FM or PCM?
02-24-2003, 05:42 PM
#26
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FM or PCM?
Boneman,
The 9CAF is packaged with an FM receiver.
The 9CAP is packaged with a PCM receiver.
The transmitter itself is capable of transmitting in either PPM or PCM, no matter which version you buy.
The 9CAF and 9CAP are the same radios, in fact when you buy the 9CAF, the transmitter has 9CAP stamped on it.
The 9CAF is packaged with an FM receiver.
The 9CAP is packaged with a PCM receiver.
The transmitter itself is capable of transmitting in either PPM or PCM, no matter which version you buy.
The 9CAF and 9CAP are the same radios, in fact when you buy the 9CAF, the transmitter has 9CAP stamped on it.
02-24-2003, 07:29 PM
#28
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FM or PCM?
Originally posted by Wooki3
I'm in cache valley. I fly at the logan airport.
I'm in cache valley. I fly at the logan airport.
I'm jealous
!Beautiful area. I was on a project near SLC that i got to visit for about 6 weeks at a time. Would love to find a job out there!
02-25-2003, 07:10 AM
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FM or PCM?
Hey David,
Thanks for the invite to go flying. I would love to go some time.
I'm doing repairs, and waiting till it warms up a bit. Only have one plane and it's grounded for a couple more weeks.
Have you noticed what brand radio most are using out there?
Want to get one that is compatable so if help is needed I can buddy box.
Thanks, Wooki3
Thanks for the invite to go flying. I would love to go some time.
I'm doing repairs, and waiting till it warms up a bit. Only have one plane and it's grounded for a couple more weeks.
Have you noticed what brand radio most are using out there?
Want to get one that is compatable so if help is needed I can buddy box.
Thanks, Wooki3
02-25-2003, 03:08 PM
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FM or PCM?
There are a lot of JR Radios out there and a few Futaba
Keep me in mind after you get your repairs done, if you need any help I would be happy to help you out i'm on JR
Take care and happy flying
David
Keep me in mind after you get your repairs done, if you need any help I would be happy to help you out i'm on JR
Take care and happy flying
David
02-26-2003, 02:39 AM
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FM or PCM?
Another good reason to get the PCM transmitter is because of the quality of the materials used. I have a 4 channel Futaba Skysport, and a 9CAP. The smoothness of the sticks and quality of the construction is by far greater, I can also fly a bit better because of this. Of course, there are some exceptions to this rule. I noticed that the JR Quattro has pretty good feeling sticks for it's price range, and undoubtedly the Multiplex transmitters are outstanding "feeling" while all they transmit is FM. The 6XAS is at the lower end, and also has very smooth control. It seems to be a manufacturer's trend that has been followed for some time.
02-26-2003, 03:05 AM
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FM or PCM?
Quote by DavidO
Sorry, but I could not resist:
If a square wave is made up of Sine Waves, then a square wave is not a sine wave.
Whoops !! But a square wave is made up of Sine Waves, therefore they are one of the same.
If a square wave is made up of Sine Waves, then a square wave is not a sine wave.
02-26-2003, 06:09 AM
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FM or PCM?
I like PCM becuse it makes range checking easier. I say this as one who checks everyone else's radio as well as his own.
It can be tricky to see FM glitching from 200 or more feet away.
I can see the surfaces move when the radio goes into failsafe. I range check all the time. If I see a big change in range from one time to the next I know there is a problem. Either local interference or model interference or radio problems, whatever.
I range check right before I fly. 2 hours later RF conditions could be different.
Getting back to the subject, I realize that many folks have grown cynical of the big radio brands pushing PCM (or for that matter, Dual Conersion). "They just want to make more money."
I just want to say that, on the test bench PCM receivers tend to receive better, and this has nothing to do with failsafe.
I don't know why this is. I have yet to get a good answer from any company rep or from anyone on RCU.
I just know what I see. PCM receivers perform better on the test bench.
PCM is not hiding anything on the bench. I hope I did not offend anyone by saying that.
There is an excellent article somewhere on Horizon's website on range testing by Eric Meyers. Most of what he says is true for all brands, not just JR. Good helpful article.
It can be tricky to see FM glitching from 200 or more feet away.
I can see the surfaces move when the radio goes into failsafe. I range check all the time. If I see a big change in range from one time to the next I know there is a problem. Either local interference or model interference or radio problems, whatever.
I range check right before I fly. 2 hours later RF conditions could be different.
Getting back to the subject, I realize that many folks have grown cynical of the big radio brands pushing PCM (or for that matter, Dual Conersion). "They just want to make more money."
I just want to say that, on the test bench PCM receivers tend to receive better, and this has nothing to do with failsafe.
I don't know why this is. I have yet to get a good answer from any company rep or from anyone on RCU.
I just know what I see. PCM receivers perform better on the test bench.
PCM is not hiding anything on the bench. I hope I did not offend anyone by saying that.
There is an excellent article somewhere on Horizon's website on range testing by Eric Meyers. Most of what he says is true for all brands, not just JR. Good helpful article.
02-26-2003, 08:55 PM
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image
Originally posted by Lynx
It's called harmonics not images. It's unavoidable in the RF world, you can help reduce it, but it can NEVER be eliminated (The only way to have a 0 harmonic wave form is to have a perfect sine wave) A perfect sine wave carries no data. I wish more companies did have good design practices. Just think of the cost of a modern day cell phone. Think of how many channels are crowded on their bandwidth and how well (considering traffic) they perform. R/C equipment manufactures are out to make a buck, not break new ground in functionality. Or we'd all have high speed bidirectional data links to whatever you fly. They keep just enough innovation in the market to keep people buying new products. (With a few exceptions) Then again the markets tough too, so it's hard to blame just corporate America.
It's called harmonics not images. It's unavoidable in the RF world, you can help reduce it, but it can NEVER be eliminated (The only way to have a 0 harmonic wave form is to have a perfect sine wave) A perfect sine wave carries no data. I wish more companies did have good design practices. Just think of the cost of a modern day cell phone. Think of how many channels are crowded on their bandwidth and how well (considering traffic) they perform. R/C equipment manufactures are out to make a buck, not break new ground in functionality. Or we'd all have high speed bidirectional data links to whatever you fly. They keep just enough innovation in the market to keep people buying new products. (With a few exceptions) Then again the markets tough too, so it's hard to blame just corporate America.
02-27-2003, 02:30 AM
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FM or PCM?
PCM versus PPM:
PCM basics:
The Positions of joy-sticks, switches and pots, originally analogue voltages are digitised by an A/D converter to a 8 to 10 bits (256 to 1024 binary) word.
For eight to ten servos means 80 -100 bits. Add to this the 16-32 bit checksum per frame, synchronisation sequences and failsafe values, and a bit number of 100 -160 becomes necessary for a complete frame.
A bit length of .3 ms (JR/Graupner and Futaba/Robbe) will produce a 30-48 ms frame time, considerably longer than the 22.5 ms PPM uses.
If even more secure bit lengths and 12 channels are used, this time is increased to 55 ms, e.g. Simprop (System 90), where only 6 channels are proportional and 6 are switch channels.
PCM - transmission:
Actual PCM uses two systems to synchronise the transfer: an extra long starting pulse made up of so many "1" or "0" bits, that it can never be mistaken for data, or the so called Half bit pulse, e.g. 2,5 bits, equally impossibly mistaken for data. Usually this is followed by a synchronisation sequence, setting the receive-clock. This is the clock that scans the middle of the bits upon reception. This explains why, at the limits of the transmission range with PPM, the servos start to glitch, because the pulse flanks vary (up to+/-30 us), while PCM keeps them quiet, having half a bit (150 us) to play with, thus avoiding faulty interpretation of a bit.
Next in line are the servo position data, usually in a few blocks, with variable contents. Then we have organisational data, like channel numbers or failsafe modus or failsafe values. Finally the checksum in the shape of a 16 bit long CRC (Cyclic Redundancy Check). It is the rest of the division of the whole message by a 16th order polynom.
Bit errors can be detected this way, but in no way corrected.
This in turn means that, even if only one single bit error has crept in the ca. 100 -160 bits total frame length, the checksum fails and the whole message is rejected.
The servo's remain in their last correctly received position until the arrival of new, correct data. If this takes too long (0.25-1 Sec), failsafe is activated in between: depending on the predefined settings a chosen (and defined in the transmitter) failsafe position or the last correctly received position.
It is a large disadvantage that a transmission failure of one % causes a break in the complete link, but this could only be solved with more complicated and redundant encoding. An audio-CD is a good example: even a 17 bit wide scratch can go unnoticed, but 30 % more data (bits) is needed to make this possible.
In our RC equipment the redundancy is provided by the high frame rate.
The servos cannot process more than 10-15 different positions per second. They receive about 45-60 pulses per second however. It is therefore not critical if a servo position has to be repeated because of a glitch.
To reduce the failure time anyway, JR/Graupner (S-PCM) and Futaba/Robbe (PCM1024) subdivided the frame using separate CRC checks. This allows rejecting only a part of the faulty frame.
PCM advantages:
Accurate and undisturbed servo movement, even if the model is far way.
Holding of the servo position during short glitches (Hold).
Moving the servo's to a predefined position in case of a longer disturbance or even complete failure of the transmitter (Fail-Safe).
Fast transmission if SPCM20 or PCM 1024 is used, similar to PPM.
Servos are not damaged by pulses that are too long/short, which could happen with PPM.
PCM disadvantages:
More expensive.
Sensitivity to adjacent channels is usually worse comparing with PPM receivers. Care has to be taken when flying near to a transmitter from an adjacent channel.
Due to different protocols, only receivers from the same brand or even type of the transmitter can be used. Innovative, flexible companies, offering automatic channel selection (SCAN-PLL), cannot use the protocol. Thus the PCM development runs a bit behind.
Checking the transmission quality is very difficult, because the hold-mode smoothes out small glitches.
The lack of early warning signs often causes trouble. Control problems that build up gradually, e.g. of a technical nature, get noticed only when the connection fails completely, usually leading to a crash.
PPM advantages:
There should be no problems using different makes of receivers with transmitters from other manufacturers.
The PPM system is cheaper.
Every 22.S-ms-Frame contains the complete servo position.
Transmission is fast enough to operate even the quickest of servos.
With PPM, the end of the transmission range is shown by the servos starting to glitch. When the pilot notices this, he/she can probably still get the model back home safely.
Momentary signal losses get smoothed out by the model's inertia.
PPM disadvantages:
Due to its simplicity, PPM transmission cannot detect errors, the receiver does not see the difference between valid and invalid servo pulses. When the range boundaries are reached, pulses get slightly longer or shorter because of noise. Servos start to move erratically. The same thing happens when antenna orientation is not optimal, when the projection of the receiver antenna is nearly down to a single point, the signal breaks down and the servos get false pulses. These short glitches go unnoticed most of the time because they are smoothed out by the servo's inertia (response time).
Improvements can still be expected in the PPM sector, like the IPD system by Multiplex, Scan-PLL by ACT or Scan2000 by Simprop.
Using a microprocessor in the receiver makes checking RC-pulses a possibility. Failsafe and Hold, exclusive advantages of PCM so far, are now also possible with PPM.
PCM basics:
The Positions of joy-sticks, switches and pots, originally analogue voltages are digitised by an A/D converter to a 8 to 10 bits (256 to 1024 binary) word.
For eight to ten servos means 80 -100 bits. Add to this the 16-32 bit checksum per frame, synchronisation sequences and failsafe values, and a bit number of 100 -160 becomes necessary for a complete frame.
A bit length of .3 ms (JR/Graupner and Futaba/Robbe) will produce a 30-48 ms frame time, considerably longer than the 22.5 ms PPM uses.
If even more secure bit lengths and 12 channels are used, this time is increased to 55 ms, e.g. Simprop (System 90), where only 6 channels are proportional and 6 are switch channels.
PCM - transmission:
Actual PCM uses two systems to synchronise the transfer: an extra long starting pulse made up of so many "1" or "0" bits, that it can never be mistaken for data, or the so called Half bit pulse, e.g. 2,5 bits, equally impossibly mistaken for data. Usually this is followed by a synchronisation sequence, setting the receive-clock. This is the clock that scans the middle of the bits upon reception. This explains why, at the limits of the transmission range with PPM, the servos start to glitch, because the pulse flanks vary (up to+/-30 us), while PCM keeps them quiet, having half a bit (150 us) to play with, thus avoiding faulty interpretation of a bit.
Next in line are the servo position data, usually in a few blocks, with variable contents. Then we have organisational data, like channel numbers or failsafe modus or failsafe values. Finally the checksum in the shape of a 16 bit long CRC (Cyclic Redundancy Check). It is the rest of the division of the whole message by a 16th order polynom.
Bit errors can be detected this way, but in no way corrected.
This in turn means that, even if only one single bit error has crept in the ca. 100 -160 bits total frame length, the checksum fails and the whole message is rejected.
The servo's remain in their last correctly received position until the arrival of new, correct data. If this takes too long (0.25-1 Sec), failsafe is activated in between: depending on the predefined settings a chosen (and defined in the transmitter) failsafe position or the last correctly received position.
It is a large disadvantage that a transmission failure of one % causes a break in the complete link, but this could only be solved with more complicated and redundant encoding. An audio-CD is a good example: even a 17 bit wide scratch can go unnoticed, but 30 % more data (bits) is needed to make this possible.
In our RC equipment the redundancy is provided by the high frame rate.
The servos cannot process more than 10-15 different positions per second. They receive about 45-60 pulses per second however. It is therefore not critical if a servo position has to be repeated because of a glitch.
To reduce the failure time anyway, JR/Graupner (S-PCM) and Futaba/Robbe (PCM1024) subdivided the frame using separate CRC checks. This allows rejecting only a part of the faulty frame.
PCM advantages:
Accurate and undisturbed servo movement, even if the model is far way.
Holding of the servo position during short glitches (Hold).
Moving the servo's to a predefined position in case of a longer disturbance or even complete failure of the transmitter (Fail-Safe).
Fast transmission if SPCM20 or PCM 1024 is used, similar to PPM.
Servos are not damaged by pulses that are too long/short, which could happen with PPM.
PCM disadvantages:
More expensive.
Sensitivity to adjacent channels is usually worse comparing with PPM receivers. Care has to be taken when flying near to a transmitter from an adjacent channel.
Due to different protocols, only receivers from the same brand or even type of the transmitter can be used. Innovative, flexible companies, offering automatic channel selection (SCAN-PLL), cannot use the protocol. Thus the PCM development runs a bit behind.
Checking the transmission quality is very difficult, because the hold-mode smoothes out small glitches.
The lack of early warning signs often causes trouble. Control problems that build up gradually, e.g. of a technical nature, get noticed only when the connection fails completely, usually leading to a crash.
PPM advantages:
There should be no problems using different makes of receivers with transmitters from other manufacturers.
The PPM system is cheaper.
Every 22.S-ms-Frame contains the complete servo position.
Transmission is fast enough to operate even the quickest of servos.
With PPM, the end of the transmission range is shown by the servos starting to glitch. When the pilot notices this, he/she can probably still get the model back home safely.
Momentary signal losses get smoothed out by the model's inertia.
PPM disadvantages:
Due to its simplicity, PPM transmission cannot detect errors, the receiver does not see the difference between valid and invalid servo pulses. When the range boundaries are reached, pulses get slightly longer or shorter because of noise. Servos start to move erratically. The same thing happens when antenna orientation is not optimal, when the projection of the receiver antenna is nearly down to a single point, the signal breaks down and the servos get false pulses. These short glitches go unnoticed most of the time because they are smoothed out by the servo's inertia (response time).
Improvements can still be expected in the PPM sector, like the IPD system by Multiplex, Scan-PLL by ACT or Scan2000 by Simprop.
Using a microprocessor in the receiver makes checking RC-pulses a possibility. Failsafe and Hold, exclusive advantages of PCM so far, are now also possible with PPM.
02-27-2003, 02:39 AM
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FM or PCM?
But to simplify the matters, one may ask:
How about the IPD System - what is it?
IPD stands for Intelligent Pulse Coding, and the receiver incorporates a processor, which analyses the incoming signal for validity. Like a PCM system, IPD filters out invalid signals.
The difference between IPD and PCM is that the receiver does not "switch off" the "dirty" signal as field strength declines, but instead widens its tolerance. This means that control becomes less precise as field strength falls away, but remains usable for longer (greater range). The result is that you can notice the approaching limit of range from the model's behaviour, whereas PCM suddenly robs you of control.
When the signal is insufficient for the receiver to interpret, a fail-safe condition occurs, thereby driving the servos to pre-selected safe positions, e.g., throttle back.
An IPD receiver sees all usual PPM formats as valid, which means that all standard FM PPM transmitters can be operated in conjunction with these receivers on the appropriate frequency.
IPD is faster than PCM because there are no check cycles.
How about the IPD System - what is it?
IPD stands for Intelligent Pulse Coding, and the receiver incorporates a processor, which analyses the incoming signal for validity. Like a PCM system, IPD filters out invalid signals.
The difference between IPD and PCM is that the receiver does not "switch off" the "dirty" signal as field strength declines, but instead widens its tolerance. This means that control becomes less precise as field strength falls away, but remains usable for longer (greater range). The result is that you can notice the approaching limit of range from the model's behaviour, whereas PCM suddenly robs you of control.
When the signal is insufficient for the receiver to interpret, a fail-safe condition occurs, thereby driving the servos to pre-selected safe positions, e.g., throttle back.
An IPD receiver sees all usual PPM formats as valid, which means that all standard FM PPM transmitters can be operated in conjunction with these receivers on the appropriate frequency.
IPD is faster than PCM because there are no check cycles.
02-27-2003, 04:05 AM
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FM or PCM?
Dirtybird
I stand corrected, I looked it up, you're right, it has something to do with the dual conversion picking up interference from the input signal adding to the noise in the demodulated signal.
DavidO
A square wave is a HIGHLY modulated sine wave. The wave forms if you look at them can not be confused for one another, and they behave totally differently.
Since you are splitting hairs, I differ with you. The image frequency is not a harmonic.
DavidO
Whoops !! But a square wave is made up of Sine Waves, therefore they are one of the same.
02-27-2003, 05:21 AM
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FM or PCM?
"I stand corrected, I looked it up, you're right, it has something to do with the dual conversion picking up interference from the input signal adding to the noise in the demodulated signal. "
Where in the world did you find that? The image frequency is simply the transmitted frequency plus or minus 910KC depending on which side (above or below) the local oscillator is.
DavidO
quote:
--------------------------------------------------------------------------------
Whoops !! But a square wave is made up of Sine Waves, therefore they are one of the same.
--------------------------------------------------------------------------------
A square wave is a HIGHLY modulated sine wave. The wave forms if you look at them can not be confused for one another, and they behave totally differently.
A square wave is defined as an infinate number of odd harmonics. You can prove this by plotting the 3rd,5th and 7th harmonic and adding them together. You will see the square wave starting to emerge.
One way of creating a sine wave is to generate a square wave then run it thru a low pass filter. You will get a nice sine wave
Where in the world did you find that? The image frequency is simply the transmitted frequency plus or minus 910KC depending on which side (above or below) the local oscillator is.
DavidO
quote:
--------------------------------------------------------------------------------
Whoops !! But a square wave is made up of Sine Waves, therefore they are one of the same.
--------------------------------------------------------------------------------
A square wave is a HIGHLY modulated sine wave. The wave forms if you look at them can not be confused for one another, and they behave totally differently.
A square wave is defined as an infinate number of odd harmonics. You can prove this by plotting the 3rd,5th and 7th harmonic and adding them together. You will see the square wave starting to emerge.
One way of creating a sine wave is to generate a square wave then run it thru a low pass filter. You will get a nice sine wave
02-28-2003, 06:26 PM
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FM or PCM?
To whom it may concern:
The Single Conversion Superht. Receiver has some drawbacks that may cause problems in model control applications.
The mixer stage produces a 455kHz output for both the incoming RF signal and also for a signal 455kHz below the local oscillator frequency. This signal is called the "image" and will cause interference if it enters the receiver.
There are also a number of other signal combinations that may cause the generation of 455kHz IF such as Second, Third, Fourth, etc., harmonics of the operating frequency and similar harmonics of the local oscillator plus and minus 455kHz may also cause problems.
Many of these drawbacks can be overcome by using a Double Conversion Superhet. Receiver.
This concept uses two Intermediate Frequencies (IF) and two crystal-controlled oscillators.
The first Intermediate Frequency is higher than 455kHz, typically 10.7MHz. Signals that could cause spurious responses are now beyond the passband of the RF stage.
A second mixer reduces the 10.7MHz to 455kHz in order to obtain a good selectivity.
Due to its complexity and increased costs, such a design is not widespread among the manufactured VHF equipment, but under some severe operating conditions it may give the only solution to reliable performance.
The Single Conversion Superht. Receiver has some drawbacks that may cause problems in model control applications.
The mixer stage produces a 455kHz output for both the incoming RF signal and also for a signal 455kHz below the local oscillator frequency. This signal is called the "image" and will cause interference if it enters the receiver.
There are also a number of other signal combinations that may cause the generation of 455kHz IF such as Second, Third, Fourth, etc., harmonics of the operating frequency and similar harmonics of the local oscillator plus and minus 455kHz may also cause problems.
Many of these drawbacks can be overcome by using a Double Conversion Superhet. Receiver.
This concept uses two Intermediate Frequencies (IF) and two crystal-controlled oscillators.
The first Intermediate Frequency is higher than 455kHz, typically 10.7MHz. Signals that could cause spurious responses are now beyond the passband of the RF stage.
A second mixer reduces the 10.7MHz to 455kHz in order to obtain a good selectivity.
Due to its complexity and increased costs, such a design is not widespread among the manufactured VHF equipment, but under some severe operating conditions it may give the only solution to reliable performance.
