Total posts : 45366
“As an interesting aside, I have read about commercial AM transmitters that are capable of dynamically increasing power on positive peaks.”
I’ve been tinkering with the AM 100 circuit which you can find in the “PDF Reading Room” section of this board, and it does that if I understand the manual correctly.
Pages 7 and 8 of that pdf talk about modulation levels and the carrier control system and show what the o-scope readings would look like.
I don’t have a scope that does rf range, I use my computer and some software for an af scope, so I can’t say how accurate their depictions of the scope trace are. I *can* say I like the sound of it and it can get fairly loud without noticeable distortion and I haven’t been running it with compression (basically because i don’t like the sound of compression).
But if you want to tinker with the basic concept, that AM 100 circuit is pretty robust.
“There is theoretically no limit to the dynamic power increase. I wonder what the FCC would make of this?” >/cite>
I would *guess* that at least at some point they’d have a problem with it. Also there is a problem with driving the circuit hard to get more audio at fringe range.. It doesn’t sound as good close in if you do. So there would definitely be a tradeoff in using it as a scheme for getting more audio level in the fringe range, I think.
Mostly it probably boils down to the usual.. If a licensed station complains then they’d look very close. Even though running “300% modulation on the positive peaks” might be *technically* legal, it’d be a very easy to just tell you not to do it and tell the licensed station that they told you not to overmodulate and that’s that.
More interesting to me is that it is a good sounding alternative to compression or using a clipper other than a .6v diode “clamp” as a hard limiter in case of some accidentally/unexpectedly high transient or peak.
I can’t really say how it affects actual range in the field, since the project is still at the bench level here. And currently on hold until I rebuild my bipolar power supply which I managed to fry one of the regulators and a couple resistors of during an embarassingly brain-dead moment.
I haven’t done “real world” tests for range yet, and have just used a wire around 2 ft long and approximately vertical right off the circuit board, to emulate a fringe level signal at the other end of the house (maybe 50 ft) and the near range being about 10 ft away. No idea how it’ll do outdoors with a decent antenna and something resembling an actual ground. Still considering options for actual antenna and case for outdoor mounting.
But what I saw when I was tinkering with it seems to indicate that pushing the “over 100% positive peak” concept too far has an adverse effect on the audio quality to a nearby receiver, though it does improve the legibility more to the fringes. So I’d say it’s possibly a tradeoff and while there theoretically might not be a legal limit, there would be practical limits so far as actually putting out a good sounding signal. Yeah, it’d be physically possible to use a circuit similar to this to do 400% or 800% or whatever, but it might result in actually not sounding good for anybody *except* a fringe listener, and pushed enough, it might not even sound good on the fringe. When Keith designed his Rangemaster, I doubt he went for the possible 125% because he couldn’t think of ways to get it any higher. Keith is a highly inventive individual and I’m sure he could think of ways to push it way past that. I’m guessing 125% was what he found to be a good compromise between getting better audio at the fringe *and* sounding good near.
Listeners expect stations they’re listening to will sound good through the whole range they can receive it and will most likely tune to another station if it starts sounding crappy, and wouldn’t care much about the technical reasons why it didn’t sound good.
Back to the FCC aspect, I think I recall reading somewhere that (as you said) it’s fairly standard practice but not when done to excess and that they particularly frown on it being used to make ads considerably louder than the usual program material. I think if it was routinely being used to *drastically* increase part15 ranges, they’d draft a new rule forbidding it or simply tell stations they can’t do it. Part15 operation is permitted so long as it doesn’t violate the rules, but at least as I understand it, we are not by any means *guaranteed* operation even if we stick to the rules if it causes any problems. If licensed stations complained because a theoretically legal (at unmodulated carrier power) part15 is getting enough solid range that the licensed station doesn’t like it, I expect that in most cases the part15 station would simply be told not to do it.
Some tradeoff between sound quality and range is acceptable of course, but if all we wanted was to get maximum range.. a CW signal and using principles like the people doing LW beacons do could give great range. But who’d want to listen?