Synthesizer IPD receivers ... made by MULTIPLEX

With the introduction of the new RX-5-SYNTH IPD CAR receiver the range of high-quality MULTIPLEX Synthesizer IPD receivers is expanded to embrace the CAR arena. RX-5-SYNTH IPD CAR receivers combine the advantages of IPD technology - another MULTIPLEX development - with Synthesizer technology: the ground-breaking innovation which lets you select channels without crystals.

Unrestricted channel selection with the Synthesizer - no crystals required!

All RX-5-SYNTH IPD CAR receivers are fitted with a programming button and LED. If you hold the button pressed in when you switch on the receiver (if the receiver is inaccessible, a remote button can be fitted using the optional cable # 8 5048), the receiver starts a Channel Search process, i.e. it looks for the channel with the strongest field strength. For this reason your transmitter must already be switched on and located adjacent to the receiver. The LED signals that it has located the corresponding channel.  

A plausibility check is then carried out (the user moves the transmitter stick repeatedly to its end-points) to ensure that the receiver has located the correct channel. The LED again confirms this, and the receiver stores the channel. The receiver can then be switched on again, and it is ready for use. The whole process takes just a few seconds. RX-SYNTH receivers can be operated with all standard commercial FM PPM transmitters on the appropriate frequency band, whether they use crystal or Synthesizer technology.

Selectable channels / frequencies*:

35 MHz (A- and B-band):

Ch 55 / 34.950 MHz ... Ch 60 / 35.000 MHz

Ch 61 / 35.010 MHz ... Ch 80 / 35.200 MHz

Ch 81 / 35.210 MHz ... Ch 90 / 35.300 MHz

Ch 182 / 35.820 MHz ... Ch 191 / 35.910 MHz

40/41 MHz:

Ch 41 / 40.575 MHz ... Ch 59 / 40.785 MHz

Ch 81 / 40.815 MHz ... Ch 92 / 40.985 MHz

Ch 400 / 41.000 MHz ... Ch 420 / 41.200 MHz

IPD - What does it mean?

The abbreviation IPD stands for Intelligent Pulse Decoding, the “intelligence” being provided by a micro-processor in the IPD receiver. The processor analyses the signals it receives from the transmitter, processes them (if necessary) and passes them on to the servos.

The advantages of IPD technology

1. Only valid signals “get through”

The IPD receiver only considers a signal valid if its length lies within the range 890 µsec to 2350 µsec. These are limit values which cater for most radio control transmitters - even those not made by MULTIPLEX.

2. Invalid signals are replaced (HOLD MODE)

If an invalid signal is picked up, the receiver rejects it and passes on the last valid signals received. It continues to do this until a “good” signal arrives again. This action alone suppresses much interference, or at least reduces its effect.

3. “Safety setting” (FAIL-SAFE) in the case of total signal failure

If no valid signal is picked up for a particular period (varies according to receiver type), the IPD receiver moves the servos to a freely programmable position (Fail-Safe settings), e.g. throttle closed and/or control surfaces to neutral.

4. IPD is compatible

IPD works on the basis of the current FM-PPM transmission process. You can exploit the advantages of IPD with most RC systems. An additional advantage of FM-PPM transmission is the high rate of signal transfer of up to 12 channels, all of equal value.

5. IPD monitors reception quality

The receiver analyses the signal, and adjusts it automatically in accordance with the current reception quality, or field strength. Powerful signals are passed on to the servos directly, but weaker signals are “post-processed”. This means that the IPD receiver calculates the nominal servo position from the last “good” signals which it picks up. This greatly reduces the effect of any interference, but - in contrast to PCM - the pilot is made aware that there is a problem. In this way the pilot receives a warning that all is not well, and he can respond appropriately.

Programming facilities, additional features

The integral LED and push-button of the RX-SYNTH-IPD receiver have additional functions:

* Setting FAIL-SAFE positions

* IPD ON/OFF for range checking

Here is a basic comparison between the current crop of receivers on the market.
Done by Jason Werner
 
PPM
Our traditional "FM" is still a framed signal that the rx processes.  The only difference is that it is an analog based signal.  So what happens is that any signal received within the "frame" for that channel is processed.  This is the famous glitch where the servos are sent screwy for an instant.
 
The advantages of FM are super high refresh rates because the data length is very small.  Also the resolution in theory can be very high because there is no loss of signal during the conversion to digital.
 
Disadvantages - the "glitch".  This spastic movement of the servos when a bad signal is decoded is why many people prefer to switch away from traditional PPM.  Also the resolution that many of the older radios had is removed by the new "digital" txs that use only 512 or 1024 bits of resolution.  But to be honest, that is a minor problem and most people would never see it.

PCM
PCM is still broadcast on the FM carrier wave.  But instead of using a PPM frame setup, a digital stream is created of x bits (1024 for most).  The digital stream consists of a header, a trailer, a set of parity bits, a failsafe section, and the actual control section.  The receiver takes this stream/word and decodes it into the control positions based on the position in the stream.  Since a small microcomputer is in the Rx, quite a bit of processing can be done during this section.
 
Advantages - no more glitch!  While the same bad data is received by the RX, the microcomputer has the smarts to reject that bad data and not move the servos.  So for that instant you simply don't get movement of the servos.   For 99% of the people out there, this is the only reason they switch to PCM!  Another advantage is better control of certain surfaces.  On PPM, all channels are of equal value.  For PCM the designer can adjust the primary channels to have more resolution than say the gear channel.  This makes for a system that can be weighted towards the primary channel resolution.
 
Disadvantages - There are several so I will list them:

IPD
IPD can be considered a mating of PPM and PCM.  While PPM has no valid "frame" or parity bits, IPD does to a certain extent.  Each frame is similar to PCM in that it has a valid length and data range.  But the frame it uses are similar to PPM.  So no header, trailer, parity, failsafe, or other bits are used.  The Rx though does contain an microprocessor that
looks at the data and determines if it is valid or not.  The failsafe positions are kept in the rx, not the stream of data!  So in the end while similar processing that is done with PCM occurs, the data rate is much higher.  Also the famous lockout does not occur because the IPD programming is to reject bad data (and use the failsafe position if enough bad data is received) but there is no pause once a good data packet is obtained.  With the higher refresh rate (smaller packets) and lack of a pause, control is
returned faster.  The other  thing that the rx does is to modulate the data rejection parameters.  If a good signal is received, then the next signal is compared on a tighter data rejection scale.  Once a bad packet is received, then the parameters move the other way.  This allows a loosening of the belt once bad data is obtained yet still allows packet information to be processed.
 
Advantages - faster refresh rate, equal value channels (similar to PPM), failsafe, bad data rejection, light weight (less parts count), compatible with ANY PPM system, adjustable packet parameters.

BTW - the "slow" response that IPD explanation hints at is once again a mating of PCM and PPM.  For example, when getting to the edge of range the signal will get a lot of bad data packets.  Since the system is designed to reject bad data and use good ones, the model's response will appear to slow down as the signals are accepted and rejected.  Also as the packet window is widened, the control response is lessened as well to prevent the dreaded glitch!