For VHF frequencies (usually between about 30MHz to 80MHz), each frequency band allocated by your government for model aircraft use is divided into a number of channels, each with a different frequency. Each channel can support only one user at a time.
VHF radios may use a variety of protocols to talk to their receiver.
In pulse position modulation (shortened to PPM), the transmitter sends a sequence of pulses, each with a different duration indicating the position a servo should take. In traditional PPM, the length of the pulse is identical to the length of the pulse sent to the servo. To accommodate multiple servos, the transmitter sends a preamble, followed by each servo pulse in turn. At the other end, the receiver notices the preamble, and then passes the received pulse to each servo in turn.
This sequence of pulses can be encoded in one of two ways.
Amplitude modulation (AM)
With amplitude modulation, the pulse is represented by the transmitter sending a stronger signal than when it is not sending the pulse. Amplitude modulation is very prone to interference, and should not be used for aircraft.
Frequency modulation (FM)
With frequency modulation, the pulse is represented by the transmitter changing the frequency it transmits on very slightly. Depending on whether the transmitter increases the frequency to represent the pulse, or decreases the frequency, the modulation is said to have 'positive shift' or 'negative shift'. Inconveniently, different manufacturers can use different shifts in different frequency bands in different parts of the world, hampering interoperability. Frequency modulation is much less prone to interference.
In general, the PPM receivers of one manufacturer will talk to the PPM transmitters of any other manufacturer, provided they are using compatible shifts. I believe that the large majority of the systems in the world are 'negative shift'; the primary defectors are Futaba and Hitec, who produce positive shift sets for the 72Mhz American market.
Pulse code modulation
With pulse code modulation (shortened to PCM), the signal to be sent is encoded as a binary stream, and the binary stream is sent to the receiver using frequency modulation. Included with the transmission is a checksum so that the receiver can check if it received the signal correctly. PCM has many advantages over PPM: it is more precise (as the signal is digital), less prone to interference, and has the ability to set failsafe positions for the servos, in case the receiver loses the signal for more than a fraction of a second. The primary disadvantage of PCM is that receivers will only work with the same brand of transmitter.
Microwave frequency radio systems, such as Futaba's FASST and Spektrum's DSM system (both of which use the 2.4GHz band) are becoming more common each day. These radios use some sort of digital checksummed signal similar to PCM systems, but have the advantage that they pick the frequency they use automatically, either by using frequency-hopping, or listening before transmitting on a free frequency.
Each servo is also connected to a 'channel' (though nothing to do with the frequency channel that the radio uses). Different manufacturers may use different channel orders, so even if a receiver will talk to radios of different brands, care need to be taken to ensure that the signals are going to the correct servos.
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