Difference btwn PCM and FM (PPM) ???
07-16-2003, 04:28 AM
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Difference btwn PCM and FM (PPM) ???
Do a quick search and you will find enough reading material on this subject to keep you busy for a month.......
Here's a nice little page that will help with a lot of questions...
I'm sure someone will chime in and correct the lock out statement though!!!!!!!!
http://www.jmack.net/html/rcmodels/pulsecode/
Here's a nice little page that will help with a lot of questions...
I'm sure someone will chime in and correct the lock out statement though!!!!!!!!
http://www.jmack.net/html/rcmodels/pulsecode/
07-16-2003, 04:57 AM
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Difference btwn PCM and FM (PPM) ???
ah...
i kinda understand..
basically both are FM , just that one is analog and the other is digital
thanx for the link !
i kinda understand..
basically both are FM , just that one is analog and the other is digital
thanx for the link !
07-16-2003, 09:42 AM
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Difference btwn PCM and FM (PPM) ???
The functional difference is that PPM will execute any signal received while PCM will validate that the signal was from a transmitter before it is executed.
07-16-2003, 11:11 AM
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Difference btwn PCM and FM (PPM) ???
Originally posted by Geistware
The functional difference is that PPM will execute any signal received while PCM will validate that the signal was from your transmitter before it is executed.
The functional difference is that PPM will execute any signal received while PCM will validate that the signal was from your transmitter before it is executed.
It only checks that it's from a transmitter like yours i.e. any Futaba PCM Tx at all on the correct channel will control a Futaba PCM Rx, two on at once will interfere with one another etc.
I'm sure that's what you intended to say
.Steve
07-16-2003, 02:49 PM
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Difference btwn PCM and FM (PPM) ???
Even the signal validation Geistware was reffering to is now available on selected PPM receivers (the same ones which provide programmable failsafe). They check the incoming signal and compare it to previous signals to verify if it is valid. If not, it is rejected and if enough are rejected it goes into "failsafe" mode.
07-16-2003, 03:05 PM
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Difference btwn PCM and FM (PPM) ???
Originally posted by strato911
Even the signal validation Geistware was reffering to is now available on selected PPM receivers (the same ones which provide programmable failsafe). They check the incoming signal and compare it to previous signals to verify if it is valid.
Even the signal validation Geistware was reffering to is now available on selected PPM receivers (the same ones which provide programmable failsafe). They check the incoming signal and compare it to previous signals to verify if it is valid.
PCM compares the latest data to the latest checksum that came with it. PPM failsafe simply discard data that is too long or too short in time length to have been generated by the Tx. Any error within the acceptable time length is treated as valid whereas PCM will spot any error within the acceptable 1024 positions.
Harry
07-16-2003, 08:34 PM
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Difference btwn PCM and FM (PPM) ???
No Harry. Strato is right. FMA sells one, it actually uses a little algorithm to generate a signature of the incoming signal, if it changes too dramatically it rejects the signal which would help protect against another transmitter on the same frequency. It would go into hold at that point. And there is a useful comparison in an RC signal when you consider all the channels together NEVER change all at the same time unless there's interference, just add a little math to that and you can assign a rough signature value to an incoming signal and reject it if it's too far off what was expected.
07-16-2003, 10:53 PM
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Difference btwn PCM and FM (PPM) ???
Originally posted by Lynx
And there is a useful comparison in an RC signal when you consider all the channels together NEVER change all at the same time unless there's interference, just add a little math to that and you can assign a rough signature value to an incoming signal and reject it if it's too far off what was expected.
And there is a useful comparison in an RC signal when you consider all the channels together NEVER change all at the same time unless there's interference, just add a little math to that and you can assign a rough signature value to an incoming signal and reject it if it's too far off what was expected.
Harry
07-17-2003, 12:00 AM
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Difference btwn PCM and FM (PPM) ???
Harry - My statement was a bit over-simplified, so that even people without the technical background would understand the basics.
Quoted from MultiplexUSA
The receiver analyses the signal, and adjusts it automatically in accordance with the current reception quality, or field strength. Powerful signals are passed on to the servos directly, but weaker signals are “post-processed”. This means that the IPD receiver calculates the nominal servo position from the last “good” signals which it picks up.
The receiver analyses the signal, and adjusts it automatically in accordance with the current reception quality, or field strength. Powerful signals are passed on to the servos directly, but weaker signals are “post-processed”. This means that the IPD receiver calculates the nominal servo position from the last “good” signals which it picks up.
Quoted from a technical "White Paper" from FMA
The Pulse Position Modulation (PPM) waveform output by a typical five-channel radio control transmitter is shown below.
As noted in the diagram, the waveform has several parameters that form a signature or "fingerprint" unique to each transmitter:
These parameters vary from one transmitter model to the next, but they are fixed for a given transmitter. Digital Signature Recognition takes advantage of those facts to assure that every frame sent by the transmitter is faithfully reproduced at the Flight System Receiver's servo outputs. Here's how DSR works:
The Pulse Position Modulation (PPM) waveform output by a typical five-channel radio control transmitter is shown below.
As noted in the diagram, the waveform has several parameters that form a signature or "fingerprint" unique to each transmitter:
- Each frame is about 20 milliseconds long.
- The synchronization pause is 4 to 6 milliseconds long.
- There are always N+1 modulation pulses, where N is the number of control channels available in the transmitter.
- Each modulation pulse is 250 to 350 miliseconds long.
- Pulses are btween 0.8 milliseconds and 2.2 milliseconds apart. (This spacing varies according to transmitter stick and control positions, and is the source of the term "pulse position modulation.")
These parameters vary from one transmitter model to the next, but they are fixed for a given transmitter. Digital Signature Recognition takes advantage of those facts to assure that every frame sent by the transmitter is faithfully reproduced at the Flight System Receiver's servo outputs. Here's how DSR works:
- DSR measures and stores your transmitter's unique signature.
- DSR then continuously checks the received signal against the stored signature.
- Then:
- DSR rejects signals from sources other than your transmitter.
- DSR reconstructs damaged frames using several proprietary error correction techniques.
07-17-2003, 12:03 AM
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Difference btwn PCM and FM (PPM) ???
It sounds to me like both FMA & Multiplex use prior signals as part of their algorithms to identify bad signals...
Where else whould it get a "signature" from???
Where else whould it get a "signature" from???
07-17-2003, 12:17 AM
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Difference btwn PCM and FM (PPM) ???
Basically, PCM takes your radio's FM signal and 'scrambles' it. Then the PCM receiver descrambles it and utilizes it. Random noise from other items are not going to be mistakenly read as proper servo instructions as can happen with an FM.
When I said your transmitter, I "MENT" the packet is valid. I was wrong in what I typed.
When I said your transmitter, I "MENT" the packet is valid. I was wrong in what I typed.
07-17-2003, 07:21 AM
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Difference btwn PCM and FM (PPM) ???
Originally posted by Geistware
Basically, PCM takes your radio's FM signal and 'scrambles' it. Then the PCM receiver descrambles it and utilizes it. Random noise from other items are not going to be mistakenly read as proper servo instructions as can happen with an FM.
Basically, PCM takes your radio's FM signal and 'scrambles' it. Then the PCM receiver descrambles it and utilizes it. Random noise from other items are not going to be mistakenly read as proper servo instructions as can happen with an FM.
The Multiplex IPD Rx does not use past data to predict the expected position of the latest data in order to determine if it is correct. It's impossible, retracts go from +100% to -100% in one cycle. The IPD determines if the radio signal strength is becoming so weak that the data will be fuzzy rather than precise, and subject to a high probability of random spikes. It then uses the latest value along with some past values to create an artificial value, I suspect it is simply using a moving average of the last handful of values, that way it will smooth out any very short time large spikes in the data. Moving averages is a technique for smoothing out high frequency cycles within low frequency cycles, in this case it will prevent a large one-off spike from making the servo jump, and it only does it when the radio signal is weak. It is not a means of predicting the next value in order to test it against the actual received value. What it does is create a delay in any genuine movement, so if the IPD was doing its post processing, and you switch flaps from fully up to fully down, they would travel slowly as the value of the moving average shifts with time, instead of going from full up to full down at normal speed. So it does not predict the correct value at all, what it does is delay the correct value by dragging the past values into it.
Harry
07-17-2003, 07:42 AM
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Difference btwn PCM and FM (PPM) ???
Originally posted by strato911
It sounds to me like both FMA & Multiplex use prior signals as part of their algorithms to identify bad signals...
Where else whould it get a "signature" from???
It sounds to me like both FMA & Multiplex use prior signals as part of their algorithms to identify bad signals...
Where else whould it get a "signature" from???
The FMA Rx does, but it is not the data values of the servo positions that it uses. What the FMA does is akin to recognising someone's voice in a crowd, but it can't in any way predict what they are going to say. What FMA looks at is the number of channels, synchro pulse length, length of gap between data values, and so on. If one of these suddenly varies, such as the synchro pulse coming after 4 instead of 6 channels, it knows it must be a fault. What it does not do is look at the last servo data values to determine the next correct range of values in order to test for interference. When it has determined from the signature that there is corruption, it uses previous values to construct a new value, but there is still guesswork and delay in there, and the test for corruption came not from the servo data but from all the data that is not servo data! Your correct value for retracts may have been UP for the last 5 minutes but that tells you absolutely zero about the next value. It has a 50/50 chance of being DOWN. If the FMA Rx is doing its post processing because it has seen interference, it will predict the next value is still UP, but at that very cycle you may have switched them DOWN, and the Rx wil continue to hold the wheels UP until it receives a good DOWN signal even though you may have been sending DOWN for some time.
Two things prevent FMA from predicting future servo data values in order to test the actual received data. Firstly the data that we create is non-linear. If FMA has solved the problems of predicting non-linear mathematics, the world's mathematicians would be ecstatic. Secondly if FMA can predict the next point in any non-algorithmic, non-linear curve, they would not be making radios. They would make more money in one week on the the stockmarket than Bill Gates will make in a lifetime! Read the literature carefully and you will see that FMA are not claiming that they can use past servo data to check future servo data. Everything in their DSR works on all the bits of the signal that are not servo data.
FMA does not sell to the UK. Shame, that Rx looks very interesting, I would buy one. I am delighted to see smaller firms like Multiplex and FMA coming up with innovative products that improve our radios. IS FMA stuff of good quality and as reliable as Mpx/Futaba/JR?
Harry
07-17-2003, 08:35 AM
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Difference btwn PCM and FM (PPM) ???
Harry, calm down. If you don't agree with what I posted, contact Futaba. This is their definition! 
Originally posted by HarryC
No it doesn't! PCM and PPM are methods of data encoding. They actually have nothing to do with radio. They are a way of converting the position of the stick into an electrical signal. They could be transmitted to the model along a wire. Since we use radio rather than wire, we need to put that PPM or PCM data onto a radio signal, the 2 methods available are AM or FM. We mostly use FM, but it carries the PPM or PCM data. PCM does not, indeed can not, scramble the FM signal. Anything that scrambles the FM signal is called interference.
The Multiplex IPD Rx does not use past data to predict the expected position of the latest data in order to determine if it is correct. It's impossible, retracts go from +100% to -100% in one cycle. The IPD determines if the radio signal strength is becoming so weak that the data will be fuzzy rather than precise, and subject to a high probability of random spikes. It then uses the latest value along with some past values to create an artificial value, I suspect it is simply using a moving average of the last handful of values, that way it will smooth out any very short time large spikes in the data. Moving averages is a technique for smoothing out high frequency cycles within low frequency cycles, in this case it will prevent a large one-off spike from making the servo jump, and it only does it when the radio signal is weak. It is not a means of predicting the next value in order to test it against the actual received value. What it does is create a delay in any genuine movement, so if the IPD was doing its post processing, and you switch flaps from fully up to fully down, they would travel slowly as the value of the moving average shifts with time, instead of going from full up to full down at normal speed. So it does not predict the correct value at all, what it does is delay the correct value by dragging the past values into it.
Harry
No it doesn't! PCM and PPM are methods of data encoding. They actually have nothing to do with radio. They are a way of converting the position of the stick into an electrical signal. They could be transmitted to the model along a wire. Since we use radio rather than wire, we need to put that PPM or PCM data onto a radio signal, the 2 methods available are AM or FM. We mostly use FM, but it carries the PPM or PCM data. PCM does not, indeed can not, scramble the FM signal. Anything that scrambles the FM signal is called interference.
The Multiplex IPD Rx does not use past data to predict the expected position of the latest data in order to determine if it is correct. It's impossible, retracts go from +100% to -100% in one cycle. The IPD determines if the radio signal strength is becoming so weak that the data will be fuzzy rather than precise, and subject to a high probability of random spikes. It then uses the latest value along with some past values to create an artificial value, I suspect it is simply using a moving average of the last handful of values, that way it will smooth out any very short time large spikes in the data. Moving averages is a technique for smoothing out high frequency cycles within low frequency cycles, in this case it will prevent a large one-off spike from making the servo jump, and it only does it when the radio signal is weak. It is not a means of predicting the next value in order to test it against the actual received value. What it does is create a delay in any genuine movement, so if the IPD was doing its post processing, and you switch flaps from fully up to fully down, they would travel slowly as the value of the moving average shifts with time, instead of going from full up to full down at normal speed. So it does not predict the correct value at all, what it does is delay the correct value by dragging the past values into it.
Harry
07-17-2003, 08:56 AM
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Difference btwn PCM and FM (PPM) ???
Originally posted by Geistware
Harry, calm down. If you don't agree with what I posted, contact Futaba. This is their definition!
Harry, calm down. If you don't agree with what I posted, contact Futaba. This is their definition!
Harry
07-17-2003, 10:03 PM
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Difference btwn PCM and FM (PPM) ???
Harry - I never said they try to PREDICT the next set of servo positions, I only said it uses previous SIGNALS. The entire data train is a signal...
Lets agree to disagree on the fine details, since the EXACT method used is not made available to the public. As I said earlier - my original post on the matter was meant to provide a ROUGH interpretation so that most people would understand the basics behind how they work.
Lets agree to disagree on the fine details, since the EXACT method used is not made available to the public. As I said earlier - my original post on the matter was meant to provide a ROUGH interpretation so that most people would understand the basics behind how they work.
07-17-2003, 10:09 PM
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Difference btwn PCM and FM (PPM) ???
Originally posted by HarryC
IS FMA stuff of good quality and as reliable as Mpx/Futaba/JR?
IS FMA stuff of good quality and as reliable as Mpx/Futaba/JR?
Check out the Company Info.
07-18-2003, 11:57 PM
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Difference btwn PCM and FM (PPM) ???
If FM radios are not PPM, then what are FM radio systems?
Originally posted by HarryC
I have just had a look at Futaba USA’s site and seen that. It also claims that FM is another name for PPM! No wonder people misunderstand radios if the distributors print things like that!
Harry
I have just had a look at Futaba USA’s site and seen that. It also claims that FM is another name for PPM! No wonder people misunderstand radios if the distributors print things like that!
Harry
07-19-2003, 01:13 AM
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Difference btwn PCM and FM (PPM) ???
Originally posted by Geistware
If FM radios are not PPM, then what are FM radio systems?
If FM radios are not PPM, then what are FM radio systems?
Frequency modulation (FM) is a method of impressing data onto an alternating-current wave by varying the instantaneous frequency of the wave. In frequency modulation the amplitude is kept constant and the frequency is modulated by the amplitude of the modulating signal. This scheme can be used with analog or digital data. There are many types of modulation, but all are variations of AM or FM.
PPM is one form of ANALOG data modulation.
PCM is one form of DIGITAL data modulation.
07-19-2003, 02:15 AM
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Difference btwn PCM and FM (PPM) ???
The following info is over-simplified for a beginner. Some of it is not "technically accurate" in a scientific sense.
Don't bash me over the head with your communications textbooks.
I am just trying to make it easier to understand.
Your Transmitter takes commands (signals) from the gimbals (sticks), pots, and switches (modified by programming in you computer, if equipped) and processes them into a collective signal, a Pulse Train. This process is called Encoding. The board, IC, or computer chip(s) that perform this function are referred to as the Encoder. The pulse train is made up of square waves (pulses) that are individually specific to the channels (i.e. elevator, aileron) of your transmitter. This is constantly cycling-updating, every 18-22 milliseconds. This Pulse Train is fed to the RF (Radio Frequency) deck, or RF module to be broadcast to the outside world (your receiver). The broadcast is commonly referred to as a Transmission Carrier, or Carrier.
Leaving this Pulse Train the way it is, you have PPM, or Pulse Position Modulation. The position of each pulse corresponds to the desired position of the surface in the model.
This pulse train can also be changed in the Encoder (in radios so equipped) into a digital format known as PCM. Pulse Code Modulation.
Both PCM and PPM are types of FM.
Confusing because most RC Hobby manufacturers refer to PPM as FM.
FM, Frequency Modulation, refers to the signal being modulated side to side, specifically 2 kHz each side of the center freq in our hobby, as prescribed by the FCC.
You can think of an FM carrier "buzzing" side to side, up and down in frequency.
PPM and PCM are just modulation schemes put on the FM carrier. This is why an FM transmitter module can be used in a PPM or PCM radio. The FM module does not care what modulation is put on its signal pin.
AM, Amplitude Modulation, refers to the signal being modulated on freq. The transmission carrier is constantly amplified and de-amplified as the the signal dictates. You could think of it as some one turning up and turning the down the power of the transmitter constantly as the Pulse Train dictates. AM uses the same Pulse Train from the Encoder. It just uses it a different process to broadcast the same Pulse train.
Most transmitters have separate Encoder and RF boards (or Modules). A tech can easily swap an AM RF board for an FM RF board. The modeler can swap an AM transmitter Module for an FM one.
Thus, some older AM systems can be converted to work with FM.
Don't bash me over the head with your communications textbooks.
I am just trying to make it easier to understand.
Your Transmitter takes commands (signals) from the gimbals (sticks), pots, and switches (modified by programming in you computer, if equipped) and processes them into a collective signal, a Pulse Train. This process is called Encoding. The board, IC, or computer chip(s) that perform this function are referred to as the Encoder. The pulse train is made up of square waves (pulses) that are individually specific to the channels (i.e. elevator, aileron) of your transmitter. This is constantly cycling-updating, every 18-22 milliseconds. This Pulse Train is fed to the RF (Radio Frequency) deck, or RF module to be broadcast to the outside world (your receiver). The broadcast is commonly referred to as a Transmission Carrier, or Carrier.
Leaving this Pulse Train the way it is, you have PPM, or Pulse Position Modulation. The position of each pulse corresponds to the desired position of the surface in the model.
This pulse train can also be changed in the Encoder (in radios so equipped) into a digital format known as PCM. Pulse Code Modulation.
Both PCM and PPM are types of FM.
Confusing because most RC Hobby manufacturers refer to PPM as FM.
FM, Frequency Modulation, refers to the signal being modulated side to side, specifically 2 kHz each side of the center freq in our hobby, as prescribed by the FCC.
You can think of an FM carrier "buzzing" side to side, up and down in frequency.
PPM and PCM are just modulation schemes put on the FM carrier. This is why an FM transmitter module can be used in a PPM or PCM radio. The FM module does not care what modulation is put on its signal pin.
AM, Amplitude Modulation, refers to the signal being modulated on freq. The transmission carrier is constantly amplified and de-amplified as the the signal dictates. You could think of it as some one turning up and turning the down the power of the transmitter constantly as the Pulse Train dictates. AM uses the same Pulse Train from the Encoder. It just uses it a different process to broadcast the same Pulse train.
Most transmitters have separate Encoder and RF boards (or Modules). A tech can easily swap an AM RF board for an FM RF board. The modeler can swap an AM transmitter Module for an FM one.
Thus, some older AM systems can be converted to work with FM.
07-19-2003, 11:08 AM
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Difference btwn PCM and FM (PPM) ???
I do not agree, while FM is the process of modulating the frequency of the carrier with a data stream, PPM and PCM use the same principle. The only difference is the format the DATA is in. True FM will have the analog position signal modulating the carrier. In PPM the pulse width (around the 1.5ms nonimal) is modulating the carrier signal.
Originally posted by mglavin
FM = Frequency Modulation
Frequency modulation (FM) is a method of impressing data onto an alternating-current wave by varying the instantaneous frequency of the wave. In frequency modulation the amplitude is kept constant and the frequency is modulated by the amplitude of the modulating signal. This scheme can be used with analog or digital data. There are many types of modulation, but all are variations of AM or FM.
PPM is one form of ANALOG data modulation.
PCM is one form of DIGITAL data modulation.
FM = Frequency Modulation
Frequency modulation (FM) is a method of impressing data onto an alternating-current wave by varying the instantaneous frequency of the wave. In frequency modulation the amplitude is kept constant and the frequency is modulated by the amplitude of the modulating signal. This scheme can be used with analog or digital data. There are many types of modulation, but all are variations of AM or FM.
PPM is one form of ANALOG data modulation.
PCM is one form of DIGITAL data modulation.