What
is the difference between PPM and PCM?
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, 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
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 refresh rate 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.
lower
resolution. On systems that output pure PPM, the resolution is higher on
PPM then 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.
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.
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