Cley

Also it appears that some if not all of the PCM systems send two frames of servo information per channel BACK to BACK so if one of the frames is damaged the other frame is right there to be received in good form......... The interference would have to be really bad for PCM to go North.

GMPheli

Cley, What you are saying does not make sense to me. Speed (response time) is an issue with PCm. You want it to respond as fast as it can. If you send duplicate frames, it would just slow everything down. Why would you send a duplicate when you can send one with more current information?

Alan

Cley

GMPheli,
Please explain the following paragraph...
Thanks!

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Robbe Futaba PCM 1024 uses an interesting solution, yielding the fastest frame rate. Differential encoding reduces the number of bits per channel for 2 frames from 20 (2*1°) to 14 (10+4) , reducing the frame rate to 14.25 ms. In the first frame an absolute value is sent , in the second one only the relative change.A complete transmission message , every channel 2 times, lasts less than 30 ms in this case. A bit error causes a 14.25 ms block to be rejected. Accuracy is ten bits.

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It would make sense to me for the purpose of reducing low level
interference problems.......
You can have bit errors and the receiver still gets all of the channels information every cycle.......... It may reduce the refresh rate some but maybe the increased noise rejection is worth it.
The servos can only process so much info anyway.........

HarryC

My Feedback: (1)

The difference in response times is worth nearly 1.5 feet for a model travelling at 100mph. For a pylon racer or pattern aerobat that is actually a noticeably visible difference in PPMs favour.

PCM used to have the advantage of failsafe whose glitch prevention could make all the difference between winning or losing a competition. Once one person had it, everyone had to have it just in case they were put at a disadvantage by a glitch in mid manoeuvre.

PCM's theoretical and laboratory advantage of signal accuracy doesn't work out to much of practical value in the real world of model flying. It's something of a feature trying to prove that it's actually a benefit, especially as failsafe is now available on PPM. I started model flying in 1977 on AM radios and in all that time I can only say honestly that I have lost a model once from interference, which may have been so bad as to crash a PCM model anyway. PCM may allow some petrol models etc to fly but many of us would say that those models are downright unsafe if they can't be used with PPM as well.

Harry

Forgues Research

My Feedback: (7)

Your absolutely right Harry,
I feel the best way, is to solve any problem, with PPM and if you must go PCM, then only after the problems have been solved.
The reason people feel PCM is better is that it actually masks a lot of the glitches until its too late.

Roger
ACM Rep....

Team ACM
Fiber Optic Servo Extensions
Forgues Propellers
Multiplex Dealer
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BigBird1

My Feedback: (4)

Just a little 2 cents worth...

I've got off into giant scale with the wazoo motors and hot ignitions...

I bought a new T6XAs to put in a Bridi Big Bee..
The range check was no more than 50 feet with the engine running...and the antenna in the fuse...put the antanna outside the fuse the range was much better 150 feet..

My flying buddy just bought himself one of those Wazoo Jr top of the line PCM's and was selling his old JR Galaxey PCM that came from Circus Hobbies...many, many, moons ago...for a good price so I bought it...
Installed the radio with the antenna in the fuse and proceeded to do a range check with the engine running antenna collapsed...500
feet and still walking away I gave up....

Their has got to be something to this PCM stuff...

Phil Cole

While PCM will generally give you a better range check distance than PPM when using the same transmitter, using range checks with the antenna down is not a valid way to compare different transmitters.

All you are comparing is how much RF leaks out of the transmitters.

Dave Bowles

My Feedback: (7)

It used to be normal Practice to range check with the ANT. removed from the TX , but it was later found not to be a universal standard and some TX would not have any range without the Ant. but did fine with it just pulled down. As far as I know it dosen't matter if its PCM or PPM , the TX broadcasts the same strength no matter which it is set on. It was also found that differant recievers range checked better than some depending on the TX used.

So I guess the question here is, Does a better range check on the ground mean better range in the air? And does Better range mean it is less effected by noise or interferance.

Sence our radios can reach much further than most of can see the aircraft , is it really an issue? do we ever get far enough away to where it would make a differance?

Obviously differant recievers have differant ranges, My little micro recivers only have a few hundred feet of range with your normal TX , I would imagine it is possible to make a PPM or PCM reciever have as much range as the TX can transmit. I guess it wasn't needed.

HarryC

My Feedback: (1)

Part of this range is PCM's advantage at getting individual servo data through whereas PPM needs the entire sequence to be clean. But a large part of this extra range is simply the "hold" part of failsafe masking the fact that there is no signal and without you realising it you have actually got no control of the model while it is briefly in hold mode. You can get this with PPM failsafe, leaving PCM with only a tiny advantage over PPM. What you are getting is a radio that is hiding from you the fact that you have got a very serious interference problem. Using PCM in this situation is a dubious solution.

PCM ought to be far better, but out here in the real world away from paper calculations and lab tests, it's got only the slimmest advantage over PPM.

Harry

BigBird1

My Feedback: (4)

Well IF i had my drothers now...I believe that I would choose the PCM over the PPM, but would not fly a reed type radio again...
(Guess that I showed my age with that one..)

Maybe its mental, but I feel a lot better using a PCM now..
Who knows...

GMPheli

----------------------------------------------------------------------------------
Robbe Futaba PCM 1024 uses an interesting solution, yielding the fastest frame rate. Differential encoding reduces the number of bits per channel for 2 frames from 20 (2*1°) to 14 (10+4) , reducing the frame rate to 14.25 ms. In the first frame an absolute value is sent , in the second one only the relative change.A complete transmission message , every channel 2 times, lasts less than 30 ms in this case. A bit error causes a 14.25 ms block to be rejected. Accuracy is ten bits.

----------------------------------------------------------------------------------
Cley, this is just a unique way of encoding to speed up the frame rate. Instead of all the frames carrying all the information, they have 2 types that alternate. One frame with all the information, the next with just what has changed, then all info, then what has changed. The frames are not identical. If either frame is corrupt, it blocks (holds for) both frames.

Alan

Cley

GMPheli said:
The frames are not identical. If either frame is corrupt, it blocks (holds for) both frames.

--------------------------------------------------------------------------------

A complete transmission message , every channel 2 times, lasts less than 30 ms in this case. A bit error causes a 14.25 ms block to be rejected. Accuracy is ten bits.
---------------------------------------------------------------------------------
GMPheli,
If what you are saying is true then the interference would have to hit the full set of code to make the channel hold for the full 2 frames, it would also mean that Futaba PCM refreshes faster than PPM wouldn't it? Did you see this?

http://www.aerodesign.de/peter/2000/PCM/pcm_systeme.gif
or this?
http://www.aerodesign.de/peter/2000/PCM/frame_ppm.gif
----------------------------------------------------------------------------
Harry C said:
PCM's theoretical and laboratory advantage of signal accuracy doesn't work out to much of practical value in the real world of model flying................. Also: PCM ought to be far better, but out here in the real world away from paper calculations and lab tests, it's got only the slimmest advantage over PPM.

----------------------------------------------------------------------------------

It's funny, I've been involved in RC since '76 and I think that the advantages of PCM over PPM are much understated.........
PCM's popularity is largely due to "REAL WORLD" experiences....
Experienced pilots know.........

Cley

Harry C,
Did you check out that aerodesign link and what do you make out of it?

HarryC

My Feedback: (1)

Sometimes gives mixed messages e.g.
"This proves the general opinion that PCM has better range than PPM to be unfounded."
"PCM-Modulation ,usually combined with excellent doublesuper receivers, provides the highest possible transmission security."

He does make the interesting point which no-one here has mentioned, that expensive PCM Rx are usually top grade dual conversion units. Clearly it would not be right to try and compare that to a budget brand single conversion PPM which I suspect that many people do. I would like to see a PCM Rx compared by a techie like him to a high quality PPM Rx like my dual conversion IPD.

I fully beleive that PCM is better. I just have never seen any evidence myself that:
1) It is better _enough_ to bother with
2) I need it. One crash due to interference since 1977 which would probably have knocked out PCM as well doesn't make me feel that I have to have PCM.

Harry

TOYMAKER

My Feedback: (6)

PCM.... gets my thumbs up. I have experienced nothing but posiitive things with the ones I have owned. However you do have to know what you are doing with setting one up properly.

I saw a guy one time taxi headlong into the pit area and thought he had control. Turned his transmitter off and the throttle went full tilt. He had his throttle failsafe reversed for lost signal. Needless to say that was not a good visual experience.

I think some people do not like PCM for the fear of not knowing how to set them up properly. If they know what the product does for them and they incorperate it into the use of thier aircraft properly then PCM becomes a real world value.

JohnW

My Feedback: (6)

Thought I was done on this thread... but guess not.

Yes, Futaba uses a unique differential compression on frames. One full frame and then one compressed frame. Average Futaba PCM frame rate is actually quicker than a similar channel PPM. In fact it is almost twice as fast. This means that as long as you use Futaba gear the complaints about PCM frame rate are unfounded.

Harry C. Why do you bother to run IPD? What make that so much better than PPM? Just curious since IPD and PCM obtain similar goals through different means. Why did IPD cut the mustard and PCM did not?

HarryC

My Feedback: (1)

I like the idea of failsafe to stop the engine from carving people up, and the short term hold just in case of the once per season glitch that I get. I have an Mpx Tx that transmits in PCM but since Mpx stopped making PCM Rx that option is not available but IPD is. I might have bought PCM Rx if they still made them but only for the failsafe. I agree that PCM gets a little bit more signal to the Rx during intermittent interference but interference has not been a problem for me. Why bother with failsafe at all then? Just that added bit of protection, I don't believe that just because I have not had a major problem for years and years there is any guarantee that my next flight will be ok and I don't want my YS91 with a wicked APC prop carving into people in the pits.

At my present club I have seen dozens of crashes in the last 5 years and not one of them has been interference. The causes have been bad flying, bad building, bad flying, nicads not charged up before coming flying, bad flying, servo failures, bad flying, Rx failures bad flying, and so on. Glitches yes, but no crashes due to interference. If we wanted to reduce the number of crashes, particularly the dangerous crashes that threaten humans, we need to regulate model flying and stop a lot of inept and careless people from flying at all! PCM alone would have made no difference.

A couple of months ago the UK mag RCMW had an article by a telecomms engineer who often flies petrol (gas) powered models and flies for one of the engine makers at the model shows. He said that PCM offered barely any real advantage over PPM, and as many of us have said here, he said it is very dangerous to use it to mask the interference being caused by spark ignition engines.

I'm all in favour of going digital, but my experience is that PCM alone (strip out the failsafe since that is now available on PPM) offers me and my club-mates no advantage.

Harry

Forgues Research

My Feedback: (7)

Harry, I have 2 Multiplex PCM receivers if your interested. I have switched to IPD's and will never look back They also have the Fail safe .

Roger
ACM Rep....

Team ACM
Fiber Optic Servo Extensions
Forgues Propellers
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GMPheli

Hey Cley:
Actually, we were both wrong. I didn't fully understand the differential part. When I said this:

GMPheli said:
The frames are not identical. If either frame is corrupt, it blocks (holds for) both frames.

--------------------------------------------------------------------------------

I meant that it would hold for the 14.25 ms block (or frame), having both the absolute and relative change in it.

2 full frames give you all channels with absolute data. But the frames are not identical. The whole reason behind this encoding scheme is to speed up the frame rate. Why send identical frames when you can send updated ones.

Alan

HarryC

My Feedback: (1)

Not exactly a way to sell your PCM kit is it!!!!!!!!!!! lol

Thanks but no, I now have 2 of the 7 channel IPD and one of the dual conversion 9 channel IPDs.

I do think it is a mistake of Multiplex to drop PCM though.

Harry

Forgues Research

My Feedback: (7)

Multiplex invented the PCM system for radio control in the first place, and now they evolved to IPD, eventually, the others will follow. they were the first with digital servos, they invented the buddy box system, and I can go on.

Roger
ACM Rep....

Team ACM
Fiber Optic Servo Extensions
Forgues Propellers
Multiplex Dealer
http://www.aerografixs.com
Model-Scan Frequency Analyzer

Cley

GMPheli,
Did you see the depiction of PCM and PPM frames that I linked above?
Also I never said that the frames were Identical.
Don't get me wrong, I keep an open mind with this stuff..
-------------------------------------------------------------------------------
I said,
Also it appears that some if not all of the PCM systems send two frames of servo information per channel BACK to BACK so if one of the frames is damaged the other frame is right there to be received in good form......... The interference would have to be really bad for PCM to go North.
----------------------------------------------------------------------------------
I guess that could be interpreted more than one way.
I would suspect that there is a pause between each 2 frame message with PCM as well as between each single frame of PPM.
Even if the absolute packet of info for a channel was corrupted and the channel held for that frame, the difference data in the next frame would move the servo from the previous good position toward the present position. I think. Maybe MonkeyBoy knows?
--------------------------------------------------------------------------------
MonkeyBoy said:
Harry C. Why do you bother to run IPD? What make that so much better than PPM? Just curious since IPD and PCM obtain similar goals through different means. Why did IPD cut the mustard and PCM did not?

-----------------------------------------------------------------------------------

I would surmise that the IPD PPM would start to show interference before PCM would, just not as drastically as regular PPM...
Maybe it's cheaper to manufacture an IPD PPM system than to manufacture a PCM system and if the manufacturers can convince the consumer that they are of similar performance then there may be a larger profit?? That would make sense.. Make it cheaper and charge the same.. Maybe the other manufacturers will follow that lead but I hope not....

Forgues Research

My Feedback: (7)

IPD will cost you less then PCM, they are faster, and they will protect you as good if not better then PCM, when you start getting some glitches, your response will get mushy, but you will get it back, as with PCM at that point your locked out, IPD is more advance then PCM, its called evolution.

Roger
ACM Rep....

Team ACM
Fiber Optic Servo Extensions
Forgues Propellers
Multiplex Dealer
http://www.aerografixs.com
Model-Scan Frequency Analyzer

Cley

I don't agree and what proof do you have? I think that Futaba PCM has a frame rate that is faster than PPM.. And PPM can never equal PCM!!
Also I believe that Futaba states on their website that they were the first to introduce PCM technology to the RC world......
Also Multi-Plex isn't even competetive in the RC industry IMO...
Most people here in the US have never even seen a Multi-Plex radio and I know that their servos don't even come close to Hitecs digitals by specs alone!
I think what IPD does is mask the interference by expanding it's tolerance to bad signals and averaging them against the good signals thus you have a sloppy situation there where PCM would continue to work just fine...

HarryC

My Feedback: (1)

Typical Cley! Massive bias against Mpx and now he is speculating about IPD performance when he has never even used or tested one. Multiplex not competitive when their Tx are vastly cheaper than Futaba/JR? Who do you know makes a 12 channel Tx with synthesised frequencies and built in scanner for less than 2/3 the price of a 9Z - Multiplex does. And aside from their radios who made the biggest selling model plane of all time? Mpx invented the programmable digital servo, were the first to use FM for model radio, were the first to introduce software modules to Txs, were the first to use rotary selector program dials years before Futaba copied it on the 9C and so on and on.

There is a huge world outside the USA Cley, most people in the USA have never seen outside their borders let alone seen a Multiplex radio. Just because you don't buy much Multiplex doesn't count for much for the majority of people on this planet.

Harry