The advantages of a Futaba PCM radio

[Guest]

Early 1980's Radio/Control transmitted each channels servo position as an analog pulse width that ranged from 1-2 milliseconds. The pulses were transmitted as a continuously "on" Radio Frequency energy carrier with their beginning and end marked by Amplitude Modulating the RF carrier "off" for 0.35 ms. The channels were sent sequentially with the .35ms end mark of one serving as the beginning mark of the next channel. A special long "Framing" pulse delineated the beginning of the channel series. The receiver used the marks to determine the channels, otherwise the receiver was dumb. Any noise or interference in the "air" was interpreted as a marker and mis-commanded the servos; the receiver couldn't tell the difference. Later FM versions in the early '90's suffered the same problems.

The Pulse Code Modulation receiver is smart; it contains a microprocessor. By clever structuring of the servo commands using mathematics and precise timing PCM facilitates accurate reception even with severe noise or interference.

It begins in the transmitter by conversion of each joystick position into a 10 bit digital word plus the extra bits to enable the receiver to mathematically verify the word. The next trick of PCM is to compact this data representing 1024 servo positions per channel into the FCC specified radio bandwidth while maintaining responsive "real time" control to fly a helicopter. The PCM data is transmitted synchronously; each bit has a particular position in time, within the Frame. The Frame continuously repeats. A precision crystal controlled clock in the receiver locks onto the transmission to maintain synchronization with the data bit by bit even during major signal drop outs. Thus the receiver can process data immediately after interference instead of waiting for a Framing pulse.

Received data is evaluated channel by channel. When the mathematics indicates an error, previously validated channel data that is stored in the smart microprocessor is substituted. If an error persists then failsafe servo operations previously specified by the operator are initiated until accurate commands are again received. The microprocessor converts the proper data into pulse widths to command the servos. No more servo jitters!

Futaba's PCM1024 scheme gives 8 channels of 10 bit resolution. Other competitive systems lack Futaba's sophisticated data compaction scheme and so must limit the resolution of some channels to 8 bits, which is 1/4 as accurate, as a trade-off to satisfy the "real time" data rates required.

Yes available on 75 mgz at Vantec.com

Amur Ship Yard

[petn7]

I remember years ago, the

I remember years ago, the debate about FM v. PCM.

Seems like the debate continues.

[Wheelerdealer]

The main problem with PCM's

The main problem with PCM's is the fail safe delay can't often be set to more than 1 sec. So after 1 second of glitches or lost signal, you sub will pop to the surface. 1 sec of glitches is very bad for a fast jet or Heli, but a sub is often perfectly controlable in this state as they move much more slowly and and you have a pitch controller to keep you level. Also PCM is no less suseptible to glitches than PPM, it just masks them by commanding the servo to hold their previous positions. This may mean that your unaware that there is interferance. My main Tx is a Graupner/JR unit that can transmit in both PCM or PPM mode, but the only RX available to me that support 12 proportional channels is PPM dual conversion rx, which has proven rock soild even with plenty of 'dirty' wiring and motors around.

[Guest]

Yes but you must admit

Yes but you must admit that the resolution and rock solid performance is better than non PCM systems,just look at the level of sofistication and for good reasons in a RF saturated area the radio always has done very well.

As for hidding glitches I would rather have them that way than while submerged.And I hardly ever use the failsafes becuase the radio is so bulletproff.

[JWLaRue]

Yes but you must admit

Yes but you must admit that the resolution and rock solid performance is better than non PCM systems,just look at the level of sofistication and for good reasons in a RF saturated area the radio always has done very well.

As a long time user of PPM radios for r/c sub use, I have n complaints as to the resolution and rock solid performance of these radios. I've never seen PCM as providing any advantage.

-ymmv,

Jeff

[KevinMC]

While I won't argue the

While I won't argue the sophistication of Futaba's PCM encoding system, it doesn't come free and is poorly understood.

Let's clear some things up. First off, the resolution of PCM is not better than what you'll find with a PPM set, in fact it can actually be worse. PPM encoding is not constrained to only 1024 steps, it can resolve far more. But there's more to control than just resolution, there's also lag to consider. Futaba PCM radios can't update the servos as fast as PPM, they're 50% slower. Servos are updated every 18mSec in a PPM system, PCM systems can only update every 32mSec. Why? Because all the "extras" that PCM provide aren't free- they cost bandwidth. If you're not actually using all those extras you're just wasting performance that you could otherwise have. (Think of it like super-sizing a Big Mac meal for the bigger drink and then throwing the fries away.)

There's another issue with Futaba's PCM system that most folks aren't aware of- they output adjacent channels simultaneously. Who cares about this? X-tail drivers certainly should. Most mixers can only handle reading one channel at a time, but if they're connected to adjacent channels (say 1 and 2, which corresponds to the two channels normally connected to the right stick on Futaba systems) they'll only be able to read one pulse per frame, so it will take two full frames to generate a control signal for the servos. (2 x 32mSec = 64mSec, three times more lag than what you'd have with a PPM system.)

Now, please don't misunderstand where I'm going with these statements. I do believe in the value of Futaba's PCM system and I use it myself. But it's not a universal solution - it has it's place. I drive all my RC models from a 9ZAP so I know what features Futaba has to offer. When it comes to transmission encoding I fly my turbine jets on PCM, but my subs will always be PPM systems. YMMV.

[bob_eissler]

You can turn the failsafe

You can turn the failsafe off on the Futaba & control which channels & how they respond to the failsafe.

[KevinMC]

No, you can't.

This is

No, you can't.

This is another common misconception about Futaba PCM. You can only choose between FAILSAFE and HOLD mode, but HOLD is not the same as being able to "turn the failsafe off". When you configure a channel for HOLD mode the receiver keeps repeating the last known good position upon loss of signal, where FAILSAFE would take the channels to a preset position. Regardless, the Rx still "creates" the missing pulses and the servos (or other connected devices) are oblivious to the fact that the signal has been lost.

Many ESCs are specifically set up to stop driving your boat when the signal goes away, but this can't work if the receiver makes up for the lost signal. If you're using this kind of setup please remember to set the throttle channel to FAILSAFE and set the position so to command a stop or other speed of your choosing.

[Guest]

I wassant trying to you

I wassant trying to you sell you guys on PCM just that it works absolutly great for me and as you know my boats are prety busy on the inside with several hydraulic pumps and even compressors.All on microswitches that way there is less cost and no reliance on other electronics that can also be damaged in humid or if there is a flood.And I dont use the failsafes any way.
I dont think a few milliseconts are going to affect a subs operation ,maybe in the shoppers I flew but not even then did I have RF problems on PCM ,but did have a few while on PPM.

Just my experiance

No mater where I run the subs do there thing.

Amur Ship Yard