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  1. #1
    S E Repton's Avatar
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    What's The Difference between PCM and PPM?

    The title says it all really. I don't understand the difference between the two and what the advantages and disadvantages are of them.

    Thanks in advance,

    Sheridan
    Still Trying To Think Of Something Funny To Put Here!

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    RE: What's The Difference between PCM and PPM?

    They are both FM signals. PCM is a more digital signal and is coded for that specific radio (i.e.) brand. God for gasoline powered aircraft. Can ignore a lot of other noise.
    PPM Is a standard FM signal. Much ceaner than AM.
    You can go to www.futaba-rc.com and go through there FAQ's and find out a lot more.

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    RE: What's The Difference between PCM and PPM?

    The data in a PCM signal is digital, ones and zeros, like a CD. The data in PPM is analogue, it is the length of time the signal is on. They are data types, nothing to do with radio, they can be sent by electrical signal, light flashing, smoke signals, sound signals and so on.

    As said in the post above, for our purposes both are sent on normal FM radio, there is no difference there. PPM is often called FM but that is false, both PPM and PCM are sent on exactly the same FM radio.

    All servos are PPM, so PCM Rx have to convert the incoming signal back into analogue PPM before they can send it to the servos.

    PCM's big advantage used to be that it alone could do error detection (though not error correction) and then go into failsafe. However the modern PPM Rx from a few firms such as Multiplex can also do error detection and have fully programmable failsafe.

    PCM can only be used with that brand's Tx and Rx, you can't mix brands, whereas with PPM you can mix brands including PPM failsafe.

    PCM Rx are more expensive than PPM or PPM failsafe Rx.

    PCM is only available on computer Tx, PPM failsafe is available to any Tx even a basic 4 ch non-computer Tx.

    At the extremes, if interference is switching rapidly on and off PCM can get a bit more of its signal through uncorrupted compared to PPM due to the way it tends to split the whole data set into smaller packets whereas PPM has to send the whole data set in one packet.

    FMA claims that its Digital Signature Recognition technology on PPM with failsafe is even better than PCM at getting the signal through interference but I have not been able to find anyone who has bought one and tried it for real.

    H.

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    RE: What's The Difference between PCM and PPM?

    The whole discussion of AM/FM PPM/PCM has been discussed here on RCU by many of us many times.

    Being a semi-retired communications tech, I can assure you that PCM is the little Radio Control hobby world's way of catching up with the big boys at Motorola, etc. When Futaba (who makes commercial communications systems as well as our RC radios) gives advantages for PCM, over traditional PPM it is worth listening to.

    You can go read Futaba's reasons, here are mine...

    Much of the commercial and Military applications went to digital years ago. These digital systems are often much more complicated than our PCM, but work on the same principles. The real world went to digital systems for several reasons: error correction, encryption, audio noise compensation, etc. Being computerized, digital conversion was nessesary early in the process.

    The primary reasons for PCM in our RC hobby were for error correction and failsafe.

    Using IC's, your transmitter (AM or FM or PCM or PPM) takes the various commands from your sticks and switches and pots and converts them into a signal. This signal cycles about every 20 milliseconds or so (100's of times a second). This cycling signal is called the "Pulse Train" and can be converted into PCM if your transmitter is capable. When the pulse train is PCM, each 20 millisecond cycle is referred to as a "data packet."

    Looking at the signal received by an RC receiver (with an occilliscope) that is being hit with spark ignition noise, it is easy to see just what the advatage of PCM is.

    The spark interference can completely mask over your transmitter's signal, but only for a several milliseconds at a time. The overall signal is very clean, but is broken up by these "spikes" caused by the noise. With PPM systems, these spikes are sent to the decoder portion of your receiver and to the servos, resulting glitches and difficulty controlling your aircraft. The high speed servo deflections can cause increased current drain (shorter battery life) and other problems.

    A PCM decoder will ignore many bad data packets as bad info and "hold last data packet" until good data packets are heard. This often happens several times a second.
    In this condition, you may not be aware of any noise at all, as controls seem unaffected.

    If a certain amount of time elapses (often selectable in the programming of the radio), the PCM receiver will instead go into a preprogrammed Failsafe setting. This allows you to kill throttle, possibly to level out elevator and ailerons, etc. and save the plane, or at least slow down its hard landing. Better than most PPM systems that would just result in unpredictable crashes.

    I should mention that many different types of noise sources can be taken care of by pcm, not just spark noise. I use it only as an example.

    Many people object to PCM for many reasons and I am sure that they will answer.
    They never built a plane I could not crash!!! Can I fly yours?

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    RE: What's The Difference between PCM and PPM?

    ORIGINAL: MO_Radio_Tech
    This signal cycles about every 20 milliseconds or so (100's of times a second).
    It is not hundreds of times a second, it is at best 50 times a second and for 8-10 channel PCM is usually less than 50 times a second. PCM is generally slower than PPM at refreshing all channels.

    The advantages of PCM which you outlined are not inherent in PCM code but are part of the processing done by the Rx. It was easier to do error detection on PCM because it is already computerised. Note that our radios do not do error correction like CDs, DVDs, computer networks and so on. Our radios can detect an error but do nothing to put it right. I have seen a Phillips demonstration of a CD player with the error correction circuit turned off - the sound was at least as bad as scratchy vinyl, the raw digital data is as corrupted as vinyl, the quality of digital comes not from the digital signal itself but from error correction because CDs can re-read the data many times, have vast amounts of extra data to help correct faults and so on. Our Rx have not got that luxury. Imagine a CD player that encountered an error and simply stopped, that is what our radios do. There are PPM Rx that now have error detection and fully programmable failsafe so the advantage of PCM as it is currently implemented in model radios is hard to see.

    Failsafe is not an inherent part of PCM code. PCM rx do not have to have error detection, they could be made without it and simply pass on the corrupted data to the servo like plain PPM does. Since both PCM and PPM can now have error detection and failsafe, what difference is there between the two systems in model aircraft?

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    RE: What's The Difference between PCM and PPM?

    I haven't read every word in this thread but I don't think anyone mentioned the most important feature of PCM over PPM. That is, the PCM receiver only needs to detect if a pulse is there or not. A pulse can be corrupted by noise and still be detected as a valid pulse. In a PPM system, noise injected in the rf link is passed on to the seros and the servos jitter. FM improved the rejection of rf noise over AM but it is still not as clean as PCM.
    Jim O

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    RE: What's The Difference between PCM and PPM?

    Actually you are right and wrong. A frame rate of 20 milliseconds is 50 times a second. PCM frame rate is just the same as its PPM counterpart. If you could see 50 times a second, it is faster than your nerve paths can carry the information from your eyes, process it in your brain and send a correction to your muscles in your fingers to move sticks. You are right that many manufacturers have come out with a faster rame rate for three channel ground radios (14-16 milliseconds) but these are not used in most planes, so the point is meaningless.

    The error detection is all in the receiver as you say, but I may disagree with your understanding of it. The PCM receiver ignores bad data packets and uses the values in the previous good data packet. If it gets bad data packets for a long enough amount of time it goes into failsafe.

    Since 8-10 channel PCM IS AS FAST as PPM, the real question is, why have PPM at all? I think it has no purpose. Why design PPM systems that have error detection and Failsafe like PCM? Just for the folks that have PPM only transmitter systems.

    Sounds like we could eventually give up PPM receivers all together.

    Prejudice agaist PCM is merely a mental roadblock.
    They never built a plane I could not crash!!! Can I fly yours?

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    RE: What's The Difference between PCM and PPM?

    I am not prejudiced against PCM, but pointing out that with PPM failsafe systems that in use there is little to distinguish between the two. Many of the advantages of digital communication such as error correction are simply not implemented in model radios. A user who was given a blind test would probably not be able to tell the difference between a PCM or a PPM failsafe system. Given that, would you buy the cheaper one that works with any Tx or the more expensive one that only works with one brand of Tx? Once you answer that, you end up asking the question why have PCM? PCM used to have the clear advantage over PPM for which it could charge a higher price and make you put up with its brand incompatibility. Now it doesn't have the advantage, all it has is high price and incompatibility. Sounds like we could eventually give up PCM altogether. In fact the first brand that used PCM on model radios already has given up PCM altogether.

    H

  9. #9
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    RE: What's The Difference between PCM and PPM?

    I would rather have the "incompatability" simply for the fact that your "X" brand radio would not trigger fail-safe on my "F" brand.
    The whole idea of PCM being able to detect errors has to do with being able to toss the bad packets that are not in the sync timing.
    PPM can't do that.
    JMHO
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    RE: What's The Difference between PCM and PPM?

    The only reason PPM is significantly cheaper than PCM is marketing. Someday this will surely change.

    Often times in the US, you can find good deals where a PCM system is as cheap as PPM. Especially when you get one second hand and don't pay full retail.
    They never built a plane I could not crash!!! Can I fly yours?

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    RE: What's The Difference between PCM and PPM?

    ORIGINAL: MO_Radio_Tech
    The only reason PPM is significantly cheaper than PCM is marketing. Someday this will surely change.
    There is indeed quite a bit of truth to that statement. The cost of additional components required to implement PCM in a transmitter and receiver is probably less than US$5 in manufacturing quantities.

    There is however, the additional cost of developing the software that must be amortized over the anticipated sales volume -- but even that's going to be no more than $30-$40 per unit.

    With PPM receivers selling for around $50 and PCM ones selling for at least twice that, I suspect that manufacturers are pricing their gear on a "what the market will bear" basis rather than a "cost-plus" one.

    However, I predict (and it's a pretty safe prediction) that PPM will eventually go the way of AM and represent a very small percentage of total radio sales.

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    RE: What's The Difference between PCM and PPM?

    Wover 98:

    "Your X brand radio would not trigger fail safe on my Y brand"

    PCM is not panacea to interference between X brand and Y brand! Let me explain.

    I did a practical test at the field recently. A club member has a top end Futaba computer radio with a Futaba PCM DC receiver. He was very proud of the systems alleged bullet proof capabilities.

    He is on frequency X.

    I have an Airtronics mid range computer radio with an Airtronics PCM receiver. Also on frequency X.

    On the ground, he turns his Tx on in PCM transmission, I turn my PCM Rx on. Complete Lock out on my PCM Rx. Vice-versa - and the same thing happens.

    This is not good. Where is the benefit? With the increasing use of PCM receivers, and the Tx's that can transmit in PCM, I now see little real safety enhancement in the use of this technology on the field.

    I think Multiplex have realised this and moved on in a clever marketing move with their IPD receivers, also FMA. In a practical sense it seems these new breed PPM/FM receivers can do everything for the flyer the much vaunted PCM receivers can. And often at lower cost. And, as Harry has noted, these receivers can be used with any PPM/FM Tx, so they won't be orphaned if you jump radio camps.

    fiery

    **I do not have any association with Multiplex or FMA - I am just a happy flyer and PCM sceptic**

    fiery

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    RE: What's The Difference between PCM and PPM?

    Go to the doctor, I think you might be falling into a coma...

    Two transmitters on the same frequency in close proximity (say at the same flying field) will always cause failure.

    Multiplex receivers can't deal with a 2nd transmitter on the same frequency
    in the same area either.

    Please dont take the coma comment as a insult, just a joke.

    Multiplex's PPM equipment is much cheaper than JR/Futaba because they are the underdog and must compete through low prices.

    Multiplex would like you to be "skeptical" of PCM, that way you are more likely to buy thier products. Notice that Hitec bought Multiplex. This marriage made since. Hitec has made its inroads into the bigboys game (effectively unseating Airtronics into the fringe) by producing and marketing radios and servos to people to use with other brands of equipment. This increased competition has been good for all of us in my opinion. It forces the other manufacturers to keep up the quality but decrease prices.

    It is not in Hitec/Multiplex's best interest to be tolerant of PCM. The propriatary nature of PCM effectively works against thier marking strategy of convincing comsumers to mix brands.

    Good luck to everyone no matter whoose goodies they buy...
    They never built a plane I could not crash!!! Can I fly yours?

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    RE: What's The Difference between PCM and PPM?

    Not so ...

    I put my Airtronics Tx into PPM/FM transmission, no problem for the Futaba PCM reciever of the other flyer on the same frequency. It doesn't hear it, even though we are on the same frequency.

    It's just that my PCM receiver will react to the other manufactuerer's PCM Tx signal, and vice-versa, by going into "lock out". Not unexpected of course. As I said, not good news ... so much for manufacturer specific proprietary encoding PCM transmission being of a practical benefit to the flyer.

    That is the point I am making.

    I'll go back to my coma now ...

    fiery

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    RE: What's The Difference between PCM and PPM?

    ORIGINAL: MO_Radio_Tech
    Two transmitters on the same frequency in close proximity (say at the same flying field) will always cause failure.
    Au contraire!

    Two transmitters on the same frequency in close proximity will very often cause problems, but if you're using one of the more modern PPM receivers with advanced features such as transmitter signal recognition (TSR) then you'd be surprised at just how much you might get away with.

    I tested a Berg DSP II receiver the other day against two transmitters (a Hitec Eclipse 7 and an Airtronics).

    Just out of curiosity, I registered the signature of the Airtronics with the Berg (by turning the Airtronics Tx on then the Berg) and it worked fine. Then, the Airtronics transmitter was turned off and the Hitec turned on -- nothing. The receiver simply would not respond to the Hitec -- which actually didn't surprise me because the Airtronics was is positive shift and the Hitec is negative.

    However, when the Airtronics was turned back on, it still had full control with no glitching or anything -- even though there were now *two* transmitters operating on the same channel and they were both equidistant from the receiver.

    In that case you could be pretty much guaranteed that a PCM receiver would have gone into failsafe and left you with no control from either transmitter.

    These new "smart" decoders aren't perfect -- but they're damned impressive!

    And another important benefit (to some) is that receivers like the Berg and FMA M5 are much smaller/lighter than *any* PCM receiver I've seen. This means that even the smallest/lightest models can gain the benefits.

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    RE: What's The Difference between PCM and PPM?

    I have one PCM RX and it is installed in my GP Christen Eagle II. This is the only plane I own running a gas-ignition engine, a Fuji BT-50SA. It was not my original intent to run PCM just because of the engine, though I understand that it probably does prevent ignition noise from interfering with my controls.

    I was convinced with the PCM RX after running a simple test. I turned on my Futaba 9C, set it to PCM encoding, and did a range check with the airplane. All was great -- range was awesome. I then took out my Hitec Eclipse 7 (older version without the PCM capability), selected the same channel on the Spectrun module, and turned it on. Side-by-side, both in close proximity of the RX and no interference from the Hitec TX.

    I did the reverse with another airplane that contained a PPM RX. I set the Hitec Eclipse to the proper channel and turned it own and range checked the airplane. Range with this combo was also acceptional. I then turned on the Futaba 9C, set to PCM encoding, and it immediately caused the airplane's controls to go haywire.

    Since the PCM is based on error-controlled receipt of data packets it can easily ignore errant signals. NOTE: One method used by many digital systems for error control is the use of check digits. A data slice is composed of a number of on-off conditions. A check digit is an extra on-off condition that is set by the TX either on or off depending on the ratio of on-off conditions in the rest of the data slice. If you have interference in the data slice there is a possibility the ratio of on-off conditions will no longer agree with the check digit -- the RX can then declare that data slice as bad and ignore its info. Expand this concept across numerous data-slices and you get a full error-control system. Each manufacturer utilizes a unique protocol (how they use the check digit(s)) and that is why their PCM RXs are not compatible with other brand TXs runnning PCM encoding.

    NOTE: This is the same concept used in modern-day highspeed digital communications, like the signal between two computers accross a network or telephone wires. It is also the same concept used in Digital Cell Phones.
    Jim Hilley
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    RE: What's The Difference between PCM and PPM?

    It would be possible to debate the merits of PPM versus PCM indefinitely but the truth is they each have their own benefits and disadvantages and people will always hold their own opinions as to which is "best".

    Thank God for choices, that's what I say :-)

    Just about the only thing I think we can all agree on is that plain old PPM gear with a "dumb" decoder (such as the many park-flier receivers and those sold by Hitec and others) are not the best choice in today's noisy RF environment -- particularly when there are cheaper alternatives that offer far better interference rejection.

    You may be flying dumb PPM and never have been hit. In such cases it's tempting to say "plain old PPM is fine for me" -- but tomorrow your plane might end up biting the dust due to a burst of interference or a wayward transmitter on the same channel as you and you'll quickly change your mind. PCM and smart PPM are like insurance -- you never need them until you do -- and if you don't have them then -- well it's just too late.

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    RE: What's The Difference between PCM and PPM?

    Two flyers on the same frquency at the same location is always a bad idea. Sooner or later due to capture ratio, one flyer will step on the other and at least one (if not both) plane will meet an early fate. There are systems with the transmitter ID recoginition and so on, but the power of one transmitter in too close of proximity to the other guys receiver will eventually make it fail.

    Even if you can get lucky and get away with this, in the US the AMA will not allow it.

    Good luck.
    They never built a plane I could not crash!!! Can I fly yours?

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    RE: What's The Difference between PCM and PPM?

    ORIGINAL: MO_Radio_Tech

    Two flyers on the same frquency at the same location is always a bad idea. Sooner or later due to capture ratio, one flyer will step on the other and at least one (if not both) plane will meet an early fate. There are systems with the transmitter ID recoginition and so on, but the power of one transmitter in too close of proximity to the other guys receiver will eventually make it fail.

    Even if you can get lucky and get away with this, in the US the AMA will not allow it.

    Good luck.
    Not even remotely my intention when running with PCM. I look for it to help me when the unthinking @#$%$^% turns their TX on while I'm flying. Channel control is still the rule on anly field I will ever fly at.
    Jim Hilley
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    RE: What's The Difference between PCM and PPM?

    Jim

    Maybe I'm missing something in this discussion, but if "old percy fruitcake" (every club of any size has one ) turns up with his "brand X" Tx on your frequency, and turns it on in PCM transmission without checking, there is every chance that your bird flying on the same frequency with your "brand Y" radio with it's "you beaut" PCM receiver will experience sky/earth zen.

    Almost all 6 ch plus Tx's these days, apart from entry level efforts, have the PCM transmission option. So, in a practical sense, how does a PCM receiver offer you any advantage? PCM won't save you here.

    That is the point I have tried to make. I don't mean to be contrary, but as far as I can see the "PCM advantage" these days is little more than marketing hype. As increasing numbers of flyers have a PCM capable Tx, it's real benefit to the R/C flyer increasingly becomes, as far as I can see in a practical sense "on the field", illusory.

    PS In Australia, as is the case in most of the world, we don't suffer this ridiculous "shift" issue that many of our contributors in North America do. Here, Futaba and Hitec in PPM/FM Tx's speak quite happiy to JR,Sanwa(Airtronics) and Multiplex Rx's - all are in "downshift". Buy any PPM/FM receiver, and it will hear your Tx.

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    RE: What's The Difference between PCM and PPM?

    My point is that right now I buy a failsafe Rx of true 9 channels (not 8+1 like Futaba) for £60 which I can still use if sometime in the future I buy a Tx of a totally different brand. If I buy a PCM Rx it will cost me £100+ and can not be used if I change my brand of Tx in the future.

    Outside North America all brands of radio work together, the shift incompatibility doesn't exist for us, so we can be quite mercenary about buying the Tx that suits us best and not have to worry about the added cost of changing all the rx as well, unless we have bought PCM rx in which case we do tend to get locked into that brand. Using PPM failsafe gives us back the total freedom to upgrade our Tx to whatever we want and not feel locked into JR or Futaba or Hitec or Multiplex tx because of the cost of replacing all the rx.

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    RE: What's The Difference between PCM and PPM?

    ORIGINAL: fiery

    Jim

    Maybe I'm missing something in this discussion, but if "old percy fruitcake" (every club of any size has one ) turns up with his "brand X" Tx on your frequency, and turns it on in PCM transmission without checking, there is every chance that your bird flying on the same frequency with your "brand Y" radio with it's "you beaut" PCM receiver will experience sky/earth zen.

    Almost all 6 ch plus Tx's these days, apart from entry level efforts, have the PCM transmission option. So, in a practical sense, how does a PCM receiver offer you any advantage? PCM won't save you here.

    That is the point I have tried to make. I don't mean to be contrary, but as far as I can see the "PCM advantage" these days is little more than marketing hype. As increasing numbers of flyers have a PCM capable Tx, it's real benefit to the R/C flyer increasingly becomes, as far as I can see in a practical sense "on the field", illusory.

    PS In Australia, as is the case in most of the world, we don't suffer this ridiculous "shift" issue that many of our contributors in North America do. Here, Futaba and Hitec in PPM/FM Tx's speak quite happiy to JR,Sanwa(Airtronics) and Multiplex Rx's - all are in "downshift". Buy any PPM/FM receiver, and it will hear your Tx.

    You are correct and I essentially rely of the fact that most "old percy fruitcake"s aren't running PCM. If most TXs are like the Futaba 9C it takes at least 2 conscious steps to switch from PPM to PCM. With the 9C you have to assign PCM to the model then cycle power to switch from one to the other.
    Jim Hilley
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  23. #23
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    RE: What's The Difference between PCM and PPM?

    I am "Old Percy Fruitcake", and I thought you might like to know my radios is a Kraft AM circa 1974. Someday I'm gonna get that narrow band work done.
    Flying Geezer

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    RE: What's The Difference between PCM and PPM?

    ORIGINAL: Flying Geezer

    I am "Old Percy Fruitcake", and I thought you might like to know my radios is a Kraft AM circa 1974. Someday I'm gonna get that narrow band work done.
    Are you saying that you routinely ignore channel control at your flying field and turned on your WIDEBAND TX and destroy other people's airplanes?

    NOTE: I could be wrong but I do believe those older WIDEBAND AM and FM TXs are currently illegal to operate in the United States. In anycase, once in a great while one of our club members comes out with a WIDEBAND setup. I believe it is common to assign the channel and both adjacent channels to that person, at least it is at our club.
    Jim Hilley
    AMA #671825
    RCCA# 758
    Twist 150 Brotherhood #14
    hilleyja@cebridge.net
    http://www.cma1193.com

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    RE: What's The Difference between PCM and PPM?

    Hey Jim, that was just a joke. I fly an Futaba 9C and watch out for the "Percy's". I know what you mean about these guys, you never know what channel(s) they are gonna' be on, or when, or what kind of wave they'll be transmitting.
    Flying Geezer


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