Dynamic carrier control

All modern high-power (>50 kW) shortwave radio transmitters incorporate dynamic carrier control (DCC) of some kind. In addition, some mediumwave transmitters incorporate DCC.

History

This amplitude modulation (and vestigial SSB modulation) energy-saving mode was devised in the late 1930s. DCC was not implemented in transmitter designs until the 1980s, because of some of the complexities of the control circuit.

• DCC's appearance is linked to NATO nations trying to save energy as a result of the ripple effects of OPEC related oil supply crises.
• DCC is still an option on all shortwave transmitters sold on the open market today, but it is almost universally mandated by those purchasing the shortwave transmitters.
• Eastern European manufacturers of shortwave transmitters never adopted DCC because Russia in particular had no energy crises, due to its abundant supply of oil.

DCC Methods

The original type of DCC causes the carrier wave level to be automatically reduced when the audio is very weak or no audio is present. During periods of silence (no audio), the carrier power is reduced by 50%, so the 250 kW transmitter is putting out a carrier of 125 kW during audio pauses. This carrier power reduction saves electricity.

The BBC has developed another type of carrier control that does the opposite of the original DCC system. The carrier is at maximum when no audio is present, and reduces the carrier by up to 75% when the audio is loudest. This system, called AMC or Amplitude Modulation Companding uses the carrier to quiet the channel when no audio is present, while still realizing power savings. DCC and AMC have been a staple in European MW broadcasting for many years, and now AMC is now being used by stations in North America.

Both of the above methods are covered by the inclusive term, modulation-dependent carrier level or MDCL. Some stations choose a mixture of DCC and AMC that uses a complex carrier vs. modulation curve, typically running less carrier for modulation in the 25% to 75% range with more carrier during peaks and pauses.

DCC can work with frequency modulation (FM), but was never adopted for high-power FM stereo transmission.

Advantages and disadvantages of MDCL methods

The main reason for using either MDCL method, or the combination of the two is to save power and thus money. Alternatively, for the same total power consumption, the coverage can be somewhat improved. The AMC method has two other advantages. It limits the peak voltages on transmitter and antenna circuits, possibly prevent damaging arcs. Also, if the degree of AMC is properly adjusted, the power drawn from the mains can be kept nearly constant with modulation. This is especially important when operating from a local generator. With conventional DCC the load variations on a generator are made worse. With both methods, the main disadvantage is marginal loss of coverage, comparable to running slightly less power.

Operational usage

When running a transmitter with DCC, these settings are recommended to optimize power savings versus listener's receiver "signal lock" loss.

• DCC (attack, decay) : instantaneous
• DCC (carrier power suppression): 3dB (for third-hop targets)
• DCC (carrier power suppression): 6dB (for first-hop targets)

Occasional misuses of DCC

• Radio New Zealand International, due to long-term funding problems, has had to resort to using 6db of carrier suppression, making reception in North America difficult.
• DCC can make DXing SW signals more difficult.

Transmitters using DCC

Here is a list of manufacturers that offer DCC in their transmitters

• RIZ Zagreb
• Ampegon AG Ex-Thales
• Nautel (MW only)
• Telefunken
• Transradio
• Continental Electronics

Possible future obsolescence

Digital transmissions such as used with Digital Radio Mondiale have a constant power level even during program pauses and once they replace standard analogue amplitude modulated signals, will make DCC unnecessary for audio broadcasting via shortwave. However, some of the same circuits used for DCC also make it easy to adjust the power level when operating in the digital mode. DRM signals use only a fraction of the power to achieve the same coverage as an AM transmitter, so the cost savings feature of DCC will not be needed.

Analog modes

• A3E AM (incl. DCC)
• H3E SSB
• R3E SSB

According to the ITU Radio Regulations, Geneva 1990, Article 4.