PCM Vs. FM Radios
09-25-2006 | 10:21 AM
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From: Luxembourg, LUXEMBOURG
RE: PCM Vs. FM Radios
with PCM I can set a failure safe settings for my servos in case I have interference or loose the connection... This is as much as I know 
09-25-2006 | 10:23 AM
#3
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RE: PCM Vs. FM Radios
Ok, first of all let's get the terminology straight. PCM radios are also FM radios. Confusing huh?? Yes. Our radios either AM or FM to broadcast their signals. Most radios these day are all FM radios. So what is PCM. This is the way the signal is encoded for broadcast. There are two major types of modulation and those are PPM and PCM. The easiest way that I could describe PPM and PCM would be to consider them languages, such as French and English. Both types of languages can be broadcast over a radio, but the person on the other end has to be able to understand the language being spoken in order to get the message being spoken. Same prinical here. PPM and PCM are the languages that are spoken by the transmitter, and you need a receiver that can understand those languages. The most common moduation is PPM, which most radios have. Higher dollar radios have PCM in them.
Ken
Ken
09-25-2006 | 11:10 AM
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From: Bend,
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RE: PCM Vs. FM Radios
PCM is more interference resistant than straight FM. It encodes a binary code to it's receiver and is supposedly only interpreted by it's receiver. That does no mean that it will not be "hit" by another signal from a transmitter on the same channel but it is some added insurance. Like was said earlier PCM radios can also be programed for fail-safe features such as throttle cut, elevator slightly up, rudder right or left, it the case of a signal interruption. Given the choice, I would always buy a PCM radio to have the choice of FM or PCM even if I never used it. They cost more though.
Andy
Andy
09-25-2006 | 11:31 AM
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RE: PCM Vs. FM Radios
Although I know the difference between the two, I was having a hard time explaining it. So I did a quick search on the web and came up with this, it's a pretty explanation of the differences between the two
I hope this helps
Ken
PPM stands for Pulse Position Modulation, and PCM stands for Pulse Code Modulation. Here is a basic comparison between the current crop of receivers on the market, by Jason Werner.
PPM
Our traditional "FM" is still a framed signal that the rx processes. The only difference is that it is an analog based signal. So what happens is that any signal received within the "frame" for that channel is processed. This is the famous glitch where the servos are sent screwy for an instant.
The advantages of FM are super high refresh rates because the data length is very small. Also the resolution in theory can be very high because there is no loss of signal during the conversion to digital.
Disadvantages - the "glitch". This spastic movement of the servos when a bad signal is decoded is why many people prefer to switch away from traditional PPM. Also the resolution that many of the older radios had is removed by the new "digital" txs that use only 512 or 1024 bits of resolution. But to be honest, that is a minor problem and most people would never see it.
PCM
PCM is still broadcast on the FM carrier wave. But instead of using a PPM frame setup, a digital stream is created of x bits (1024 for most). The digital stream consists of a header, a trailer, a set of parity bits, a failsafe section, and the actual control section. The receiver takes this stream/word and decodes it into the control positions based on the position in the stream. Since a small microcomputer is in the Rx, quite a bit of processing can be done during this section.
Advantages - no more glitch! While the same bad data is received by the RX, the microcomputer has the smarts to reject that bad data and not move the servos. So for that instant you simply don't get movement of the servos. For 99% of the people out there, this is the only reason they switch to PCM! Another advantage is better control of certain surfaces. On PPM, all channels are of equal value. For PCM the designer can adjust the primary channels to have more resolution than say the gear channel. This makes for a system that can be weighted towards the primary channel resolution.
Disadvantages
Proprietary - Each company's PCM is only compatible with PCM rxs and txs of their own make. Most aftermarket rxs will not work on proprietary PCM txs. So your are locked in to one brand or even one rx!
Slow refresh rate compared to PPM. The actual data stream is larger (2- 3 times) the size of PPM. Many pattern fliers used to switch to PPM simply because they felt the added speed of PPM helped them. I know that on one of my giant scale planes I can tell the difference between the PCM update and PPM update. Only one plane, but there is a difference.
lower resolution. On systems that output pure PPM, the resolution is higher on PPM then PCM. But for most of us this is not a problem. And for the killer! The PCM lockout. When the PCM rx gets too many bad packets it then goes into a lockout or failsafe position. This continues for several refresh cycles after a valid packet is received. Most are around 1 second long before control is returned. It is this lockout that has most heli fliers staying with PPM! imagine hovering and having the system lock out for a second. A glitch can be just as bad, but at least then the control is instantly returned and not held up for a second or so
PPM
Our traditional "FM" is still a framed signal that the rx processes. The only difference is that it is an analog based signal. So what happens is that any signal received within the "frame" for that channel is processed. This is the famous glitch where the servos are sent screwy for an instant.
The advantages of FM are super high refresh rates because the data length is very small. Also the resolution in theory can be very high because there is no loss of signal during the conversion to digital.
Disadvantages - the "glitch". This spastic movement of the servos when a bad signal is decoded is why many people prefer to switch away from traditional PPM. Also the resolution that many of the older radios had is removed by the new "digital" txs that use only 512 or 1024 bits of resolution. But to be honest, that is a minor problem and most people would never see it.
PCM
PCM is still broadcast on the FM carrier wave. But instead of using a PPM frame setup, a digital stream is created of x bits (1024 for most). The digital stream consists of a header, a trailer, a set of parity bits, a failsafe section, and the actual control section. The receiver takes this stream/word and decodes it into the control positions based on the position in the stream. Since a small microcomputer is in the Rx, quite a bit of processing can be done during this section.
Advantages - no more glitch! While the same bad data is received by the RX, the microcomputer has the smarts to reject that bad data and not move the servos. So for that instant you simply don't get movement of the servos. For 99% of the people out there, this is the only reason they switch to PCM! Another advantage is better control of certain surfaces. On PPM, all channels are of equal value. For PCM the designer can adjust the primary channels to have more resolution than say the gear channel. This makes for a system that can be weighted towards the primary channel resolution.
Disadvantages
Proprietary - Each company's PCM is only compatible with PCM rxs and txs of their own make. Most aftermarket rxs will not work on proprietary PCM txs. So your are locked in to one brand or even one rx!
Slow refresh rate compared to PPM. The actual data stream is larger (2- 3 times) the size of PPM. Many pattern fliers used to switch to PPM simply because they felt the added speed of PPM helped them. I know that on one of my giant scale planes I can tell the difference between the PCM update and PPM update. Only one plane, but there is a difference.
lower resolution. On systems that output pure PPM, the resolution is higher on PPM then PCM. But for most of us this is not a problem. And for the killer! The PCM lockout. When the PCM rx gets too many bad packets it then goes into a lockout or failsafe position. This continues for several refresh cycles after a valid packet is received. Most are around 1 second long before control is returned. It is this lockout that has most heli fliers staying with PPM! imagine hovering and having the system lock out for a second. A glitch can be just as bad, but at least then the control is instantly returned and not held up for a second or so
Ken
09-25-2006 | 11:48 AM
#7
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RE: PCM Vs. FM Radios
There are a couple of not quite correct statements in the above. PCM is no more immune to interferance than PPM is, it just sits and waits for a good signal to come along. If one does in a few milliseconds, then it continues to work as asked, if not it goes into failsafe which can be set up by the user to be a specific control position. If the interferance is intermittent, you will get what appears to be very slow reaction from the plane, maybe enough to save it from crashing. If using PPM, you can now also get the same failsafe features in some equipment (check out FMA for instance) but it is not common. One big advantage of PPM is that, if interferance is present, you usually know it before you take off and any sensible reaction is to abort the flight. PCM requires more bandwidth than PPM so limits the period between subsequent channels, i.e. aileron, elevator, throttle, etc., to a longer period than PPM uses. While this is not normally a critical issue, if you try to cram to many channels into a frame, frame rate will be much less for PCM.
09-25-2006 | 11:55 AM
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From: Claremont,
ON, CANADA
RE: PCM Vs. FM Radios
ORIGINAL: RCKen
And for the killer! The PCM lockout. When the PCM rx gets too many bad packets it then goes into a lockout or failsafe position. This continues for several refresh cycles after a valid packet is received. Most are around 1 second long before control is returned. It is this lockout that has most heli fliers staying with PPM! imagine hovering and having the system lock out for a second. A glitch can be just as bad, but at least then the control is instantly returned and not held up for a second or so
And for the killer! The PCM lockout. When the PCM rx gets too many bad packets it then goes into a lockout or failsafe position. This continues for several refresh cycles after a valid packet is received. Most are around 1 second long before control is returned. It is this lockout that has most heli fliers staying with PPM! imagine hovering and having the system lock out for a second. A glitch can be just as bad, but at least then the control is instantly returned and not held up for a second or so
This may have been true for first generation systems but hasn't been the case for many years and is very easy to prove. Take two radios on the same frequncy one ppm, the other pcm. Use the ppm to flood the pcm RX into lockout, turn this ppm radio off while continuing to twitch the sticks on the pcm to determine when control is returned. I dare you to measure the delay, it's a frame or two, nowhere near as slow as the response time of the human nervous system....
Further, failsafe (what is incorrectly referred to as lockout) will only occur after an extended period of garbled frames, on some radios this time period is programmable, for most it's 30ms.
Another fact is PCM's resistance to noise. The simple fact is a PCM radio will remain unaffected in a noisy environment that has long ago sent any PPM radio into the ground with zero control. The old fable about "flying through" interference couldn't be more false. Any interference a PPM unit could "fly through" would be flown through with control by a PCM radio.
It's not lockout (sic) that causes crashes, it's interference.
In a shoot down, I'd rather have my aircraft fly a power-off predictable path to the ground than have it twitching about overhead with roaring at full throttle. How about you?[sm=72_72.gif]
09-25-2006 | 01:34 PM
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From: Claremont,
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RE: PCM Vs. FM Radios
ORIGINAL: BarracudaHockey
Most serious heli guys use PCM.
Most serious heli guys use PCM.
09-25-2006 | 03:27 PM
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From: Claremont,
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RE: PCM Vs. FM Radios
Again, be wary of sources of information, there is much misinformation out there.
"Atmospheric conditions fouling up 1s and zeros" is not a science I'm familiar with. The simple fact is "ones and zeros" are much easier to detect than discrete positional shifts in the same PPM modulation used to send both signals ... hence the improved signal to noise ratio of PCM.
This link may provide you with a better understanding of PCM. [link=http://en.wikipedia.org/wiki/Pulse-code_modulation]Wikipedia - PCM[/link]. Keep in mind that modern PCM radios have surpassed the resolution of standard servos....
"Atmospheric conditions fouling up 1s and zeros" is not a science I'm familiar with. The simple fact is "ones and zeros" are much easier to detect than discrete positional shifts in the same PPM modulation used to send both signals ... hence the improved signal to noise ratio of PCM.
This link may provide you with a better understanding of PCM. [link=http://en.wikipedia.org/wiki/Pulse-code_modulation]Wikipedia - PCM[/link]. Keep in mind that modern PCM radios have surpassed the resolution of standard servos....
09-26-2006 | 02:26 AM
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From: Warialda NSW, AUSTRALIA
RE: PCM Vs. FM Radios
G'day Mate,
Just one more point that has not been mentioned here, so far, is the fact that when I changed my girlfriend/ students plane from PPM to PCM I noticed an increase in range with the aerial down, because as usual after changing receivers in her plane I did a range check, it went from about 30 paces, before any jittering, to at least 70 on PCM, JR 770S receiver, & I still had not gone to failsafe, meaning I still had good signal.
Also here in Aus there is only $19.00 difference between PPM & PCM, $119.00 for PPM & $129.00 for PCM, in JR brand anyway. And I think the advantages far outweigh the disadvantages, when you cnsider the price difference.
Just one more point that has not been mentioned here, so far, is the fact that when I changed my girlfriend/ students plane from PPM to PCM I noticed an increase in range with the aerial down, because as usual after changing receivers in her plane I did a range check, it went from about 30 paces, before any jittering, to at least 70 on PCM, JR 770S receiver, & I still had not gone to failsafe, meaning I still had good signal.
Also here in Aus there is only $19.00 difference between PPM & PCM, $119.00 for PPM & $129.00 for PCM, in JR brand anyway. And I think the advantages far outweigh the disadvantages, when you cnsider the price difference.
