Understanding Radio Language
for RC Airplanes

"Which is better?
Pulse Position Modulation or Pulse Code Modulation?"

The two different radio languages used by RC airplane transmitters and receivers to communicate are Pulse Position Modulation (PPM) and Pulse Code Modulation (PCM).

The carrier frequency (or radio waves) can be thought of as a telephone line that provides a way for the transmitter and receiver to communicate regardless of what "language" they use.

Both PPM and PCM radio languages use the same "telephone line" or 72Mhz frequency band to communicate, whether it be AM or FM. It is important to know that that PPM is the only radio language that a standard servo understands.

Pulse Position Modulation used to be the only radio language used with FM(Frequency Modulation). So Pulse Position Modulation (PPM) was referred to as "FM".

When PCM technology came along, people continued to refer to PPM as "FM". The new Pulse Code Modulation radio language was referred to as "PCM". Most people still refer to these two radio languages as "FM" and "PCM". This is confusing because an FM radio system can be either Pulse Code Modulation or Pulse Position Modulation as both operate on the same 72Mhz frequency band.

Both Pulse Position Modulation and Pulse Code Modulation languages have their pros and cons. One is not necessarily better than the other. There are distinct differences that you should take into account when choosing your radio system.

To fully appreciate the following discussion, I highly recommend that you take the time to read these brief explanations of Pulse Position Modulation and Pulse Code Modulation. These are short reads that will definitely help you digest the info below.

Pulse Position Modulation vs. Pulse Code Modulation

Some folks are under the impression that PCM is better for resisting interference. This is not the case. Pulse Code Modulation and Pulse Position Modulation both broadcast on the same frequencies and are subject to the same interference. How the systems deal with the interference is the difference.

Have you ever been watching digital or satellite television when the signal gets interference? What happens? The picture will either freeze, go blank, or garbage will be displayed on the screen. Digital is an “all or nothing” technology. If you are watching analogue television from the local broadcast tower the picture will become fuzzy as the interference increases, but you can still watch the program even if you don’t have a perfect picture.

When a Pulse Code Modulation system encounters interference that distorts the digital signal, the receiver will simply not transmit this signal to the servos. The servos controlling the control surfaces will stay at the position of its last known good signal. If the interference is only present for a split second the pilot will not even notice the interference. The interference is still there but is masked.

When a PCM system encounters interference for a prolonged period of time it can be programmed to enter a fail-safe mode. Each user can program the system to put the servos in a predetermined position in the event of a signal loss or prolonged interference. For instance, the user could program all control surfaces to be neutral and for the throttle to be minimized.

You may be able to save your model if the interference occurs when the airplane is cruising along horizontally when it enters fail-safe mode. But what happens if the airplane is doing an aerobatic maneuver when it enters fail-safe mode and the nose of the airplane just happens to be directed towards the ground at that precise time? You guessed it! The main purpose behind fail-safe mode is not to save the airplane, but to protect an innocent bystander from having their face chewed up by an out of control airplane.

When a Pulse Position Modulation receiver encounters interference it transmits this interference directly to the servos. Depending on the severity of the interference it may be possible for the receiver to interpret the transmitters signal enough to get the airplane to the ground in one piece. Just like a fuzzy TV reception, the control over the airplane may not be perfect but it may be possible to land the airplane.

Since a PCM system masks interference, the pilot never knows when he is experiencing slight interference. A pilot using a PPM system will see the slight interference in the form of small glitches and can land the airplane before the interference gets worse.

Since PCM systems are more expensive and seem to be more risky, why would anyone want to buy a PCM system? In some situations a small glitch could send an aircraft to the ground. A helicopter hovering inverted 12” off the pavement would definitely be a situation where you can’t have any glitches! A 3-D plane touching the tail on the pavement while doing a torque roll would be out of luck if the airplane glitches even for a millisecond! If the pilot is participating in an aerobatic competition where every move must be perfect, he will probably want a PCM system. In these cases it is better to have PCM which "ignores" slight interference that may cause "glitches".

If you are new to the hobby I recommend that you get a regular FM RC radio system (PPM). These systems are cheaper than PCM systems. They are also less risky as they allow you to have a fighting chance to save your model should you encounter moderate interference. You will also find that the majority of the people at your flying club will also have PPM systems.