Implementing MDCL

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Back to the transmitter now: again, with a 50kW carrier, the transmitter can develop 25kW of sideband power. But what if the transmitter itself could analyze the incoming audio, so that it could determine just how much carrier power is really needed to generate the appropriate amount of sideband power? If the program audio was speech, for example, and there were gaps in the audio, wouldn't it make sense to be able to reduce the carrier power during those gaps in speech when there isn't going to be any need for modulation? The answer to that question is yes, and that basically describes what DCC does. DCC was developed during the early 1980s by Asea Brown Bovari, a Swiss company that manufactured large MW transmitters at the time.

DAM is an implementation of DCC and works as described, but during moderate modulation levels. (See Figure 1.) An audio peak detector's output is followed by appropriate filters that set the attack and delay; subsequent output is then used for the input of a lookup table, the output of which sets the carrier level. From the description on how this system works, it's not hard to see that it would be most effective with speech programs, since there are always gaps in speech. It's also interesting to note that when the carrier power is reduced, the modulation depth is effectively increased; so in the field, a receiver's audio output will make the audio sound somewhat louder during the brief instances that the carrier power is reduced.

Figure 1. DAM showing power and voltage compared to modulation. Courtesy Nautel. Click to enlarge.

Figure 1. DAM showing power and voltage compared to modulation. Courtesy Nautel. Click to enlarge.

With a heavily processed music program, one can see that DAM likely wouldn't be effective in saving power since there would be few opportunities to reduce the carrier power. If the AM station in question has a highly processed program, then the MDCL method that would save power would be AMC (amplitude modulation companding). Unlike DAM (or DCC) which just reduces the carrier power, AMC reduces the carrier power and the power generated by the modulation, so the modulation percentage stays constant, even with varying carrier power. (See Figure 2.)

Figure 2. Typical AMC response. Courtesy Nautel. Click to enlarge.

Figure 2. Typical AMC response. Courtesy Nautel. Click to enlarge.

You may at first ask, Why would I want to reduce my carrier power during peak modulation? Won't that make the station sound weaker on receivers? According to research on this topic, the answer is, Yes, but it's barely audible. Think about it: The time that noise in the output of an AM receiver will be most noticeable is during no modulation. AMC does not reduce the carrier level at this point at all. The time that noise is least noticeable in the output of an AM receiver is at the highest modulation levels. This is an effect of the way human hearing works; noise is masked in the presence of other loud sounds. AMC takes advantage of this; at the time that you can most afford it (highest modulation levels) the overall carrier and modulation power are reduced. The noise floor would increase, but in practice that isn't perceptible by the listener.

- continued on page 3

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