Chuck said:Even so, where I live (Texas) 40-50 miles is more or less considered to be local. Unfortunately, this technology doesn't seem to work very well at that distance. Perhaps HD really does make a new definition of "Local Radio."
I was referring to w9wi's high-on-the-dial "local" AMs, 5 kW on 1430 and 50 kW on 1510. Totally different from the big class C FMs I presume you're talking about in Texas.
The consensus so far seems to be that pretty much no AMs work well in HD at 40 or 50 miles, even flamethrowers like WBZ.
I am no fan of the AM system. I think the FM system holds some promise. There's no good technical reason why it shouldn't work at 40 or 50 miles on a class C signal, assuming relatively clean adjacent channels and a decently-configured transmission system. (Based on anecdotal reports, this seems to be an issue in the DFW market at present.)
When it comes to defining "local" signals, I'm less interested in distance (given all the variables in AM ground conductivity, FM classes, etc) than in received signal strength.
For an AM station, any system that's even remotely usable ought to function to around the 2 mV/m contour. The Ibiquity system doesn't. My experience suggests that at least 10 mV/m is needed for it to decode on most radios. How many stations have 10 mV/m coverage of all, or most, of a sprawling market like Dallas or Houston? Very few, if any.
For an FM, NPR's research has suggested that the limit for reliable mobile use of HD is somewhere in the 64 dBu range. Receiver performance seems to have improved somewhat since those early tests. My experience says most of the car receivers that are out there now can decode down to about 60 dBu. That may not hit the fringes of a station's current analog coverage, but it's respectable, and there's at least some hope for incremental improvement with higher HD injection levels and better transmit antennas.
And, Chuck, come over and say hi at Barry's next luncheon, won't you?