The author, WA2EWT, is a broadcast consultant in St.
The FCC has
formally approved a “green” technique for AM radio stations that want to reduce
their operating costs. The technique is known as Modulation Dependent Carrier
Level, or MDCL. Public notice was issued on Sept. 13, extending what had been
an experimental program in Alaska to all U.S. AM stations.
broadcasters have been familiar with the concept for many years under the name
of Dynamic Carrier Control. The purpose of this article is to explain to U.S.
AM stations a technology already familiar to their colleagues abroad.
power bills, controlling electricity usage is especially important to the
international broadcaster. Dynamic Carrier Control, offering significant
savings in power bills for high-power broadcast transmitters, was proposed more
than 75 years ago. It took the power of the integrated circuit digital signal
processor (DSP) and pulse-step modulation (PSM) to bring it to fruition.
savings are impressive when DCC is added to 100, 250 and 500 kW transmitters.
modern super-power (output power greater than or equal to 100 kW) shortwave,
and most medium-wave (AM band) transmitters, incorporate DCC capability. Total
power consumption typically is reduced by 15 to 30 percent, and the effects are
virtually undetectable to the listening audience.
As a benchmark,
the average household in the U.S. consumes about 11,000 kilowatt-hours of
electricity each year. Assuming an average power savings of 25 percent with
DCC, a single modern 250 kW transmitter, operated 12 hours per day,
saves 342,000 kilowatt-hours per year, or the equivalent of powering 30 average
If we multiply
this by the over 1,000 shortwave, super-power transmitters positioned around
the world, we save enough electrical energy to power over 30,000 homes, a small
AM modulator impresses the program onto the high-frequency radio wave.
BACKGROUND ON AMPLITUDE MODULATION
Amplitude modulation occurs when a voice or
music signal’s varying voltage (fm)
is applied to a carrier frequency (fC). The
resultant AM signal consists of the carrier frequency, plus upper and lower
sidebands, known as double sideband amplitude modulation (DSB-AM), more
commonly referred to as plain AM.
When a carrier
is amplitude-modulated, only one-third of the total RF power is contained in
the information-bearing sidebands. The other two-thirds of the RF output power
is contained in the carrier, which does not contribute to the transfer of
information. The intelligence is in the sidebands, and the carrier is aptly
named for its purpose of merely carrying the modulation.
With a complex
modulating signal, such as voice or music, the sidebands generally contain only
20 to 25 percent of the overall signal power; thus the carrier consumes 75 to
80 percent of the total output power, making AM a very inefficient mode of
This raises the
question: “How can the transmitted waveform be modified to reduce power without
reducing received quality in simple AM receivers?”
WHAT DOES THE CARRIER DO FOR US?
The carrier in any AM
system has two functions: to translate the information signal to a higher
frequency that is suitable for transmission and to suppress static noise
and interference during silent intervals in the program. When the amplitude of
the audio signal is significantly less than that required for 100 percent
modulation, it is possible to save on the power consumption of the transmitter by
reducing the amplitude of the carrier, without affecting demodulation of the
sidebands. Power savings of 15 to 30 percent (depending on program material)
are feasible if carrier power is reduced 3 to 6 dB (carrier reduced one-half to
one-fourth) of the rated output power when modulation is low, with little or no
effect on reception of the signal. DCC is designed to keep the carrier at
sufficient amplitude to achieve 100 percent modulation, even during the
low-level portions of the audio program.
WHAT IS DCC?
spectrum of AM signal shows the wasteful carrier power compared to the
information-bearing sideband power.
Traditionally, AM systems have provided a
fixed carrier level and applied modulation up to 100 percent (or +125 percent
positive). Using DCC, during periods of low or no modulation, power consumption
can be significantly lowered by automatically reducing the carrier level and
restoring the carrier level when modulation later increases.
is adjusted for full carrier power output when the audio input appears at 100
percent modulation. If the audio input level falls, the carrier level will
dynamically adjust itself to maintain modulation at 100 percent. If modulation
is totally removed, the carrier level will fall to a pre-selected minimum level
(–3 dB to –6 dB). Typically a front-panel switch or remote control offers
selectable levels and types (different transfer curves) of DCC. With a typical
0.1 ms attack and 200 ms release time, DCC retains the ability to modulate to
positive 125 percent as we do now. At +125 percent modulation, the carrier
remains at the same level corresponding to 100 percent modulation.
ORIGIN OF DCC
This amplitude modulation energy-saving
mode was devised in the late 1930s. DCC was not implemented in transmitter
designs until the 1980s, because of the complexities of the control circuitry.
If we journey back in time to the 1930s to the village of Gliesmarode in
Germany, we find the first trace of DCC with Professor Pungs, who was involved
in the development of broadcasting technology. His system was known as “HAPUG”
Modulationsverfahren, incorporating the last name of each of the three
inventors: Harbich, Pungs and Gerth. This system never made it beyond the
experimental stage, due to a lack of computing and processing technology.
PULSE STEP MODULATION
transfer function between the program audio and the actual level of the carrier
has evolved to a curve that reduces the carrier power by a factor of four in
the mid-volume range, but retains half-power at low audio moments.
The advent of PSM in
the 1980s and DSPs in the 1990s allowed the implementation of DCC in all
modern, high-power transmitters today. Using signal processing, we can readily
break the modulation voltage waveform into a large number of steps. These small
increments are used to turn a series of switches on or off. Thus modulation
losses are now reduced to very low switching losses, increasing the efficiency
of the modulator itself to better than 95 percent.
The modulator typically consists of 48 series-connected
modules, mimicking the sampling levels, which are switched into and out
of operation to superimpose high-level audio onto the high-voltage DC anode (plate). The switching is accomplished with insulated gate bipolar
transistors (IGBTs). A low-pass filter follows the series-connected modules,
which removes the switching signals and allows the DC and audio signals to pass
to the RF amplifier. To further refine the linearity, each power supply step is
pulse-width modulated, creating a smooth transition from one step to the next.
PROGRAMMING TYPE AFFECTS POWER SAVINGS
As might be expected, actual power saving
over that of non-DCC usage is highly affected by audio program content, with
talk programs giving more power-saving than most music programming. Even the
short blank spaces between spoken words allow the carrier to be reduced in
conjunction with the decrease in modulation power.
programming dwells longer at modulation peaks, as evidenced by the higher
degree of modulation index. Audio processing, which artificially increases the
average level of modulation, tends to reduce power savings and increases the
total power consumption. Tests using processed music programs consistently
produced a power saving of 18 percent, whereas talk programs produced
power-savings in excess of 22 percent.
DOES DCC AFFECT THE LISTENERS?
In AM transmission systems, the
signal-to-noise ratio (SNR) is largely determined by sideband power, not by
carrier power. Thus, if an AM transmitter is modulated at 50 percent, a
listener will have to turn the volume up 6 dB (x4) to get the same loudness as
a transmitter modulated at 100 percent, consequently bringing up the background
noise level by the same amount. If the carrier level of the same transmitter is
lowered to obtain 100 percent modulation, no change in sideband power will
occur. A receiver’s automatic gain control (AGC) circuitry will bring
the volume up much as a volume control does, and background static noise will
also increase by the same amount. Thus the SNR remains constant.
advent of the digital signal processor (DSP) and the solid-state pulse-step
modulator (PSM) have allowed DCC to emerge as a ‘green’ technology.
seriously discuss the quality of the received signal over audio, they are most
likely talking about local AM or FM, where small imperfections are readily
noticed. The quality of audio over shortwave or international medium-wave is
inherently low, due to atmospherics, fading and noise. Laboratory tests on
subjective listening quality using DCC, even with co-channel interference,
showed no degradation.
Today, with so many Web-controlled, remote
monitoring receivers, it is easy for a broadcaster to access their signal audio
via the Internet. This allows the broadcaster to monitor their signal and make
their own assessment of DCC under real-world propagation effects when
transmitting to remote areas on shortwave. DCC tends to have little effect on
listener perception because of slow receiver AGC. Experience has shown that
when running a transmitter with DCC, response time is best set to fast
attack/slow decay (0.1 ms attack / 200 ms release) and carrier suppression, with
no modulation, is set to –6 dB for first-hop targets and –3 dB
for third-hop targets.
HOW DOES DCC AFFECT THE TRANSMITTER?
DCC extends the life of transmitter
components because the transmitter operates below its design capability. One
first notices that the power amplifier (PA) current meter now starts to wiggle
with DCC energized. PA current will decrease with low-level audio as the
carrier level is dynamically in accordance with the average modulation. Without
DCC, the majority of PA current was needed just to produce the large
constant-amplitude carrier power.
power-line loading can cause audio modulation on the AC mains if the power
lines are exceptionally long to the broadcast station. This is especially true
at large transmitting centers, where two or more transmitters are
simultaneously broadcasting over different bands, but using the same audio
program. When powered by diesel generators, feedback control circuitry in the
generators must be modified to handle these increased load swings on the AC
WHO OFFERS DCC?
and acceptance by broadcasters is indicated by the fact that all of the
super-power shortwave transmitter manufacturers — Continental Electronics in
Dallas, RIZ-Transmitters in Croatia, Thomson Broadcast & Multimedia in
Switzerland and Nautel in U.S./Canada — offer it as built-in or optional.
Nautel has long offered DCC on its NX series of high-power medium-wave models.
Harris Corp. offers DCC in its line of high-power AM transmitters.
The addition of DCC can pay for itself in as little as a few months,
based on the broadcast power level and the price of oil. Transmitters more than
30 years old evidenced an overall efficiency of only 70 percent. With the use
of switch-mode Class D and pulse-step modulation, the overall (AC power-to-RF power)
efficiency has improved to nearly 80 percent for high-power shortwave and
nearly 90 percent for high-power medium-wave transmitters.
FOLLOW THE MONEY TRAIL
DCC’s appearance can be linked to NATO
nations trying to save energy as a result of the ripple effects of OPEC-related
oil supply crises. Eastern European manufacturers of shortwave transmitters
never adopted DCC because Russia,
in particular, had no energy crises due to its abundant supply of oil.
broadcasters are indeed interested in “going green,” and even more so in the
future as power bills continue to esculate. Not only do all the manufacturers
of high-power transmitters offer DCC, but all the broadcasters use it to help
contain expenses. DCC was pioneered by the broadcasting giant British
Broadcasting Corp. (BBC) and AEG Telefunken in the late 1980s.
As the cost of electricity steadily increased,
others such as Voice of America (VOA), Trans World Radio (TWR), HCJB (Voice of
the Andes), Radio France
Internationale (RFI), Deutsche Welle (DW), Radio Canada
International (RCI), Far East Broadcasting Co. (FEBC), Australian Broadcasting
Corp. (ABC Radio Australia), Radio New Zealand (RNZ) and Radio Netherlands
Worldwide (RNW) quickly joined the fray. As a simple matter of economics, it is
to be expected other broadcasters will add to the numbers, as they are able to
make the switch.
In April 2011, the FCC authorized the use of DCC on
non-commercial AM stations in Alaska as long as notification was given to the
commission. The high cost of electricity in the state was a factor in the
decision to allow stations to use DCC experimentally.Power reductions
of 30 to 35 percent were reported, with no deterioration of received sound and
no complaints about reception from listeners.
The experimental operation appears to
have been a success, confirmed by the recent FCC action to allow DCC for all
U.S. AM stations.
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