I was glad to see the FCC issue its tome on IBOC recently … and after wading through it several times, I found something curious in the multicasting discussion in Section 33, where the commission discusses the loss of the blend-to-analog feature when a station chooses to multicast.
Testing by NPR demonstrated the supplemental channel will have coverage equivalent to the main channel, it told the commission, though the tradeoff is loss of the blend-to-analog feature and rapid tuning. “Any supplemental channel will require several seconds for tuning and will experience muting of the audio in the event of signal loss,” stated the FCC.
Any? Seems to me years ago when engineers first discussed multicasting, the gist was that the first multicast channel (HD2) would escape those ills of long tuning time and muting but that other supplemental channels could be affected.
I asked Mike Starling of NPR Labs to clarify.
He believes all multicast channels behave similarly in that:
1) If tuning directly to the channel there is a “digital acquisition” delay, but you’ll experience none if first tuning to the analog for six seconds while the radio “acquires digital”; and
2) All multicast channels have “no analog fallback,” unlike the HD1 simulcast channel, however typically no analog fallback is an issue only when leaving the coverage area.
As to how robust an HD2 is to an HD3, he says “HD2 is reported by ‘some’ stations as being more robust, with wider coverage, than an HD3 running on the extended hybrid carriers,” however the lab believes this is a station-specific issue and not an inherent characteristic of the HD-R design.