For some modelers, this can be a confusing subject. I will try to explain the difference between them here. When shopping for a radio, you will find that people refer to a few different types of modulation. They are referring to the way the electronic control information is sent from your transmitter to the receiver in your model by means of radio waves.

AM — AM, or Amplitude Modulation, was the primary means of modulation in R/C until recently. The control information is transmitted by varying the amplitude of the signal.

FM — FM, or Frequency Modulation, is now the common method and is less prone to interference than AM. Information is transmitted by varying the frequency of the signal.

PCM — In PCM, or Pulse Code Modulation, FM is still utilized, however the control information is in the form of a digital word rather than just a pulse width as in standard AM or FM. Using PCM adds additional protection against interference from various sources.

How Does It Work?

We now know that AM stands for amplitude modulation. Let's say you and I went to a bar for "Happy Hour". When we first get there I start shouting the word "Hello!" to you from across the bar. If I shout it a little louder, you to turn to your left. And if I shout a little quieter, you to turn to your right. If I shout faster, you turn more. If I shout slower, your turn less.

Again, FM stands for frequency modulation. So now imagine another person is shouting the word "left", "center", and "right" at you from across the bar. You respond to the verbal commands accordingly. Again, you turn more or less as his shouting becomes more or less frequent.

Under ideal conditions, both of these radio systems will work equally well. In fact, under most conditions, these radio systems are virtually indistinguishable from each other. But conditions are not going to be ideal all the time.

Now that it is a little later, the bar is starting to fill up with other people. The background noise may easily drown out the difference in how loud I am shouting "Hello!" to you, but it takes a lot more noise to drown out the difference between "left", "right" and "center" that the other person is shouting to you. Herein lies the strength of the FM radio system. Background noise only affects the amplitude of the signal that is being transmitted. By depending on frequency change, instead of amplitude change, the FM system is far better at rejecting background noise and interference.