PCM vs FM

[Canadiank-9]

Can someone direct me to a web pag or explain the difference between these two. As well as the pro's and cons of each?

Thanks
Andrew

[JimMcIntyre]

There many, most are more opinionated than informative.

I fly pcm, primarily because it provides reliable fail-safe and a better signal-to-noise ratio but also because it provides repeatable positioning.

[JimBrown]

I'll try a simplified explanation, as I understand it.

PCM is FM. Or rather, a form of FM. The FM that you're thinking of is known as PPM. Both methods use an FM signal to carry the information that moves the servos. It's the way the information is encoded that is different.

PPM stands for Pulse Position Modulation. It is an analogue signal. A servo's position is determined by the width of a pulse. There is a pulse for each channel on the transmitter, plus some other pulses for timing. The transmitter sends the pulses in a specific order. The receiver measures the width of the pulses and sends a signal to the servo telling it where to move.

PCM stands for Pulse Code Modulation. It is a digital signal. It still uses pulses, but instead of the pulse width determining the servo position, each channel gets 9 or 10 pulses (depending on the system,) representing the individual bits of a number. The number determines the servos position. Again, the transmitter sends the pulses in a specific order and format, along with other pulses for timing and checksums. The receiver decodes the pulses, compares it to the checksum, figures out which numbers are for which servo, and sends a signal to the servo to tell it where to move.

It goes something like this (very simplified):

For PPM - The stick position is read as a voltage from the position of the potentiometer. The voltage is converted into a pulse width. The pulse width is sent out the antenae. The receiver receives the pulse width. The receiver converts the pulse width to a voltage. The voltage is sent on the signal wire to the servo. The servo runs the motor to turn the potentiometer in the servo to match the voltage from the receiver.

For PCM - The stick position is read as a voltage from the posistion of the potentiometer. The voltage is converted into a number between 0 and 511 (for 9 bit systems) or 1023 (for 10 bit systems.) The number is converted to a series of pulses (low pulse for a 0 bit, high pulse for a 1 bit) plus timing and checksum pulses. The receiver receives the train of pulses. The receiver decodes the pulses to a number. The number is converted to a voltage. The voltage is sent on the signal wire to the servo. The servo runs the motor to turn the potentiometer in the servo to match the voltage from the receiver.

The "advantage" of PCM is that, because it is a digital signal, the receiver can reject bad data (checksum does not match.) When a bad data packet is rejected by the receiver, the receiver uses the last good data it received to keep the servos where they are. If too many bad packets in a row are received, the receiver can be programmed to do something (failsafe settings.) If there is only a momentary loss of signal, ie, the receiver gets some bad data due to interference or no data at all, but then starts getting good data again before the failsafe kicks in, the servos will not move during that period of bad/no data. If a PPM receiver loses the signal, or gets interference, it will still try to interpret the signal it does get, resulting in the servos moving randomly (the infamous inflight "glitch".) The "disadvantage" of PCM is that it can mask a bad signal (no "glitch".) The solution to that is to set the failsafe to hold all servos except the throttle. The throttle should be set to go to idle or shut off when failsafe is engaged.

And then there's the new kid on the block. Multiplex's IPD technology. This is a hybrid of PPM and PCM. The signal between the transmitter and the receiver is PPM, but the IPD receiver has a lot more smarts built in than a regular PPM receiver. It can detect wildly fluctuating pulse widths (due to interference or loss of signal) and can reject them. The IPD receiver can also have failsafe settings programmed into it. However, unlike PCM, which sends the failsafe settings over the air from the transmitter to the receiver each time the system is powered on, the IPD transmitter must be plugged into the IPD receiver to send the failsafe settings. This is normally done once and the receiver then remembers the settings.

As I said, this is a greatly simplified explanation. There is some good info at http://www.aerodesign.de/peter/2000/PCM/PCM_PPM_eng.html, but it is geared toward the European scene.

Hope I didn't bore you with this. :wink:

Regards,
...jim

[Bob Hudson]

Once again, goggle to the rescue,

I was surprised at how many hits !!

http://www.google.ca/search?q=pcm+vs+ppm&ie=UTF-8&hl=en&meta=

[Hughes500E]

I would agree with Jim myself as well.
I hear one day PCM will be our only choice :lol:

[VE1FH]

Jim's rundown is pretty much spot on, except to reinforce and repeat (I hear the mistake made so many times) that both schemes use frequency modulation. As Jim points out, PPM and PCM are two different ways of encoding a joystick position into a stream of data that a servo will understand. The radio system uses frequncy modulation to put the PCM or PPM data on the radio signal.

You could transmit PPM and PCM data using an AM transmitter and receiver (just like you can transmit voice using AM or FM) .... FM has a bunch of advantages in this application that make it better for the job.

If you look at the circuit diagrams of a PCM receiver and a PPM receiver, you'll see they're identical up to the output of the discriminator ... the PCM box will have more complex circuitry after that point. It has to figure out the checksums, store failsafe positions, and have some "smarts" to hold it all together.

It's really clear when you look at the transmitter. Futaba users will see their fancy computer radio like an 8U or a 9C is pretty much identical to an FM Conquest from the output of the encoding system to the antenna. BIG difference before the transmitter board ....

John

[tech1]

There is one more way to look at this.

PPM VS PCM both have their advantages and disadvantages. There is no arguement there.

If you are purchasing a new radio I would say to buy the PCM version if the extra price is not a problem............. Why???

Because most PCM transmitters will transmit either PCM or PPM. This is a setting you assign to the specific model memory so the radio will allow you to use each one on a model by model basis. This means later on if you purchase more receivers for the rest of your fleet you will have the option to buy either type of receiver and just set the TX type for that one model.
The PCM system will be more flexible in the future from a hardware standpoint.

[SilverFox]

For PCM - The stick position is read as a voltage from the posistion of the potentiometer. The voltage is converted into a number between 0 and 511 (for 9 bit systems) or 1023 (for 10 bit systems.) The number is converted to a series of pulses (low pulse for a 0 bit, high pulse for a 1 bit) plus timing and checksum pulses. The receiver receives the train of pulses. The receiver decodes the pulses to a number. The number is converted to a voltage. The voltage is sent on the signal wire to the servo. The servo runs the motor to turn the potentiometer in the servo to match the voltage from the receiver.

Good explanation except the last sentence. The pulse is sent to the servo , not a voltage level. The servo decodes the pulse width and moves the arm appropriately.

The rest looks bang on.
Thanks.

[tech1]

Nah, PCM goes with more receivers than PPM