On Mon, Jan 16, 2006 at 05:43:25PM -0800, Jim Laurel wrote: | I ran a few informal tests the other day. I set a pair of JR 9303 | TXs with synth modules to the same frequency, one on PPM and one on | PCM. Then, I laid out two receivers - a JR R700 and an R770. | Results? WIth both transmitters on, servos on the PPM R700 were | totally out of control. No surprise there. But the R770 was totally | quiet, only responding to the PCM TX.
Note that this test is only really valid if you are actually moving the servos. PCM should *never* glitch under any conditions (except under the *extremely* unlikely case where the interference generates a frame with a valid CRC) but just because it's not glitching, that doesn't mean you have control. (I'm sorry if you already know all of this.) My Futaba 9C controller has a servo test mode that slowly moves all servos back and forth. I don't know if the JR radios have anything equivilent, but this is just perfect for range tests and such. As long as the servos are smoothly moving back and forth, all is well. If they start stuttering, hesitating or jerking, then things are iffy, and if they stop completely, nothing is getting through. | At the field results were the same. The R770 seemed to ignore the | PPM transmitter altogether, responding perfectly to inputs from the | PCM TX only. I wouldn't rely on that. At the RF level, PCM is no different than PPM (all that differs is what signal is sent, not how it is sent), and both will happily interfere with each other if on the same channel. | My theory is that new folks who may not be so keen on | frequency control yet are likely to be using PPM. That is probably true, but I don't see it as being useful. | PCM gives me an extra margin of protection from frequency conflicts. My theory would be that the PCM RX just went into failsafe (or `hold last position') and this was mistaken for `no interference'. Or maybe the PCM signal was just stronger than the PPM signal and you just got lucky. Don't get me wrong -- PCM does provide some benefits (and there are some drawbacks too) and being able to kill the throttle automatically during interference on a powered plane is a huge benefit, but they're still not a magic bullet against interference. | Has anyone else tried this? Or had a PCM RX glitched on the same | frequency by a PPM TX? Is it unrealistic to think that PCM gives you | an extra level or protection from interference from rogue PPM receivers? It's an extra layer of protection if you've set up your failsafe positions properly and your plane can benefit from them (i.e. put your polyhedral glider into a gentle circle, kill any motor) and never glitching is certainly a good thing (having a glitch start a motor on the ground is especially bad) but it IS unrealistic to assume that PCM gives you any more protection against interference than that. Yes, your plane may handle the interference better (going to failsafe or hold position rather than glitching) but either way, you still don't have control. - Doug McLaren, [EMAIL PROTECTED] Some tips on how too write good: -Exaggeration is a billion times worse than understatement. -Don't be redundant; don't use more words than necessary; it's highly superfluous. -Be more or less specific. RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
