RC FAQ: Radio & Battery FAQ page

PPM stands for Pulse Position Modulation, and PCM stands for Pulse Code Modulation. Here is a basic comparison between the current crop of receivers on the market.

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 low cost. Also the resolution (in theory) can be very high because there is (theoretically) no loss of signal during the conversion to digital. This is only true with analog radios, though. Digital radios put PCM and PPM on a level playing field as far as resolution goes.

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.
Slower Refresh rate - A PPM Rx sends a signal to the servo once every 20-25ms. A PCM Rx sends pulses almost twice as often. This is true with my Futaba systems--I have measured it with my oscilloscope.

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.
Faster refresh rate - While a PPM Rx only sends out a servo pulse once every 20-25ms, the PCM Rx's are almost twice as often, at about 10-15ms. For Futaba gear, anyway, the thought that PCM's have slower refresh time is a myth! I have measured it with my oscilloscope.

Disadvantages - There are several so I will list them:
Proprietary - Each company's PCM is only compatible with PCM rxs and txs of their own make. Most aftermarket rxs will not work on proprietary PCM txs. So your are locked in to one brand or even one rx!

Slow response time compared to PPM. The actual data stream is larger (2- 3 times) the size of PPM. Many pattern fliers used to switch to PPM simply because they felt the added speed of PPM helped them. I know that on one of my giant scale planes I can tell the difference between the PCM update and PPM update. Only one plane, but there is a difference. Note that I have not yet checked this to verify it.

lower resolution. On systems that output pure analog PPM, the resolution is higher on PPM than PCM. But for most of us this is not a problem. And for the killer! The PCM lockout. When the PCM rx gets too many bad packets it then goes into a lockout or failsafe position. This continues for several refresh cycles after a valid packet is received. Most are around 1 second long before control is returned. It is this lockout that has most heli fliers staying with PPM! imagine hovering and having the system lock out for a second. A glitch can be just as bad, but at least then the control is instantly returned and not held up for a second or so. On the other hand, PCM lockouts are FAR fewer and much farther between than PPM glitches.

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!

The IPD system - what is it?
IPD stands for Intelligent Pulse Coding, and the receiver incorporates a processor which analyses 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. 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 behavior, whereas PCM suddenly robs you of control. When the signal is insufficient for the receiver to interpret, a fail safe condition occurs, thereby driving the servos to pre-selected safe positions, e.g., throttle back.

An IPD receiver sees all usual PPM formats as valid, which means that all standard FM PPM transmitters can be operated in conjunction with these receivers on the appropriate frequency. IPD is faster than PCM because there are no check cycles.

Practical testing has shown that IPD and PCM produce comparable results when interference occurs. But we are warned not to get complacent. Like PCM, IPD is not an excuse for incompetent R/C installation and careless cable deployment, and thorough suppression of electric motors and ignition systems is still essential.
Roger, Multiplex dealer http://www.aerografixs.com

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