In the early 21st century, 2.4 gigahertz spread spectrum RC control systems have become increasingly utilized in control of model vehicles and aircraft. Now, these 2.4 GHz systems are being made by most radio manufacturers. These radio systems range from a couple thousand dollars, all the way down to under US$30 for some. Some manufacturers even offer conversion kits for older digital 72 MHz receivers and radios. As the emerging multitude of 2.4 GHz band spread spectrum RC systems usually use a frequency-agile mode of operations, like FHSS that do not stay on one set frequency any longer while in use, the older exclusive use provisions at model flying sites needed for VHF-band RC control systemss frequency control, for VHF-band RC systems that only used one set frequency unless serviced to change it, are not as mandatory as before.
The first general use of radio control systems in models started in the early 1950s with single-channel self-built equipment; commercial equipment came later. The advent of transistors greatly reduced the battery requirements, since the current requirements at low voltage were greatly reduced and the high voltage battery was eliminated. In both tube and early transistor sets the models control surfaces were usually operated by an electromagnetic escapement controlling the stored energy in a rubber-band loop, allowing simple on/off rudder control and sometimes other functions such as motor speed.
Crystal-controlled superheterodyne receivers with better selectivity and stability made control equipment more capable and at lower cost. Multi-channel developments were of particular use to aircraft, which really needed a minimum of three control dimensions , as opposed to boats, which required only two or one.
As the electronics revolution took off, single-signal channel circuit design became redundant, and instead radios provided proportionally coded signal streams which a servomechanism could interpret, using pulse-position modulation .
More recently, high-end hobby systems using Pulse-code modulation features have come on the market that provide a computerized digital bit-stream signal to the receiving device, instead of the earlier PPM encoding type. However, even with this coding, loss of transmission during flight has become more common, in part because of the ever more wireless society. Some more modern FM-signal receivers that still use PPM encoding instead can, thanks to the use of more advanced computer chips in them, be made to lock onto and use the individual signal characteristics of a particular PPM-type RC transmitters emissions alone, without needing a special code transmitted along with the control information as PCM encoding has always required. (source : wikipedia)