PCM stands for Pulse Code Modulation.
The position of joy-sticks, switches and pots, originally analogue voltages are digitised by an A/D converter to a 8 to 10 bits (256 to 1024 decimal) word. For eight to ten servos means 80 -100 bits. With a further 16-32 bit checksum per frame, synchronisation sequences and failsafe values, and a bit number of 100 -160 becomes necessary for a complete frame.
A bit length of 0.3mS (JR/Graupner and Futaba/Robbe) will produce a 30 48mS frame time, considerably longer than about 20mS the PPM uses. If even more secure bit lengths and 12 channels are used, this time is increased to 55mS, e.g. Simprop (System 90), where only 6 channels are proportional and 6 are switched channels.
Actual PCM uses two systems to synchronise the transfer: an extra long starting pulse made up of so many "1" or "0" bits, that it can never be mistaken for data, or the so called half bit pulse, e.g. 2,5 bits, equally impossibly mistaken for data. Usually this is followed by a synchronisation sequence, setting the receive-clock. This is the clock that scans the middle of the bits upon reception. This explains why, at the limits of the transmission range with PPM the servos start to glitch, as noise causes the pulse flanks to vary (up to+/-30 us), while PCM keeps them quiet, having half a bit (150 us) to play with.
The checksum in the shape of a 16 bit long CRC (Cyclic Redundancy Check) provides an effective way to detect bit errors, but in no way corrects them. This in turn means that, even if only one single bit error has crept in the ca. 100 - 160 bits total frame length, the checksum fails and the whole message is rejected. The servos remain in their last correctly received position until the arrival of new, correct data. If this takes too long (0.25-1 Sec), failsafe will take place, and depending on the predefined settings, a chosen (and defined in the transmitter) failsafe position or the last correctly received position will be activated.
To reduce the failure time, JR/Graupner (S-PCM) and Futaba/Robbe (PCM1024) subdivided the frame using separate CRC checks.
This allows rejecting only a part of the faulty frame.
PCM advantages:
Servo movements without glitch, even if the model is far way.
Holding of the servo position during short glitches (Hold).
Moving the servo to a predefined position in case of a longer disturbance or
even complete failure of the transmitter (Fail-Safe).
Fast transmission if S-PCM20 or PCM 1024 is used, similar to PPM.
Servos are not damaged by pulses that are too long/short, which could happen with PPM.
PCM disadvantages:
More expensive.
Sensitivity to adjacent channels is usually worse comparing with PPM receivers. Care has to be taken when flying near to a transmitter from an adjacent channel. Due to different protocols, only receivers from the same brand or even type of the transmitter can be used. Checking the transmission quality can be difficult, because the hold-mode smoothes out small glitches.
The lack of early warning signs often causes trouble. Control problems that build up gradually, e.g. of a technical nature, get noticed only when the connection fails completely, which may lead to a crash.
PPM advantages:
The PPM system is cheaper.
There should be no problems using different brands of receivers with different
transmitter manufacturers.
Transmission is fast enough to operate even the quickest of servos.
With PPM, the end of the transmission range is shown by the servos starting
to glitch. When the pilot notices this, he/she can probably still get the model
back home safely.
PPM disadvantages:
Due to its simplicity, PPM system cannot detect errors, the receiver does not
see the difference between valid and invalid servo pulses. When the range
boundaries are reached, pulses get slightly longer or shorter because of noise.
Servos start to glitch. This may happen when antenna orientation is not optimal,
when the projection of the receiver antenna is nearly down to a single point, the
signal breaks down and the servos get false pulses.
These short glitches go unnoticed most of the time because they are smoothed
out by the servo's and the model's inertia (response time).
Improvements can still be expected in the PPM sector, like the IPD system by
Multiplex, Scan-PLL by ACT or Scan2000 by Simprop.
Using a microprocessor in the receiver makes checking RC-pulses a possibility.
Failsafe and Hold, exclusive advantages of PCM so far, are now also possible
with PPM.
The position of joy-sticks, switches and pots, originally analogue voltages are digitised by an A/D converter to a 8 to 10 bits (256 to 1024 decimal) word. For eight to ten servos means 80 -100 bits. With a further 16-32 bit checksum per frame, synchronisation sequences and failsafe values, and a bit number of 100 -160 becomes necessary for a complete frame.
A bit length of 0.3mS (JR/Graupner and Futaba/Robbe) will produce a 30 48mS frame time, considerably longer than about 20mS the PPM uses. If even more secure bit lengths and 12 channels are used, this time is increased to 55mS, e.g. Simprop (System 90), where only 6 channels are proportional and 6 are switched channels.
Actual PCM uses two systems to synchronise the transfer: an extra long starting pulse made up of so many "1" or "0" bits, that it can never be mistaken for data, or the so called half bit pulse, e.g. 2,5 bits, equally impossibly mistaken for data. Usually this is followed by a synchronisation sequence, setting the receive-clock. This is the clock that scans the middle of the bits upon reception. This explains why, at the limits of the transmission range with PPM the servos start to glitch, as noise causes the pulse flanks to vary (up to+/-30 us), while PCM keeps them quiet, having half a bit (150 us) to play with.
The checksum in the shape of a 16 bit long CRC (Cyclic Redundancy Check) provides an effective way to detect bit errors, but in no way corrects them. This in turn means that, even if only one single bit error has crept in the ca. 100 - 160 bits total frame length, the checksum fails and the whole message is rejected. The servos remain in their last correctly received position until the arrival of new, correct data. If this takes too long (0.25-1 Sec), failsafe will take place, and depending on the predefined settings, a chosen (and defined in the transmitter) failsafe position or the last correctly received position will be activated.
To reduce the failure time, JR/Graupner (S-PCM) and Futaba/Robbe (PCM1024) subdivided the frame using separate CRC checks.
This allows rejecting only a part of the faulty frame.
PCM advantages:
Servo movements without glitch, even if the model is far way.
Holding of the servo position during short glitches (Hold).
Moving the servo to a predefined position in case of a longer disturbance or
even complete failure of the transmitter (Fail-Safe).
Fast transmission if S-PCM20 or PCM 1024 is used, similar to PPM.
Servos are not damaged by pulses that are too long/short, which could happen with PPM.
PCM disadvantages:
More expensive.
Sensitivity to adjacent channels is usually worse comparing with PPM receivers. Care has to be taken when flying near to a transmitter from an adjacent channel. Due to different protocols, only receivers from the same brand or even type of the transmitter can be used. Checking the transmission quality can be difficult, because the hold-mode smoothes out small glitches.
The lack of early warning signs often causes trouble. Control problems that build up gradually, e.g. of a technical nature, get noticed only when the connection fails completely, which may lead to a crash.
PPM advantages:
The PPM system is cheaper.
There should be no problems using different brands of receivers with different
transmitter manufacturers.
Transmission is fast enough to operate even the quickest of servos.
With PPM, the end of the transmission range is shown by the servos starting
to glitch. When the pilot notices this, he/she can probably still get the model
back home safely.
PPM disadvantages:
Due to its simplicity, PPM system cannot detect errors, the receiver does not
see the difference between valid and invalid servo pulses. When the range
boundaries are reached, pulses get slightly longer or shorter because of noise.
Servos start to glitch. This may happen when antenna orientation is not optimal,
when the projection of the receiver antenna is nearly down to a single point, the
signal breaks down and the servos get false pulses.
These short glitches go unnoticed most of the time because they are smoothed
out by the servo's and the model's inertia (response time).
Improvements can still be expected in the PPM sector, like the IPD system by
Multiplex, Scan-PLL by ACT or Scan2000 by Simprop.
Using a microprocessor in the receiver makes checking RC-pulses a possibility.
Failsafe and Hold, exclusive advantages of PCM so far, are now also possible
with PPM.
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