Thread: FM or PCM??
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Old 02-11-2004, 10:10 PM
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Default RE: FM or PCM??


> In the begining every PPM system came with a failsafe system.
> It was deemed to be unnessesary and dropped. In fact one of
> the manufactors, PCS, told me that it caused more crashes than
> it saved

"In the beginning"?

When digital proportional gear first began appearing on the scene, I was involved in the design of several systems and also serviced quite a bit of commercially made RC gear.

I don't recall *ever* seeing a PPM system that had any kind of fail-safe system.

Back in those days, all the encoding/decoding was done with discrete transistors and there was no such thing as "memory" -- so how would such a failsafe system have worked?

To the best of my knowledge, there was no "failsafe" assocaited with PPM until the advent of microprocessor-based decoders -- which only appeared 20 year so after the advent of PPM.

I expect the reality is that the PPM/PCM debate will continue virtually for ever -- just as there are still audiophiles who swear that valves and vinyl are superior to today's latest digital audio discs and solid-state amps -- and who knows, in some cases they may be right.

The reality is that the quality of today's radio gear is very high and 99.9% of the time, people won't notice the difference between PPM and PCM in everyday flying.

I expect that the only time we'll get rid of this argument is once we've all switched to spread-spectrum equipment that should, for all intents and purposes, significantly reduce the problem of interference from external sources. By that time, all systems will be PCM (or a more advanced variant thereof) anyway.
If you were involved with the design of the first digital systems you will recall Doug Springs Digicon and Bonners Digimite and the first Logictrols. They all had a failsafe system. It consisted of limit switches built into the servos and a delay system in the receiver. If the transmisssion was interferred with and the last pulse did not appear when expected the delay circuit would switch to the other line to the servo and cause the servo to drive to where the limit switches stopped it. That was at the neutral point on all of the servos except the MC servo which was driven to one end. The result was very similar to the failsafe of today
Jerry Pullen and Clif Weirick produced the PCS system that appeared in 1964 and was one of the first to eliminate the failsafe system. PCS was later absorbed by Kraft
There are many ways to implement a "memory".
I had a very effective failsafe in the pulse systems I flew in the late '50s. It used a relay and a rubber band.