 could not be seen below the noise floor of the FM band and when the power was increased on 102.9 its HD carriers just were observable above the noise floor. Demonstrating what was already known, that the noise floor of the FM band was limiting the coverage of the very low power HD carriers and the existing FM noise floor was sufficient to mask the effects of the power increase. Based on these favorable tests CBS, Greater Media and Clear Channel went on to test in three markets with stations that had multiple severely short spaced first adjacent channels and to seek, through rules modification a power increase to improve digital radio coverage. Almost simultaneously with the FCC’s decision to grant a 6 dB HD power increase, Dan Mason had a vision for a sports multicast where multiple sports programming options would be available to HD radio listeners. CBS chose WJFK as a candidate station to implement a 4 channel multicast along with a simultaneous HD power increase. CBS radio is home to some of the best sporting brands in radio and it was decided to make some of these brands available to the listeners of WJFK. On March 8, 2010, WJFK commenced full time quadcasting of “The FAN Sports Network” bringing DC sports fans WJFK-FM and programming from three other CBS markets: WJZ-FM in Baltimore (HD2), WFAN-AM in New York (HD3) and WIP-AM in Philadelphia (HD4). CBS had been successfully delivering WFAN’s sports programming to HD multicasts in Tampa, Orlando and West Palm Beach over the CBS corporate WAN prior to the decision to develop the quadcast for WJFK. Each originating station employs a Broadcast Electronics Audio Vault automation system to insert cover material during live play-by-play, and an Ando Media machine to cover in-market spots, insert network or local spots and encode the audio for delivery by the corporate WAN to WJFK. The audio from each station is decoded at WJFK where it passes through an Audio Vault workstation for local station identification, EAS and local Washington DC public affairs programming. The fully packaged program is delivered as AES audio to an HDI-100 Importer where the signals are encoded and multiplexed with the PAD (program associated data) for each multicast channel and transported as a single IP stream over a T1 link to WJFK transmitter. WJFK chose to continue to use its StarLink STL to transport the main program channel as AES audio to the transmitter. At the transmitter a Harris FlexStar HDE-200 Embedded Exporter takes the incoming AES main channel audio stream, provides diversity delay for the analog channel, encodes the Main Program Service for the main channel and multiplexes it with the incoming multicast from studio. In order to accommodate the additional multiplex channel, WJFK employs iBiquity’s extended hybrid mode with the P3 partition adding an additional 24 kbps of capacity for a total of 120 kbps throughput. The fully packaged HD stream is delivered to a FlexStar HDx exciter where it is combined with the analog channel for amplification by a Harris HPX30 transmitter. The HPX30 operates in the low level combined mode where the analog and digital signals are simultaneously amplified in a common amplifier. The transmitter is providing WJFK with an analog transmitted power of 22.5 KW ERP and 985.3 W ERP of digital while meeting the more restrictive emissions mask that is imposed on stations operating with elevated HD digital power. Since March 8, WJFK has been successfully operating from its Falls Church, VA transmitter site with four program channels and with its digital signals at the -14 dBc level. The system has been proven to be stable, the audio quality of the multicasts is excellent and the digital coverage significantly improved. Ray T. Mahorney WA4WGA _______________________________________________ Medianews mailing list [email protected] http://lists.etskywarn.net/mailman/listinfo/medianews
