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HD Radio Boosters and Single-Frequency Networks
The effect of the digital-only boosters on analog radios was tested along I-93:
As you can see, the route was taken driving towards the booster (digital only) and away from the main site (hybrid transmission). As iBiqity's test vehicle moved along this route, the digital-to-analog power ratio varied from -6dBc to +20dBc as the distance closed from 5.65 miles to 1.3 miles, and then increased to four miles as the route passed the booster site. The test van was outfitted with a newer Delphi radio, an older Chrysler OEM radio, and an analog clock radio. The newer Delphi radio was able to withstand the IBOC sidebands, and continued receiving the analog signal from the main site; the older Chrysler radio suffered, "exhibiting audible interference from the beginning of the run to end of the run. Close to the booster, the audio was all but inaudible." The analog clock radio's audio was not evaluated because of the receiver's extremely poor selectivity.
Another set of tests were performed during the WKLB booster experiment: a small amount of analog FM (synchronized to the main) was transmitted from the booster site, to see if that would mitigate the effect of the higher-than-usual IBOC carrier levels (with respect to analog) in proximity to the booster site. According to the report, the results were "mixed.” From the text: "Using this approach, with similar main and booster analog signal levels, all receivers will suffer from the multipath-like interference."
The results of these three experiments seem to indicate that while it is possible to build a digital-only SFN, that implementation of such systems is still a ways off. As time goes forward and more and more new analog receivers appear, the effects of digital only, on analog reception, near the digital booster transmitter itself, will diminish.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at email@example.com.
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