Radio interference
01-29-2002 | 06:06 AM
#26
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Radio interference
PCM will and does offer a higher magrgin for succesfully transmitting and receiving a valid signal train prior to a failsafe/lockout activation as compared to PPM, period. Its a technical fact not fiction or a fish story...
PCM does not work in the presence of interference that would wreak havoc on PPM systems.
I completely disagree with this statement. The inverse is an accurate depiction of what is realized with PCM.
If the interference is bad enough to mess up PPM, the PCM system would simply go into failsafe OR hold the last setting.
Not necessarily. PCM’s ability to ignore interference that would harm PPM would offer greater resistance to interference and maintain control of the model. In the event of major interference without doubt PPM is all over the sky, while PCM is either able to interpret the modulated code and perform without harm or a failsafe/hold or combination thereof will be initialized. The resulting action of PPM is entirely unpredictable, with PCM it’s a given. Now what will work best for an individual/model individually and or cumulatively is a matter subject to interpretation.
My premise is that we want nothing in our aircraft that causes radio interference. Most of the time, if something goes bad it happens gradually. A wire start rubbing and wears through - a pushrod starts to come loose at a metal joint - all kinds of thing can cause interference. Given this scenario, PPM will glitch, giving you an indication that something is wrong and needs to be found and corrected. PCM will appear to work fine - appear is the operative word here. PCM will know that there is interference and work through the problem. Given that you do not know there is a problem and it will probably get worse, your first indication that you have a problem is when you go into failsafe or the system holds the last command - either way you have a big problem.
You make some valid points here. Most are common situations. Proper installation, routine maintenance and frequent radio ground checks will uncover most of these trouble areas. Your ascertain that PPM will forewarn you is only a maybe it may also shoot you down due to the higher likelihood of RF noise damaging the carrier/pulse position signal wave. The pulse positions can be shifted slightly by RF noise between the TX and RX. Depending on the severity of the RF noise/noise's being picked up by the RX dither or glitching may occur and be realized via eradict servo movement. PCM pulse trains do not realize this damage of the carrier wave as does PPM’s pulse position commands. PCM is looking for the existence of binary coded words; they can change in amplitude or width. This does not change the coded format. ----___----_-_-_----___-__ Is all read the same, if the code is unrecognizable a failsafe is initiated. It takes a major RF incursion for this to happen… Well beyond the time PPM went south…
This issue has been argued and discussed ad nauseum - all over the place.
I here that. Its like this you make your choices, you fly with them and you die with them, or not. There are advantges and what some see as disadvantages asscociated with the use of PCM... The same argument is true of PPM...
I have yet to loose an airplane to a PCM lockout. I did loose one many years ago with PPM due to some RF noise generated outside the aircraft. There is no gaurantee either way, unless of course you leave the model at home...
PCM does not work in the presence of interference that would wreak havoc on PPM systems.
I completely disagree with this statement. The inverse is an accurate depiction of what is realized with PCM.
If the interference is bad enough to mess up PPM, the PCM system would simply go into failsafe OR hold the last setting.
Not necessarily. PCM’s ability to ignore interference that would harm PPM would offer greater resistance to interference and maintain control of the model. In the event of major interference without doubt PPM is all over the sky, while PCM is either able to interpret the modulated code and perform without harm or a failsafe/hold or combination thereof will be initialized. The resulting action of PPM is entirely unpredictable, with PCM it’s a given. Now what will work best for an individual/model individually and or cumulatively is a matter subject to interpretation.
My premise is that we want nothing in our aircraft that causes radio interference. Most of the time, if something goes bad it happens gradually. A wire start rubbing and wears through - a pushrod starts to come loose at a metal joint - all kinds of thing can cause interference. Given this scenario, PPM will glitch, giving you an indication that something is wrong and needs to be found and corrected. PCM will appear to work fine - appear is the operative word here. PCM will know that there is interference and work through the problem. Given that you do not know there is a problem and it will probably get worse, your first indication that you have a problem is when you go into failsafe or the system holds the last command - either way you have a big problem.
You make some valid points here. Most are common situations. Proper installation, routine maintenance and frequent radio ground checks will uncover most of these trouble areas. Your ascertain that PPM will forewarn you is only a maybe it may also shoot you down due to the higher likelihood of RF noise damaging the carrier/pulse position signal wave. The pulse positions can be shifted slightly by RF noise between the TX and RX. Depending on the severity of the RF noise/noise's being picked up by the RX dither or glitching may occur and be realized via eradict servo movement. PCM pulse trains do not realize this damage of the carrier wave as does PPM’s pulse position commands. PCM is looking for the existence of binary coded words; they can change in amplitude or width. This does not change the coded format. ----___----_-_-_----___-__ Is all read the same, if the code is unrecognizable a failsafe is initiated. It takes a major RF incursion for this to happen… Well beyond the time PPM went south…
This issue has been argued and discussed ad nauseum - all over the place.
I here that. Its like this you make your choices, you fly with them and you die with them, or not. There are advantges and what some see as disadvantages asscociated with the use of PCM... The same argument is true of PPM...
I have yet to loose an airplane to a PCM lockout. I did loose one many years ago with PPM due to some RF noise generated outside the aircraft. There is no gaurantee either way, unless of course you leave the model at home...
01-29-2002 | 04:49 PM
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From: Topeka,
KS
Radio interference
...been in the hobby 21 years and i agree with RC Pilot - PCM crashes planes!...
...we used to fly at a soccer field with a high voltage power line and 2-way radio tower close by - i flew Futaba Conquest PPM and my buddy flew Futaba Super Seven PCM...
...if i flew too close to the power lines, occasionally i would get a glitch - it would happen maybe 2-3 times in 30-60 flights - but flying 120 sized glow, my buddy crashed 3 planes in a matter of a couple of months with his PCM system when his radio would go into failsafe - he blamed it on Futaba, i think PCM was the culprit - i had no problem dealing with a momentary glitch but when his radio went into failsafe it was bye-bye airplane...
...we used to fly at a soccer field with a high voltage power line and 2-way radio tower close by - i flew Futaba Conquest PPM and my buddy flew Futaba Super Seven PCM...
...if i flew too close to the power lines, occasionally i would get a glitch - it would happen maybe 2-3 times in 30-60 flights - but flying 120 sized glow, my buddy crashed 3 planes in a matter of a couple of months with his PCM system when his radio would go into failsafe - he blamed it on Futaba, i think PCM was the culprit - i had no problem dealing with a momentary glitch but when his radio went into failsafe it was bye-bye airplane...
01-30-2002 | 11:05 PM
#28
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From: Platteville, WI
Noise from elevator servo
Marc - reqarding your servo chatter, you might try moving the plane outside away from any house wiring. Sometimes the electrical fields from indoor house wiring can cause a servo to jitter.
Also, make sure the receiver antenna is separated as far as possible from the long servo wiring. Try tapeing it full length to a vertical balsa stick. Also, a range test would be good to do with the elevator servo disconnected. The servo jitter may be an indicator that the radio system will fail a range test. You may have a bad receiver or something.
Things like a separate battery for servos, optical isolaters and ferrite beads can prove helpful but are not always neccessary. To really be certain, sometimes an oscilloscope is helpful, and even then it often comes down to trial and error.
************
GaryB - I've been in RC for over 40 years. I operated one of the first repair centers for digital radio systems for about ten years. I totally agree with mglavin. PCM is always a more reliable data transfer system than PPM.
When your friend lost aircraft near powerlines it could have been many things ; 1)Bad battery; 2) Bad receiver; 3) Bad transmitter; 4)Interference on the same channel etc. The biggest single cause of radio failure is the battery pack. PCM systems may somtimes go into some failsafe mode on battery fail. To say absoluletly that his loss was due to PCM isn't based on very solid evidence.
I don't mean to be argumentive, but a lot of newcomers read these threads and I get a little concerned that well meant, but somewhat incorrect information, could be unintentionally passed on to them. We should try to base our comments on facts as much as possible.
I do think each of our experences are important too and it's fine to pass them on so that everyone can voice an opinion and readers can draw their own conclusions.
Also, make sure the receiver antenna is separated as far as possible from the long servo wiring. Try tapeing it full length to a vertical balsa stick. Also, a range test would be good to do with the elevator servo disconnected. The servo jitter may be an indicator that the radio system will fail a range test. You may have a bad receiver or something.
Things like a separate battery for servos, optical isolaters and ferrite beads can prove helpful but are not always neccessary. To really be certain, sometimes an oscilloscope is helpful, and even then it often comes down to trial and error.
************
GaryB - I've been in RC for over 40 years. I operated one of the first repair centers for digital radio systems for about ten years. I totally agree with mglavin. PCM is always a more reliable data transfer system than PPM.
When your friend lost aircraft near powerlines it could have been many things ; 1)Bad battery; 2) Bad receiver; 3) Bad transmitter; 4)Interference on the same channel etc. The biggest single cause of radio failure is the battery pack. PCM systems may somtimes go into some failsafe mode on battery fail. To say absoluletly that his loss was due to PCM isn't based on very solid evidence.
I don't mean to be argumentive, but a lot of newcomers read these threads and I get a little concerned that well meant, but somewhat incorrect information, could be unintentionally passed on to them. We should try to base our comments on facts as much as possible.
I do think each of our experences are important too and it's fine to pass them on so that everyone can voice an opinion and readers can draw their own conclusions.
01-30-2002 | 11:27 PM
#29
Banned
Radio interference
Thanks for the help on troubleshooting this...will test it out in the next few days time permitting to see if I can narrow it down. I flew the plane and all was ok in the air luckily. Range check at the field was ok. How far should I get before chatter though on a Hitec Eclipse Tx? That antenna collapses all the way into the Tx itself. 30 feet? 50 feet?
01-31-2002 | 05:57 PM
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From: Topeka,
KS
Radio interference
...i'm pretty much a "frequent flyer" - never cared much for building, but in 21 years i've logged thousands of flights on 20+ Futaba and Hitec radios at various flying sites in 3 states - i've never crashed a plane due to radio interference and never experienced more than a few intermittent glitches flying glow planes with both AM and FM radios...(the majority of the flying has been in a metro area of around 160,000 population)...
...maybe i've just been lucky - or maybe you have to fly in a large metropolitan area or fly the big gas ignition engines to have a lot of interference problems?...(i know electrics have more interference problems)...
...maybe i've just been lucky - or maybe you have to fly in a large metropolitan area or fly the big gas ignition engines to have a lot of interference problems?...(i know electrics have more interference problems)...
01-31-2002 | 11:33 PM
#31
The DEBATE
To follow up on what Gary just added, I too am a frequent flier. 15 some years with RC - CL before that. About a dozen different radios over the years - Futaba and JR - AM, PPM, PCM. Unlike lucky Gary, I have lost 4 planes due to radio problems - 2 with PPM and 2 with PCM. With the PPM plane losses, both were because some low life turned their radio on - while my planes were in the air. Both PCM losses were due to interference that was not discovered until the interference got bad enough that both planes went into failsafe. One hit a roof in a residential area - the other hit the ground in the middle of our field - in the middle of nowhere. When I get a chance, I will post and sell several PCM receivers I have. Now I stricly use my Futaba 9Z WC2 in the PPM mode with Hitec or Futaba PPM receivers - two in each Giant Scale. Works for me.
Dan
Dan
02-01-2002 | 04:29 AM
#32
More DEBATE
Whyes -
My good buddy that I fly with is big into Giant Scale - even more than me. Larry was a firm believer in PCM - extra protection he said. Well, over the past four years, he has lost 5 35% Carden Extras and 2 1/3 Lasers - all when his planes went into failsafe. Guess what he is doing for the coming year? A 9Z like I use and two Hitec Quantum receivers per big bird. Ya'll can listen to people argue about electrons and modulations and stuff, but from my own experiences and buddies that I fly with, I'm betting my $3,500.00 birds on two PPM receivers and one 9Z each. When it is the $$$ out of my own pocket, I'm gonna use what I have seen work.
Dan
My good buddy that I fly with is big into Giant Scale - even more than me. Larry was a firm believer in PCM - extra protection he said. Well, over the past four years, he has lost 5 35% Carden Extras and 2 1/3 Lasers - all when his planes went into failsafe. Guess what he is doing for the coming year? A 9Z like I use and two Hitec Quantum receivers per big bird. Ya'll can listen to people argue about electrons and modulations and stuff, but from my own experiences and buddies that I fly with, I'm betting my $3,500.00 birds on two PPM receivers and one 9Z each. When it is the $$$ out of my own pocket, I'm gonna use what I have seen work.
Dan
02-01-2002 | 03:35 PM
#34
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From: Vernonia, OR,
Radio interference
Let take a vote. How many are using PCM in large giant scale airplanes. I for one would not consider a large gas engine airplane without PCM. Would be interesting to find out how many at the TOC fly PPM.
02-01-2002 | 05:25 PM
#35
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From: Florida
PCM is Better than PPM
I flew my first RC plane in '81 RC boats in '76.. I use PPM on my glow planes with fine results (occasional glitches)but have seen through the years that PCM is definitely the way to go with gas planes. Most gas ignitions are shielded well enough nowadays to get away with using PPM at least for awhile.
IT IS IMPORTANT TO SET-UP A GAS PLANE PROPERLY and to range check PCM properly. In my opinion using PPM on a gas plane is IMPROPER METHOD from the start. No wonder some people crash 11 planes in 4 years. They simply do not know what they're doing. I fly at 2 fields with about 15 GIANT SCALE AEROBATIC GAS PILOTS. About 40 35%-40% gas planes between us and all of us use PCM exclusively (Futaba and JR). None of us have ever crashed from radio interference. Everybody around here knows to use PCM on gas planes. I would never fly my 40% cap 232 on PPM no way!http://ourworld.cs.com/cleybennett2/...?mtbrand=CS_US
IT IS IMPORTANT TO SET-UP A GAS PLANE PROPERLY and to range check PCM properly. In my opinion using PPM on a gas plane is IMPROPER METHOD from the start. No wonder some people crash 11 planes in 4 years. They simply do not know what they're doing. I fly at 2 fields with about 15 GIANT SCALE AEROBATIC GAS PILOTS. About 40 35%-40% gas planes between us and all of us use PCM exclusively (Futaba and JR). None of us have ever crashed from radio interference. Everybody around here knows to use PCM on gas planes. I would never fly my 40% cap 232 on PPM no way!http://ourworld.cs.com/cleybennett2/...?mtbrand=CS_US
02-02-2002 | 01:39 AM
#36
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From: Platteville, WI
Range Tasts
Not to belabor the point, but in Cal Orr's "Radio Spectrum" article in the March 2002 RCM, he discusses the mandatory range test that he requires at all Giant Scale races. The requirement is 200 feet of range with the engine running and the antenna collapsed.
It makes no differece which brand of radio or style of modulation a participant is using.
He also mentions tecniques for range testing and suggests that losing more than 10% of range because of the engine running should be corrected.
-Marc, be careful if anyone suggests 50 feet as acceptable range with the antenna down. Radios will differ somwhat, but my 11 Futaba radios all go 300 feet with the antenna collapsed, and I'll bet yours should go between 200 and 300 feet.
Also, you fellows using Futaba PCM don't need to utilize "fail safe" unless you want to. It's an option in the setup menu for the 8UA & 9ZAP radios. Don't know about JR or other brands though.
BTW, I have flown in quite a few giant scale races and I don't believe I ever saw more than a couple of racing setups with PPM receivers.
It makes no differece which brand of radio or style of modulation a participant is using.
He also mentions tecniques for range testing and suggests that losing more than 10% of range because of the engine running should be corrected.
-Marc, be careful if anyone suggests 50 feet as acceptable range with the antenna down. Radios will differ somwhat, but my 11 Futaba radios all go 300 feet with the antenna collapsed, and I'll bet yours should go between 200 and 300 feet.
Also, you fellows using Futaba PCM don't need to utilize "fail safe" unless you want to. It's an option in the setup menu for the 8UA & 9ZAP radios. Don't know about JR or other brands though.
BTW, I have flown in quite a few giant scale races and I don't believe I ever saw more than a couple of racing setups with PPM receivers.
02-02-2002 | 02:38 AM
#37
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From: Florida
It's all in the encoding
Whyes said:>>>>>>>>>>>>>>>>If you are getting ANY interference with PPM ,you are getting the same interference with PCM . >>>>>>>>>>>>>> I don't agree with that statement at all.
I mean the signal may be affected in an identical way but the PCM system will still function perfectly while the PPM system will not. Both systems use pulses on the carrier wave to communicate the control information....PULSE POSITION MODULATION!!! This system is reliant on the pulse positions or timing! PULSE CODE MODULATION!!! This system uses a code of pulses some there and some not to communicate each servos information and is not as reliant on the signal timing as PULSE POSITION MODULATION!
I mean the signal may be affected in an identical way but the PCM system will still function perfectly while the PPM system will not. Both systems use pulses on the carrier wave to communicate the control information....PULSE POSITION MODULATION!!! This system is reliant on the pulse positions or timing! PULSE CODE MODULATION!!! This system uses a code of pulses some there and some not to communicate each servos information and is not as reliant on the signal timing as PULSE POSITION MODULATION!
02-02-2002 | 03:00 AM
#38
More, The DEBATE
Gary B - I like the Hitec strictly because of the price, given that I think that the Quantum is as good as or better than Futaba.
iflyrvs1 - The TOC guys use PCM in their planes. BUT, I was at several meets this past summer yakking with several of them (TOC guys) and flying each others planes. It got real interesting when I asked them some questions about radio setups. All that I talked to used 2 receivers on PCM. WHY? Cause that is what they were given. Well golly gee lets think about that - I am sponsoring a world class 3D pilot - what these world class pilots use and fly with all the other wannabees are gonna buy - lets give this world class guy our top of the shelf / most expensive / best markup products for free - and we'll make it up when all the other wannabees go and buy the same thing.
My buddy fell into that trap - used a single PCM receiver - didn't pay attention to noise - and lost a bunch of big expensive birds over the past several years. In the same time, I lost one to PCM failsafe (interfernece that got worse until I found out about it when failsafe kicked in) and one due to an idiot turning on while I was flying.
SO, 2 PPM receivers for me, unless Futaba or JR or Airtronics wants to sponser me - Please send me two TXs, @ 20 PCM receivers, 40 or so digital servos, harnesses, switches, hats, t-shirts, and yeah, I'll tell everyone that brand XX is the best in the world and you need to go buy the exact same or you won't be any good. Yeah, that would work for me.
Dan
iflyrvs1 - The TOC guys use PCM in their planes. BUT, I was at several meets this past summer yakking with several of them (TOC guys) and flying each others planes. It got real interesting when I asked them some questions about radio setups. All that I talked to used 2 receivers on PCM. WHY? Cause that is what they were given. Well golly gee lets think about that - I am sponsoring a world class 3D pilot - what these world class pilots use and fly with all the other wannabees are gonna buy - lets give this world class guy our top of the shelf / most expensive / best markup products for free - and we'll make it up when all the other wannabees go and buy the same thing.
My buddy fell into that trap - used a single PCM receiver - didn't pay attention to noise - and lost a bunch of big expensive birds over the past several years. In the same time, I lost one to PCM failsafe (interfernece that got worse until I found out about it when failsafe kicked in) and one due to an idiot turning on while I was flying.
SO, 2 PPM receivers for me, unless Futaba or JR or Airtronics wants to sponser me - Please send me two TXs, @ 20 PCM receivers, 40 or so digital servos, harnesses, switches, hats, t-shirts, and yeah, I'll tell everyone that brand XX is the best in the world and you need to go buy the exact same or you won't be any good. Yeah, that would work for me.
Dan
02-02-2002 | 02:41 PM
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Big Question?
One BIG Question...
From where the hell this interference comes from, on 72MHz band?
Also is this legal or illegal or randomly generated?
Okay if they are generated by spark ignition or any random source of Wide Band RF, then why this intereference is sometimes so prolonged that we lost expensive planes???
Is 72MHz safe enough for R/C Now?
From where the hell this interference comes from, on 72MHz band?
Also is this legal or illegal or randomly generated?
Okay if they are generated by spark ignition or any random source of Wide Band RF, then why this intereference is sometimes so prolonged that we lost expensive planes???
Is 72MHz safe enough for R/C Now?
02-02-2002 | 03:22 PM
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From: Hammond,
IN
Radio interference
Hello RCPilot 100:
No offense, but if your buddy lost over 5 planes in a year due to radio problems, it is not likely going to be fixed by choosing PPM. The problem rests with your buddy! It just might be his method of installing radios or he keeps using a particular receiver that has a defective component etc.
Also, here's a heads up for others who may be having some glitches. I just finished a 33% Cap with a single Futaba PCM receiver, Hitec 5945 digital servos, redundant 2700 NiMH 4-cell packs for the receiver, 2700 NiMH 4-cell pack for the ignition, 3 JR HD charge switches, and JR HD extensions and Y-harness. Everything bench tested fine. First day out flying, good range test and no glitches, everything OK. Next day, the rudder was glitching and acting very strange for the first couple of minutes on the ground. Then it fixed itself.......hmmm. Flew 4 flights glitch free.
At home, peak charged the receiver packs and rediscovered the glitching problem on the rudder. Worked the servos for a couple of minutes and the problem again fixed itself.
The problem was caused by a JR Y-harness (with amplifier) on the rudder. It was very sensitive to the high voltage of a peak charged 4-cell pack. Changed out to a standard Y-harness, peak charged the packs again, and the rudder was rock solid. The first day when I flew the plane, I hadn't peak charged the packs, so I didn't see the voltage related problem. Also, when I was bench testing, I wasn't using a peaked pack.
No offense, but if your buddy lost over 5 planes in a year due to radio problems, it is not likely going to be fixed by choosing PPM. The problem rests with your buddy! It just might be his method of installing radios or he keeps using a particular receiver that has a defective component etc.
Also, here's a heads up for others who may be having some glitches. I just finished a 33% Cap with a single Futaba PCM receiver, Hitec 5945 digital servos, redundant 2700 NiMH 4-cell packs for the receiver, 2700 NiMH 4-cell pack for the ignition, 3 JR HD charge switches, and JR HD extensions and Y-harness. Everything bench tested fine. First day out flying, good range test and no glitches, everything OK. Next day, the rudder was glitching and acting very strange for the first couple of minutes on the ground. Then it fixed itself.......hmmm. Flew 4 flights glitch free.
At home, peak charged the receiver packs and rediscovered the glitching problem on the rudder. Worked the servos for a couple of minutes and the problem again fixed itself.
The problem was caused by a JR Y-harness (with amplifier) on the rudder. It was very sensitive to the high voltage of a peak charged 4-cell pack. Changed out to a standard Y-harness, peak charged the packs again, and the rudder was rock solid. The first day when I flew the plane, I hadn't peak charged the packs, so I didn't see the voltage related problem. Also, when I was bench testing, I wasn't using a peaked pack.
02-02-2002 | 03:28 PM
#41
Big QUESTION
FCC -
This interference can come from many sources - the biggest offender I see is the plane itself, especially on big birds. We are dealing with some kind of system to zap the spark plugs, which if it is not installed correctly, can cause heartburn. Have a part of that ignition system rub on something and you can cause more interfernece. Long leads can act as antenna and take garbage back to the receiver. Digital servos and large amounts of power going through the receiver can cause problems I don't think are fully understood yet - this is one of the reasons I use two receivers. Anywhere in your big bird that something starts vibrating - and our big ones can vibrate - you have the possibility of generating garbage that can affect you receiver. My perspective is to eliminate all potential sources in the aircraft and you have good odds that you won't lose your plane - unless someone turns on your frequency while you are flying. That has happened to me twice in 15 + years - cost me an Astro Hog and a Sukhoi. The guy admitted the Astro Hog and paid for a new kit - the scum that did my Sukhoi in slithered away - and the club that hosted the fly in didn't have the courtesy of saying they were sorry.
Dan
This interference can come from many sources - the biggest offender I see is the plane itself, especially on big birds. We are dealing with some kind of system to zap the spark plugs, which if it is not installed correctly, can cause heartburn. Have a part of that ignition system rub on something and you can cause more interfernece. Long leads can act as antenna and take garbage back to the receiver. Digital servos and large amounts of power going through the receiver can cause problems I don't think are fully understood yet - this is one of the reasons I use two receivers. Anywhere in your big bird that something starts vibrating - and our big ones can vibrate - you have the possibility of generating garbage that can affect you receiver. My perspective is to eliminate all potential sources in the aircraft and you have good odds that you won't lose your plane - unless someone turns on your frequency while you are flying. That has happened to me twice in 15 + years - cost me an Astro Hog and a Sukhoi. The guy admitted the Astro Hog and paid for a new kit - the scum that did my Sukhoi in slithered away - and the club that hosted the fly in didn't have the courtesy of saying they were sorry.
Dan
02-02-2002 | 03:37 PM
#42
PCM vs PPM
Diablo -
You are right - my buddy didn't do a good job of checking all the small stuff to insure he wasn't causing himself grief. The PCM 'ate' the garbage his plane was generating in the way of interference for a while - and when it couldn't anymore - failsafe - smack - needs new plane. My whole issue is that if there is interference, I believe that PPM will allow the problem to manifest itself by way of glitches, shimmies, whatever, way before PCM. PCM may appear to work correctly in the presence of interference than PPM, but you have no sliding scale to know that you do have it with PCM - your first indication is failsafe. From all the flying I have done with others, I have not myself recovered from a failsafe condition nor have I seen it allow someone to save their plane once it kicked in. I believe that once you have an indication that something is amiss - get it fixed - PPM give you an earlier warning that PCM.
Dan
You are right - my buddy didn't do a good job of checking all the small stuff to insure he wasn't causing himself grief. The PCM 'ate' the garbage his plane was generating in the way of interference for a while - and when it couldn't anymore - failsafe - smack - needs new plane. My whole issue is that if there is interference, I believe that PPM will allow the problem to manifest itself by way of glitches, shimmies, whatever, way before PCM. PCM may appear to work correctly in the presence of interference than PPM, but you have no sliding scale to know that you do have it with PCM - your first indication is failsafe. From all the flying I have done with others, I have not myself recovered from a failsafe condition nor have I seen it allow someone to save their plane once it kicked in. I believe that once you have an indication that something is amiss - get it fixed - PPM give you an earlier warning that PCM.
Dan
02-04-2002 | 04:21 PM
#43
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From: Florida
Big Question
One BIG Question...
From where the hell this interference comes from, on 72MHz band?
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
FCC
For what it's worth I don't think that all interference is just picked up by ones antenna as a signal.....I think that for the most part the interference that affects our planes is affecting our radio signal while it is in electromagnetic form..Before it hits the antenna..
A radio signal is an electromagnetic wave that can travel through most objects...
When electricity passes thru a wire (spark plug wires) it radiates an electromagnetic field to the sides.......
You guys know that magnetic turns to electric and electric turns to magnetic right? So part of your radio signal travels right thru your plug wires and part thru the surrounding electromagnetic field and gets to your receiver antenna at exactly the same time as the rest of the signal?? I wouldn't bank on that! Maybe the distorted part of the signal blends in with the non distorted signal and kind of resonates to create a signal that is slightly off or fluctuating... PPM times off of the signal and PCM keeps reading the codes... Now if part of the signal is totally gone or unreadable the PCM will have holds before failsafe kicks in. So with intermittent interference you may only experience holds and never have a failsafe condition.
From where the hell this interference comes from, on 72MHz band?
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FCC
For what it's worth I don't think that all interference is just picked up by ones antenna as a signal.....I think that for the most part the interference that affects our planes is affecting our radio signal while it is in electromagnetic form..Before it hits the antenna..
A radio signal is an electromagnetic wave that can travel through most objects...
When electricity passes thru a wire (spark plug wires) it radiates an electromagnetic field to the sides.......
You guys know that magnetic turns to electric and electric turns to magnetic right? So part of your radio signal travels right thru your plug wires and part thru the surrounding electromagnetic field and gets to your receiver antenna at exactly the same time as the rest of the signal?? I wouldn't bank on that! Maybe the distorted part of the signal blends in with the non distorted signal and kind of resonates to create a signal that is slightly off or fluctuating... PPM times off of the signal and PCM keeps reading the codes... Now if part of the signal is totally gone or unreadable the PCM will have holds before failsafe kicks in. So with intermittent interference you may only experience holds and never have a failsafe condition.
02-05-2002 | 09:14 AM
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Radio interference
after writing a bit about reactive coupling, EMI and near-field radiation, PPM and PCM frame synchronization, and PCM error-handling, I decided that it just wasn't worth it.
02-05-2002 | 12:46 PM
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Ch # 22
I think it is SW Ohio they do not allow use of CH 21 or 22 because of a local cell phone interference..?? I think, they think, that is where it is coming from...Have you moved up the band a ways in ch #s in all your change over problems..This is all spectulation, but can it be the same in other parts of the country?? Just a thought... Brian Smith
02-07-2002 | 04:26 AM
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Too good to share??
ftomteen4cat said on a related thread:
As far as I know, PPM encoding just varies the pulse width from 1 to 2us. To me, that is a pretty good description of PWM. (pulse width modulation) Pulse Position Modulation would use frame pulses or a sync pulse and move the pulse in time, relative to the frame or sync, but oh well I didn't name it.
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Maybe you should contact Futaba corp or Japan Radio and discuss this oversight on their part.
I read somewhere that PPM pulses are 400 us long and that the time between them varies between 1ms and 2ms.. PWM refers to one signal, one channel.
Pulse position Modulation refers to each individual channels position in the serial frame in relation to the sync. All our radios are multichannel so PPM is the correct useage.
The term PPM is used to describe all channels as a result of multiplexing them serially into one coherent signal. Is this true??
As far as I know, PPM encoding just varies the pulse width from 1 to 2us. To me, that is a pretty good description of PWM. (pulse width modulation) Pulse Position Modulation would use frame pulses or a sync pulse and move the pulse in time, relative to the frame or sync, but oh well I didn't name it.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Maybe you should contact Futaba corp or Japan Radio and discuss this oversight on their part.
I read somewhere that PPM pulses are 400 us long and that the time between them varies between 1ms and 2ms.. PWM refers to one signal, one channel.
Pulse position Modulation refers to each individual channels position in the serial frame in relation to the sync. All our radios are multichannel so PPM is the correct useage.
The term PPM is used to describe all channels as a result of multiplexing them serially into one coherent signal. Is this true??
02-07-2002 | 06:28 AM
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Radio interference
No, not too good. I just didn't see a point in highlighting the statements you've made regarding PPM and PCM that are based upon nothing more than your assumptions. Although it might benefit the general community, I felt the potential damage to your integrity and credibility just weren't worth it. The number of points I disagreed with you on made it unlikely that you would have taken my post in the spirit in which it was intended. You probably would have viewed it as an attack, though it certainly would not have contained sarcasm.
Your statement about position modulation is valid. There is a variable starting pulse and negative 300us channel separation pulses shifted in time. However, the positive portion of the waveform consists of modulated pulse-widths. You can call it what you want. (duration of the negative pulses does not appear to be critical, as timing indicates the demux is postive-edge triggered) PPM, PWM, or PCM are modulation schemes, methods of adding some sort of intelligence to a signal. It's not necessary that they be the output of a mux.
What RCPilot100 is talking about is EMI and near-field RF radiation. This radiation wreaks havoc on our receivers by it's sheer magnitude and broad spectrum. The interference is induced and reradiated in the receiver, servos, leads, and antenna. This near-field induction is called evanescent or reactive coupling, and is not limited to H field emissions. As for artifacts causing distortion "in electromagnetic form" vs. being mixed, erm.. sure man. Your theory that "the distorted part of the signal blends in with the non distorted signal and kind of resonates to create a signal that is slightly off or fluctuating" is pure poetry.
Your statement about PCM not being reliant on signal timing is false. Both PCM and PPM rely on synchronization with each frame.
Your statement about PPM discarding corrupted frames and PCM chugging right along is actually backwards. PCM will discard a subframe after an error of a single bit. PPM will continue to process the frame, although if channel 1's pulse was so badly corrupted that it was to go unnoticed, channel 2's pulse could be sent to channel 1, and so on, until the beginning of the next frame. In the event of an error, a PCM receiver will simply hold the last valid frame's data in the decoder until the failsafe timeout. This is why short errors can go by unnoticed in a PCM system.
There is nothing more robust about the PCM waveform as it travels through the air. I can't say whether anyone puts better componentry in their PCM receivers, but front-end components aside, the robustness stems from PCM's CRC checks, the receiver's ability to hold an output in the event of a corrupted frame, and the fact that jitter errors do not affect the servo's position in a PCM system. While jitter can be evident in PPM systems, the frame repetition rate is high enough that it's effect is minimal.
While I appreciate your attack, I suggest that you claim "expertise" in another area of the hobby. I'll admit to being wrong once in a while, and I welcome it when people enlighten me, provided they do so in a professional manner. Disillusionment is a part of growth in engineering. If I believe in my point, I'll argue it, in a professional manner. You however, seem to enjoy attacking people with what little you know.
Have a nice day
Your statement about position modulation is valid. There is a variable starting pulse and negative 300us channel separation pulses shifted in time. However, the positive portion of the waveform consists of modulated pulse-widths. You can call it what you want. (duration of the negative pulses does not appear to be critical, as timing indicates the demux is postive-edge triggered) PPM, PWM, or PCM are modulation schemes, methods of adding some sort of intelligence to a signal. It's not necessary that they be the output of a mux.
What RCPilot100 is talking about is EMI and near-field RF radiation. This radiation wreaks havoc on our receivers by it's sheer magnitude and broad spectrum. The interference is induced and reradiated in the receiver, servos, leads, and antenna. This near-field induction is called evanescent or reactive coupling, and is not limited to H field emissions. As for artifacts causing distortion "in electromagnetic form" vs. being mixed, erm.. sure man. Your theory that "the distorted part of the signal blends in with the non distorted signal and kind of resonates to create a signal that is slightly off or fluctuating" is pure poetry.
Your statement about PCM not being reliant on signal timing is false. Both PCM and PPM rely on synchronization with each frame.
Your statement about PPM discarding corrupted frames and PCM chugging right along is actually backwards. PCM will discard a subframe after an error of a single bit. PPM will continue to process the frame, although if channel 1's pulse was so badly corrupted that it was to go unnoticed, channel 2's pulse could be sent to channel 1, and so on, until the beginning of the next frame. In the event of an error, a PCM receiver will simply hold the last valid frame's data in the decoder until the failsafe timeout. This is why short errors can go by unnoticed in a PCM system.
There is nothing more robust about the PCM waveform as it travels through the air. I can't say whether anyone puts better componentry in their PCM receivers, but front-end components aside, the robustness stems from PCM's CRC checks, the receiver's ability to hold an output in the event of a corrupted frame, and the fact that jitter errors do not affect the servo's position in a PCM system. While jitter can be evident in PPM systems, the frame repetition rate is high enough that it's effect is minimal.
While I appreciate your attack, I suggest that you claim "expertise" in another area of the hobby. I'll admit to being wrong once in a while, and I welcome it when people enlighten me, provided they do so in a professional manner. Disillusionment is a part of growth in engineering. If I believe in my point, I'll argue it, in a professional manner. You however, seem to enjoy attacking people with what little you know.
Have a nice day
02-07-2002 | 03:22 PM
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Another Know it all?
ftomteen4cat said on a related thread:
As far as choosing one over the other, if you're going to get hit, you'll get hit either way. What you want the plane to do when you get hit is a matter of preference. It seems to me that current receivers have much better rejection than they used to. I remember going out hoping I wouldn't get hit that day.. (pre-'91 rx) Given the cost difference, I've stuck with PPM.
- I just read the interference thread where mglavin said that PCM rx's afford a 20db improvement in S/N ratio. (learn something every day) That would give PCM the edge in noisy RF environments, at least in my mind. -
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Well? Which is it? Futaba states on their website that PCM has noise advantage over PPM! Maybe you should give them a call and tell them that their website is incorrect! What makes you think that I'm being sarcastic?? All of the most experienced pilots know that it's true about PCM and can readily see holds while they're flying or range checking. Just cause you don't understand it doesn't mean that it's not true. Experienced people spend the extra money on PCM because they know it's worth the added security for an expensive plane. It's not just holds. PCM gives you extended functional range also. It has to do with what happens to the signal when it starts to weaken and how each encoding system is read.
As far as choosing one over the other, if you're going to get hit, you'll get hit either way. What you want the plane to do when you get hit is a matter of preference. It seems to me that current receivers have much better rejection than they used to. I remember going out hoping I wouldn't get hit that day.. (pre-'91 rx) Given the cost difference, I've stuck with PPM.
- I just read the interference thread where mglavin said that PCM rx's afford a 20db improvement in S/N ratio. (learn something every day) That would give PCM the edge in noisy RF environments, at least in my mind. -
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Well? Which is it? Futaba states on their website that PCM has noise advantage over PPM! Maybe you should give them a call and tell them that their website is incorrect! What makes you think that I'm being sarcastic?? All of the most experienced pilots know that it's true about PCM and can readily see holds while they're flying or range checking. Just cause you don't understand it doesn't mean that it's not true. Experienced people spend the extra money on PCM because they know it's worth the added security for an expensive plane. It's not just holds. PCM gives you extended functional range also. It has to do with what happens to the signal when it starts to weaken and how each encoding system is read.
02-07-2002 | 04:04 PM
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Re: Another Know it all?
Where did I say this was incorrect? You should read learn to read better. I am talking about the air interface. They are talking about the whole system.
02-07-2002 | 06:19 PM
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Definition of PCM from online encyclopedia
Developed in 1939 by the English inventor Alec H. Reeves, pulse code modulation is the most important form of pulse modulation because it can be used to transmit information over long distances with hardly any interference or distortion; for this reason it has become increasingly important in the transmission of data in the space program and between computers.
