The position of the controls has to be turned into some sort of signal. The pots make a varying voltage, which is a signal but not very good for our purposes. We could convert it into morse code, but that is not a good code for our purposes. There are many types of data code we could use. We have settled on PPM which is analogue and PCM which is binary. Both are simply data codes, nothing to do with radio. We could send them from one place to another by drawing, by light flashing, by smoke signal, by sound, by electricity down a wire, etc. PCM can be transmitted by clapping your hands.

We want to send the PPM or PCM data by radio. There are two ways of putting data onto a radio wave, AM or FM. So there are 4 model radio systems to choose from - PPM AM, PPM FM, PCM AM and PCM FM. No-one makes a PCM AM as far as I know. A lot of people talk about FM versus PCM but as we have seen above, that is impossible, many people say FM when they should say PPM.

Dual conversion is a way of dealing with a problem created by the way receivers work. Receivers generate their own frequency a little off the frequency thay are receiving. The two frequencies mix and make many results, one of which is a much lower frequency of 455khz which still contains the data but the lower frequency is much easier to filter and amplify. This mixing is done once and is single conversion. The snag is that there are two incoming frequencies, yours and another, that will create a 455khz signal in the Rx, and there are various other multiples and fractions of frequencies that can also create these false images. So if the receiver does the mixing twice, first to 10.7Mhz and then down to
455khz it removes the false images just leaving your correct signal. Doing the frequency mixing twice is dual conversion. Note that it does nothing about interference on your proper frequency, what it does is block other specific frequencies from creating false 455khz signals in the Rx.

H