Dawson: AMC Can Be a Very Useful Technique
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:
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
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.
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
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.
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.
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.
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
Benjamin F. Dawson III, P.E.
Hatfield & Dawson Consulting
North Seattle, Wash.