Mendenhall Expands on MDCL Discussion
J. Fred Riley
wrote to Radio World recently with a critical view about the AMC algorithm used
in some modulation-dependent carrier control technologies.
Mendenhall of Harris Corp. expands on Riley’s comments.
Referring to the
recent letter to the editor
from J. Fred Riley, I would like to make a couple of additional comments about
Fred’s point about
the “top down,” AMC algorithm reducing the total peak envelope power (PEP) is
correct. The carrier is reduced by 3 dB during modulation peaks which does
reduce the total PEP of the transmitter, but the full carrier power is restored
during lapses in audio, thereby masking the impact on fringe area S/N. This
perceptual masking effect on the audio S/N would not be the same if the
transmitter power output was simply reduced by 3dB with conventional AM modulation.
Fringe area listening tests documented by several different broadcasters
confirm that there is a negligible impact on received audio S/N or coverage
when using the AMC –3 dB algorithm.
AMC seems to have
the least impact on simultaneous IBOC, HD Radio operation possibly because the
AMC algorithm causes less variation in the total integrated power output of the
transmitter than the “bottom up” algorithms like ACC or DCC. Less variation in
total received power will have less impact on the receiver’s AGC/AVC circuit
trying to follow the variations in transmitted carrier level. More tests with
different receivers are needed to confirm this.
The December 1988
BBC Engineering Report “Implementation of Amplitude Modulation Companding in
the BBC MF National Networks” (BBD RD 1988/15) provides a very complete
analysis of AMC vs. a fixed reduction of carrier power and the relative impact
Fred is also correct
in pointing out that AMC gives the greatest power consumption reduction with
heavily processed audio that maintains high average AM modulation levels. I
have seen up to 37% power savings with AMC vs. up to 21% with ACC using the
same transmitter, audio processing and audio content. If a broadcaster is not
operating with HD Radio and wants to maintain the full PEP of the transmitter,
the option to use ACC or DCC is available from practically all of the
Use of any of the
MDCL algorithms gives the broadcaster the opportunity to significantly reduce
the AM transmitter power consumption without having to accept any significant
impact on coverage or audio quality.
The bottom line is
that the broadcaster will need to experiment with several different algorithms
to determine which one gives the best trade-off in audio quality and coverage
for that particular station’s operating mode and audio program content.
Geoff Mendenhall, P.E.
VP Transmission Research and Technology
Harris Broadcast Communications