Some people prefer PPM to PCM due to the above reason, but in general, PCM receivers (and transmitters) are a lot more expensive than PPM

Servos

Servos are mechanical motors which are linked to the receiver and to the helicopter's control surfaces via machanical linkages. Again, there are many different kinds of servos ranging from 3kg torque to as high as 9kg torque. Generally, the higher torque servos are used for advanced flying like aerobatics and 3D, and the normal or standard servos for general flying.
For a helicopter, you require a minimum of 5 servos.

Throttle
Collective Pitch
Tail Rotor
Cyclic Left/Right
Cyclic Forward/Backward
Gyro

A gyro, is an electro-mechanical device used in a helicopter to help semi-automate the response of the tail rotor. In the case of an R/C chopper, the device fits electrically between the receiver and the servo that controls the pitch of the tail rotor blades. A sensor measures any unwanted change in yaw of the aircraft and will correct the situation by increasing or decreasing the tail rotor pitch to stabilize the movement.
It is strongly recommended that a gyro be used in every helicopter. Without a gyro, the tail can spin around uncontrollably, making it spin and crash. Controlling an overly sensitive tail of a helicopter is most frustrating and the gyro can help make the flying much easier and more pleasurable. Gyros are made by most radio manufacturers for operation compatable with their systems.

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