RCPilot100
Posts: 582
Joined: 1/12/2002
From: Chelsea,
MI, USA
Status: offline
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Ok, I'll give you MY perspective on PPM versus PCM, without all the technical stuff that can be argued about six ways from Sunday. I will give my simplified description that I am sure will get debated, but is more or less the way it works.
Both transmit signals the same way, via FM transmission. With a PPM Tx, the signal that represents stick movement is sent to the receiver which sends signals to the servos which cause movement. When all is ok, the servos move in proportion to the stick movement. When there is interference, the servos glitch or jerk or shimmy. Basically, you see the interfernece with PPM - this is what is important to note.
With PCM, the stick movement is first coded and then sent to the PCM receiver. The receiver decodes the signal and checks that the signal is OK using a checksum in the coded signal. Pretty cool stuff here. When the checksum is confirmed, the signals are sent to the servos to represent stick movement. But, when the checksum is not OK, which is caused by some kind of interference, no signal is sent to the servos. Given that the interference continues for a sufficient period of time, the receiver goes into failsafe. With this way of doing business, fly through brief periods of interference. Only when it stays bad for an extended period of time does the plane go into failsafe. Failsafe without a BTA autopilot does nothing for you - you either stay in the condition of your last control inputs before interference, or you go to a preset condtion that you set up.
Now, the real world difference between the two. On one of my 35% Extras, I was using a PCM setup. Intermittent interference - and I didn't know it was there - the plane flew right through it. But, it kept getting worse, and I didn't know it until the receiver went into failsafe. It went into failsafe while I was coming in for a landing. Far downwind at 1/4 throttle lined up nice with the runway, it went into failsafe and kept going - at 1/4 throttle - across the runway - past the runway - and into the ground about 300 feet beyond - @ $1,000.00 damage. What I finally found out was that I had an intermittent condition that caused the interference - for about a year - and I didn't know it because the way PCM works, it masked the fact that it was there. Had it been a PPM setup, a year prior I would have noticed glitches and grounded the plane until I figured out what was going on.
Consider, my Extras are @ $3,500 apiece, 6-12 months to get one ready to fly. I just got a new 9Z WC2 with a PCM receiver. I sold the PCM receiver and bought 2 Hitec Supremes. Both my Extra's will be set up this way this summer as well as my 1/3 scale Laser 200. After trial and error, seeing what the other guys use that fly big birds, and much research, I am going to use this setup on about $10,000 worth of airplanes.
Dan
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Radio interference - 
