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Modulation Dependent Carrier Level Methods

modulation dependent carrier level methods

Today’s AM broadcasters reduce carrier level to save money on their power bills. Here is a summary of the modulation dependent carrier level methods. 

We all know that at 100% modulation, two-thirds of the power used in an AM transmission is wasted in the carrier. But we need carriers for reception and demodulation. Improvements in transmission technology and receiver detectors make it possible to reduce the carrier level at most levels of modulation. Broadcasters can save a lot of money on the power bill. And most listeners will not notice a difference.

In this article, we will talk abut the modulation dependent carrier level methods used by broadcasters. MDCL is the umbrella term covering a variety of techniques that work by reducing the carrier level, and sometimes the sideband power, as well.

Adaptive Carrier Control, or ACC, is the method that just reduces the carrier level while keeping sideband power constant. ACC has also been called Dynamic Amplitude Modulation and Dynamic Carrier Control. These originated in Germany and Switzerland during the 1980’s.

In general, Adaptive Carrier Control reduces carrier power with decreasing audio level. However, if modulation hits or exceeds 95%, carrier level is boosted enough to prevent distortion caused by over-modulation. Even though it is varied, the carrier level is never reduced to the point where there is not enough energy to support negative modulation without clipping and distortion. In fact, when modulation is very low or none, the carrier may be increased to provide silence.

Obviously, Modulation Dependent Carrier Level methods require very good automatic control. Typically, these are AGC systems with fast attack (1 ms) and slow decay (250 ms) times. At the same time, these techniques require receivers to have very good AGC themselves to maintain stable levels of loudness.

Modulation Dependent Carrier Level Methods – AMC

The method that varies both carrier and sideband power is called Amplitude Modulation Companding. AMC lowers the carrier power for increased audio level. If audio is less than 10%, power is increased to maintain quieting or SNR. An AMC system operates with full carrier for no or low modulation and reduces both carrier and sidebands with increasing modulation. Typically the maximum carrier and sideband compression is adjustable between 0 and 6 dB. AMC varies both sidebands and carrier. There is no apparent change in loudness.

All of these systems are highly configurable, and can be adjusted to suit program content. For example, talk radio will tailor there systems differently from music formats. Some broadcast engineers even report improved loudness with less carrier power.

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