Radios, FM / PPM / PCM ?

If I can add to this thread.

PCM does reject noisy signals compared to a FM/PPM receiver. Some people like this feature, and some people do not.

By the way, for those interested, here are the acronyms and what they mean:

AM - Amplitude Modulation (there are not many AM transmitters or recivers any more. A modulation scheme for the output signal.

FM - Frequency Modulation (mostl transmitters and receivers are FM now. A modulation schem for the output signal.

PPM - Pulse Position Modulation, an encoding system where the position of the pulses denote where a servo should position its output arm.

PCM - Pule Coded Modulation, an encoding system where a number represents where a servo should position its output arm. Note that PCM systems can also include some error checking code so that a receiver can tell if it ever receives 'bad' data.

The reason some people prefer FM/PPM (FM and AM systems are both PPM) over PCM is that they feel that they can start to tell if they are getting some iterference because their servos will 'jump' or be 'jittery'.

PCM systems will never show a 'jumpy' or 'jittery' servo when some interference is present. The servo will either move as you command it, or it will remain still.

Today most all R/C transmitters and receivers use either PPM or PCM encoding. Most all use FM modulation. So technically, it's more accurate to say FM/PPM or FM/PCM. However we tend to simplify things and simply say "FM" to mean "PPM", and then say PCM for the 'other' method. Confusing, no? :shock:

The feature thing that PCM brings is a setting which you can program to state what you would like the servo to do if it does get interference or loses the signal from the transmitter. There are typically two settings available:

1) Hold the last valid position
or
2) Set the servo to a pre-determined position

#2 is generally called a 'fail safe' setting in contrast to #1 which is usually referred to as a 'hold' setting.

The idea behind 'fail safe' is not to save your model, but to minimize the impact of a crash to help it be safer to others. As an example, if you had a PCM system, you might want to program your settings to place the throttle as a fail safe setting of idle, yet place the flight controls in a 'hold' setting. Some others prefer to place the flight controls in a fail save setting of neutral elevator, rudder, ailerons, etc.

The important thing if you get a PCM receiver is to remember to program these settings. Do not simply leave the settings at their factory default settings! Otherwise you could see your engine go to full throttle suddenly when your aircraft is on the ground if you get some interference. This could be dangerous to you if you were tuning your engine or to other by standers.

Manufacturers are getting better about the default settings of PCM systems so this may vary with new radios. But there it is important to check into it.

The other thing which I can mention is that a dual conversion receiver is almost always better to have compared to a single conversion receiver. The type of interference which a dual conversion receiver can reject is very real at an R/C flying site. This is referred to as 'image rejection'.

This is not to say that a single conversion receiver is unsafe. But given the choice, I would always choose a dual conversion receiver if all else is equal. Normally a receiver is only 'single conversion' when size or weight is an issue. So small, ultra-light park flyer type receivers tend to be single conversion designs to save in their weight. This is because it takes more parts to build a dual conversion receiver.

Be aware that there are many designs of both dual and single conversion receivers out there. If you stick with a reputable brand that has a track history of good performance, and you use the receiver for it's it's intended use, you will not have any problem no matter if you have a dual or single conversion receiver.

I would be more concerned with using a receiver designed for a park flyer in a larger aircraft. The reason is that these receivers tend to have limited range and you could lose control if you fly futher away, which is the tendancy with a larger aircraft.

I like PCM systems as I feel it gives me some what of a choice to say what will happen to my flight surfaces if I receive interference or loss of signal, but I'm always careful to preset the hold/fail-safe settings.

Not to cloud this discussion, but you should be aware that some new technology receivers are starting to build in 'hold/fail-safe' like features into a standard PPM receivers. So be aware that you can get 'PCM like' features in a receiver that will work with your standard 'FM' or PPM transmitter. Talk about confusing! :shock:

As far as what recievers will work with which transmitters, you can basically say that most if not all of one brand of receiver will work with that same brand of transmiter.

Of course, if you have a 9 channel receiver, and a 4 channel transmitter, you can only control the first 4 channels of that receiver. So you'd be waisting your money to purhcase a 9 channel receiver for use with that 4 channel transmitter.

But if you have a 9 channel transmitter, you can use 4 channel receivers if you only need 4 functions.

As far as 'cross brand' receivers working with different transmitters, the answer is "It depends".

There are two major differences in receivers today. One is called 'positive shift' and the other is 'negative shift'. I know that Futaba, Hitec and I believe a few others use 'negative shift'. JR and Airtronics use positive shift. So the direction "or positive/negative shift' of the signaling must be compatible. This is why you can't use most JR receivers with a Futaba transmitter (and vice versa).

Lately, some transmitters have come out which are programable to use positive or negative shift. There are also some newer 'universal' receivers which will automatically detect the shift of the transmitter and 'just work', without you having to do anything.

From out point of view, there is not advantage to using 'positive' or 'negative' shift. It just means we have to be careful to purchase a compatible receiver.

Luckily, the industry standardized on servo signaling a long time ago. So most of today's servos (either 'standard' or digital servos) will work with all receivers. The only difference is that there is a slight difference in the plugs which are on the end of the cable.

I like the Futaba plug the best as it's least prone to being plugged in backwards. However the JR style is more 'universal' in that I can plug a JR servo into a Futaba receiver without any changes (but I have to be careful not to plug it in backwards). But I can not plug a Futaba servo into a JR receiver without making a minor adjustment (or modification) to the servo connector.

Keep in mind that with any of this stuff, there are always advances being made, and what may be true yesterday, may be different tomorrow. An example is with the new 'spread spectrum' type of systems. These will be a whole new animal which we will all have to learn about. About the only compatible thing with the spread spectrum systems (so far) are that they use the same servo signaling, so we can use all of our current servos with their receivers (as of today, tomorrow may bring different rules).

I know this is a long post, but I think it covered all the questions which were asked.