What is the difference between PPM and PCM?
What is the difference between Dual conversion and Single conversion receiver ?
How to connect Aero-Mate Receiver and servo?
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What is the difference between PPM and PCM?
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.
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 low cost. Also the resolution (in theory) can be very high because there is (theoretically) no loss of signal during the conversion to digital. This is only true with analog radios, though. Digital radios put PCM and PPM on a level playing field as far as resolution goes.
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.
Slower Refresh rate - A PPM Rx sends a signal to the servo once every 20-25ms. A PCM Rx sends pulses almost twice as often. This is true with my Futaba systems--I have measured it with my oscilloscope.
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.
Faster refresh rate - While a PPM Rx only sends out a servo pulse once every 20-25ms, the PCM Rx's are almost twice as often, at about 10-15ms. For Futaba gear, anyway, the thought that PCM's have slower refresh time is a myth! I have measured it with my oscilloscope.
Disadvantages -
There are several so I will list them:
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 response time 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.Note that I have not yet checked this to verify it.
lower resolution. On systems that output pure analog PPM, the resolution is higher on PPM than 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. On the other hand, PCM lockouts are FAR fewer and much farther between than PPM glitches.
1. Synthesized receiver gets rid of the inconvenience of using the crystal. Receivers with crystal still use the technology of 1970¡¯s which have to switch the frequency by changing the crystal. Users have to bring dozens of crystal to flying field. In addition, every changing of the crystal may damage the electron devices. While it is only need to push the button, you can switch to any channel with synthesized receiver.
2. Every transmitter and receiver has production tolerance inevitably. But the production tolerance of the crystal affects it most. Even the most precise crystal has the tolerance of 1©\2KHz, and sometimes it will be 4KHz. This is a big tolerance to a 10KHz narrow receiver. Synthesized receiver will correct the tolerance because it can automatically follow the frequency tolerance. It is what will make R/C receiver has the best performance.
What is the difference between Dual conversion and Single conversion receiver ?
Dual conversion receiver can eliminate mirror frequency interference efficiently. Usually single conversion receiver uses IF of 455MHz, and there is 1MHz bandwidth of RF frequency (72.010MHz-72.990MHz). 0.445*2=0.910MHz, so if the transmitters of 72.010MHz and 72.910MHz (or 72.930MHz) turn on at the same time, single conversion receivers will be interfered from each other. While dual conversion receiver uses IF of 10.7MHz, so RF frequency of 1MHz bandwidth will not happen mirror frequency interference.
Further remote control distance means more reliable control. Even the reliability can compare beauty with PCM receiver. PCM receiver has good reliability because it uses CRC code to fill up the wrong codes which leading by shortage of receiving range. The signal will always seem to be fine ostensibly, but CRC code of PCM began to fill up 'bad data cavity' which happens at times while exceeding half of the distance described by manufacturer. PCM code wastes a lot of time to receive more data to fill up the cavity of 'bad data'. So usually we feel that PCM receiver has no glitch, while its response speed is less than PPM receiver.
What is 2.4G?
Recent developments have provided a new and more secure frequency - 2.4GHz - for use with radio controlled models. Basically, 2.4G technology allows the transmitter and receiver to be 'locked-on' to the same frequency ensuring much improved reliability and peace of mind when flying.Is 2.4GHz a better option than 35Mhz?
The advantages of 2.4GHz systems are explained in the information below.Improved reliability
Operating at 2.4 Gigahertz puts the radio control out of the frequency range of any 'noise' caused by the other electronic components on your helicopter - such as the motor, speed controller and any metal to metal noise - eliminating interference and glitching that can affect a 35 megahertz system.Improved performance
The high data rate of 2.4 GHz systems offers quicker, more responsive control, when compared to a standard 35 MHz system.Safety and security
Obviously, the last thing you need when piloting an RC helicopter is anything that might cause loss of control or a crash. In urban areas you may not be aware if your neighbour is also flying on the same frequency as you. The built-in security of the 2.4G system allows you to maintain radio control at all times, ensuring your safety and that of others around you.Lower power consumption
Within the same control range (up to 250m), a 2.4G system will consume only a fraction of the power required by a 35mhz system - about 4mW compared to 750mW - saving ¡ês on AA batteries - and allowing you to fly longer!Club flying
The main characteristic of 2.4G equipment from a RC model club¡¯s point of view is that no frequency control is needed. Eliminating the need to check everyone else's channel numbers, prior to flying. No more worries about turning up at a club with the wrong crystal in your Tx.Council By-laws
With the growth in the popularity of remote control models there comes an inevitable increase in 35mhz frequency conflicts. Councils are already introducing restrictions on where you can use model helicopters on park and municipal land. There are reports that in some areas flying will only be allowed if the RC models are equipped with 2.4Ghz systems.2.4G Spread Spectrum Technology - how does it work?
Instead of transmitting on one channel at a time, both the transmitter and receiver are constantly hopping from channel to channel - at over 120 times a second! When you initialy 'pair' your transmitter to your receiver, they initiate the synchronised sequence of channel hopping.
With Aero-Mate 2.4GHz effectively 256 channels are available and the sets automatically set themselves to an unused frequency when switched on. Operation is constantly self monitored and the set will move to an unused frequency if any interference is detected.
Any other benefits?
Because 2.4G RC systems work on higher frequency short wave length, the transmitter antenna is only about 15cm long and flexible - avoiding bends and breakages that can occur with traditional 35Mhz telescopic aerials! The receiver antenna is much shorter too - allowing for much neater installation of the radio gear in the helicopter.Any downsides?
The only downside may be that you won't be able to blame that crash on a radio glitch anymore!How to connect Aero-Mate Receiver and servo?
.........Tx Rx........ |
Futaba/Hitec |
JR/SANWA |
1 |
Aileron |
Throttle |
2 |
Elevator |
Elevator |
3 |
Throttle |
Aileron |
4 |
Rudder/Battery |
Rudder/Battery |
5 |
Gear |
Gear |
6 |
Aux1 |
Aux1 |
7 |
Aux2 |
Aux2 |
8 |
Aux3/Battery |
Aux3/Battery |
