IPD Receivers
I have flown FM, Futaba 1024 PCM and Multiplex IPD receivers for many years now and feel no advantage of resolution of one over another, and I fly F3A which requires very precise equipment. I have not encountered any interference at my flying field that I felt, but if there is interference then the IPD receiver does become less precise as shown in the following advertisement, but less precision and some control may possibly be better than failsafe and no control:
"The new IPD process is a MULTIPLEX development which combines the best features of two different systems: the advantages of PPM - speed and compatibility - with the safety of PCM - to detect and suppress interference.
As fast as PPM
Compatible with other PPM transmitters
Detects invalid signals
No servo jitter with transmitter switched off
Servo Hold function
Servo Failsafe function
No servo travel beyond set limit values.
IPD receivers can be operated with all standard FM PPM transmitters on the appropriate frequency. In the event of radio interference, IPD receivers will help you maintain control, and in the event of unrecoverable interference, the receiver’s behaviour is similar to that of PCM receivers, with the servos moving to a pre-set failsafe position.
IPD stands for Intelligent Pulse Decoding, and the receiver incorporates a processor which analyzes the incoming signal for validity. Like a PCM system, IPD filters out invalid signals.
The difference between IPD and PCM is that the receiver does not 'switch off' the 'dirty' signal as field strength declines, but instead widens its tolerance. If the signal weakens, the receiver software starts to calculate an average pulse width using the values of subsequent pulses, which are normally slightly different caused by the jitter of the signal. The so called "floating average" suppresses the effect of jitter. As an additional help, the resolution of the signals, sent to the servos, is also reduced. Those two effects result in a slower reaction of the model, which you can take as an warning of interference. This "visible" warning enables you to take action (e.g. change direction of flight, hold the tx up, ...) This means that control becomes less precise as field strength falls away, but remains usable for longer (greater range). The result is that you can infer the approaching limit of range from the model’s behaviour, whereas PCM suddenly robs you of control.
An IPD receiver sees all usual PPM formats as valid, which means that all standard FM PPM transmitters on the same frequency can be used with IPD receivers
IPD is faster than PCM because there are no check cycles.
The user programs the fail-safe positions of all channels into the IPD receiver. The transmitter is used to set all servos to the desired positions.
Practical testing has shown that IPD and PCM produce comparable results when unrecoverable interference occurs. But please don’t get complacent: Like PCM, IPD is not an excuse for incompetent RC installation and careless cable deployment, and thorough suppression of electric motors and ignition systems is still essential."