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AM dynamic carrier control?
A true story!
Idon’t think it’s any surprise to anyone in our business that AM broadcasting
can be a pretty expensive proposition. Aside from the usual transmitter and
equipment costs, there’s often a great deal of transmitter site land tied up, extensive
civil works and big power bills to boot.
Those power bills don’t look to be getting any easier to handle. According to
the local utilities, we’ve been living in a subsidized bubble and the end of the
“easy times” is approaching rapidly, perhaps never to return. In B.C. we’ve been
told to expect fifty per cent increases in electric power costs over the next few
years, just for starters.
Yikes!
When one’s already subjected to transmitter power bills in the thousands of
dollars per month, how is one to make ends meet when all this comes to pass?
For an unusual answer to this problem, one might turn to the story of Chuck
Lakaytis, the director of engineering for the National Public Radio stations in
Alaska. NPR runs a network of stations, including many AM stations, throughout
the more populated parts of Alaska. They’ve already been hit with huge power
bill increases as most of the power generated there comes from diesel generators.
And the increasing fuel transport costs coupled with the increased costs
of the fuel itself have hit them hard.
There’s no end in sight.
They’ve contemplated shutting down their AM rigs and replacing them with
FM for the power savings, but in the remote north nothing gets out into the remote
areas like AM.
Lakaytis has been experimenting with, and has become a proponent of, a
technique called Dynamic Carrier Control. Simply put, this is a modification of
standard amplitude modulation designed to save on power consumption. While
this sounds kind of quaint to our ears, evidently BBC and other heavyweight
broadcasters overseas have been working on this for decades and have found
algorithms that will reduce the power bills but result in transmissions that
sound good on standard AM receivers. If you’ve listened to BBC on LW, MW or
SW overseas in the last 30 years, chances are you’ve been listening to one of
these broadcasts without realizing it.
There are two contrasting techniques out there: the BBC has AMC, and the
Germans and Swiss have been tinkering with DAM and DCC. AMC reduces carrier
power during peaks of modulation and restores the carrier to full power
during silence in order to get the receiver into full quieting. The carrier level
is reduced in such a fashion that the receiver’s AGC is prompted to increase
ENG
INE
ERI
NG
by Dan Roach
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BROADCAST DIALOGUE Technology Insider • April 19, 2011 14