Continuing the recent discussions here on Radio World’s website about modulation-dependent carrier level control algorithms, Ben Dawson of Hatfield & Dawson Consulting Engineers writes:
Since I am at least partially responsible for the events leading to the interest in MDCL (I helped nourish the Alaska Public Broadcasting experimental authorizations, the first U.S. implementations), I have some observations about it, leading on from the comments of my good friends Fred Riley and Geoff Mendenhall.
Fred and Geoff both correctly describe the characteristics of the types of MDCL, but neither of them discusses some of the implications and considerations of the uses of these techniques.
Full-carrier double-sideband AM transmission is a very inefficient system of information transmission. The carrier, which provides no actual information, serves only one purpose: It makes demodulation simple, allowing simple, inexpensive practical receivers. Single-sideband transmission, which dominates HF communications (as contrasted with broadcasting), is a practical solution to this problem, but requires receivers that are not simple or widely available. FM (narrowband for communication, wideband for program transmission) serves the same purpose at higher frequencies, and provides other advantages as well where bandwidth is available. (And, of course, digital transmission, practical only since inexpensive solid-state devices have become ubiquitous, has even further advantages.)
But much broadcasting at MF still relies on the inexpensive receiver, and MDCL methods allow this while not requiring better receivers. In my view, each situation calls for the broadcaster to evaluate the respective systems for applicability to that situation.
However, neither Fred nor Geoff discusses an advantage of AMC. Because AMC systems produce power levels (and therefore peak voltage levels) only slightly higher than unmodulated carrier, the voltage stresses on older antenna systems often produced by modern transmitters (which can produce really high positive peak modulation) can be minimized.
Indeed, were a new installation to be operated only with AMC, antenna components — capacitors, guy and feedthrough insulators, inductors, transmission lines — it could be appropriately downsized and result in realistic savings. And the transmitter itself could also be reduced in cost if it were never required to make 100% or more positive peak modulation with full carrier.
My father taught me that over-engineering anything is just as bad as designing something that fails (a tenet that Henry Petroski of Duke University has written about), so in some cases I think AMC is a very useful technique despite my friend Fred’s reservations!
Benjamin F. Dawson III, P.E.
Hatfield & Dawson Consulting Engineers
North Seattle, Wash.