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I’m a bit late to this thread, but have some comments regarding bandwidth/frequency response.
As has already been noted, there is nothing inherent in the process of amplitude modulation to limit bandwidth. Current FCC regulations set the upper frequency limit for audio content on the AM broadcast band at 10KHz. But, and this is a big BUT, that has not always been the case, and in fact that rule does not apply to us under Part 15.
In days of yore, when radio began, the state of technology was the limiting factor in fidelity. By the 30’s or so, microphones and speakers were the limiting factor in frequency response. By the mid-late 50’s true HiFi was coming of age, and 50-10K response was common, and 30-15K was attainable. By the time the 70’s had rolled around, the ‘gold standard’ for frequency response in the audiophile world had settled as 20Hz-20KHz, which is also assumed to be the extreme limits of human hearing.
Anyhow…. at some point in all this, probably in the 50’s or early 60’s, the FCC designated 15KHz as the limit for AM Broadcast. A tad short of ‘audiophile” quality, but nonetheless still excellent. By the late 60’s or early 70’s, stations had begun to discover the “benefits” of simple compression and equalization in attempts to make their station sound “louder” or “better” than the competition. That practice has, of course, continued to this day, with ever-more-complex audio processing being applied for virtually all broadcast audio (am, fm, digital, analog, tv, internet, you-name-it).
At some point, and without trying to research it I’ll again take an educated guess and say perhaps late 80’s, and maybe coincident with the appearance of C-QUAM stereo, the FCC cut the bandwidth on AM BCB down to its present 10KHz.
But we, operating under Part 15, are not subject to that, and therefore can offer (to those listeners who can receive and appreciate it) a wideband 15K or even 20K signal, provided our transmitters are capable. And most transmitters sold for Part 15 use are indeed capable of at least 15K or better, as they do not employ any steep or “brick wall” filtering in the audio path.
One exception to that would be Radio Systems and LPB transmitters that may have been used in as TIS stations: If you repurpose one for Part 15 use (carrier-current, most likely) that was previously used for TIS, you’ll want to bypass the “TIS Filter” which limits bandwidth down to 3KHz. I’m not sure if this filter is there for practicality or if there is an FCC rule limiting TIS stations to 3K audio bandwidth, but you’ll definitely want to bypass that filter (if present) unless your station is to be talk-only.
OK… ’nuff rambling on my part.