As a longtime part 15er it has gotten to feel like there is nothing new to think about, but that all changed with the March issue of Radio which on page 16, RFENGINEERING by Jeremy Ruck, PE, presents the topic: Synchronous AM Boosters Could Help Revive Senior Band.
True, we have had the Hamilton Rangemaster1000 with a synchronous option, but I think I heard recently that that feature has been dropped.
While the article discusses the long history and modern advancements in synchronizing AM transmitters for operation on a single frequency... to de-tune the carrier beat-frequency and lock the audio...what got my attention was a very brief description as part of the textual caption underneath a photo of the original towers for WBZ in Boston:
"WBZ and WBZA were synchronized: the carrier was generated at WBZ, divided downward to an audio frequency, fed over a phone line, then remultiplied at WBZA to create a precisely locked carrier."
That's the only information disclosed in the article about what sounds like a wild and amazing way of achieving synchronization that could probably be accomplished as a part 15 do-it-yourself project!
So far I've tried to reverse-engineer this idea using my limited knowledge of a few things...
An AM carrier has a bandwidth of 10 kHz, so we need that much space in the audio band for our down-divided signal;
The audio base-band is 20 kHz, so we need half of it for our encoded carrier;
Special equalized audio phone lines are normally good for 15 kHz bandwidth, so our down-converted carrier needs to be centered within this space.
What I cannot guess is whether the carrier is pre-modulated with program audio or is sent as a pure signal with modulation injected at both transmitter sites and also synchronized (somehow).
Sound like fun?