Pulse Code Modulation -

The Digital Radio Language

Pulse Code Modulation PCM is a digital radio language used by PCM radio systems. PCM is very similar to Pulse Position Modulation(PPM), except the positions are decoded by digital numbers rather than time on vs. time off.

It would be a good idea to take a couple seconds to read the quick explanation of Pulse Position Modulation in order to fully understand the following discussion.

Keep in mind that even though PCM transmitters and receiver communicate with the PCM language, all servos speak the PPM language. So, a PCM receiver translates PCM into PPM for the servo to understand.

72Mhz Carrier Frequency - The Phone Line

PCM and PPM systems operate over the same 72Mhz carrier frequency. Just think of this FM carrier frequency as a phone line. PCM and PPM are just different radio languages, but use the same "telephone line" to communicate.

With a PCM system, the receiver performs two tasks. The first task is obviously to receive the signal from the transmitter. But a PCM receiver also acts as a translator. The transmitter tells the receiver what to do in Pulse Code Modulation language. The receiver translates the Pulse Code Modulation language to Pulse Position Modulation language before sending the information to the servos.

So what is Pulse Code Modulation?

Instead of using the amount of time that the pulse is on to dictate a position(like PPM does), PCM uses binary numbers to represent each position of the servo. Binary numbers are integers or whole numbers. There is a finite or limited amount of numbers available to represent the servo position based on how many bits the system has available. The amount of numbers available will be 2 to the power of #bits. If it is a 10-bit system there will be 2^10=1024 numbers available.

Let’s just say we have a 10-bit system with 1024 servo positions. This is digital because there are only 1024 different positions that the servo can be positioned, with nothing in between those numbers. In contrast, a PPM system has an infinite amount of positions available which makes it is analogue.

Advantages of Pulse Code Modulation

The advantage of a digital signal is that it can be reproduced perfectly. The information is always decoded as a 0 or 1. There are no gray areas in between to create noise. This means that the information arriving at the receiver is exactly the same as the information leaving the transmitter. In contrast, a PPM analogue signal could be distorted by slight interference before it arrives at the receiver. The Pulse Position Modulation receiver will pass all of the information it gets to the servos, including interference.

Disadvantages of Pulse Code Modulation

The advantage of a PCM radio system is that it gives the pilot much finer and more precise control over the airplane. However, there is a huge drawback. A PCM receiver must translate the signal from Pulse Code Modulation language to Pulse Position Modulation language before sending the information to the servos.

If one small byte of information is corrupt, even if it is only for one servo, the translator inside the receiver gets all confused! The "translator" part of the receiver looks at ALL of the servos and says “I don’t understand a word the transmitter is saying, so all of you hold your position until I get the next message!” If only a couple of messages or sequences of information is corrupt, the pilot will never notice because the servos will stay put at the position of the last known good signal for a very short period of time (20ms for each bad or corrupt message).

If the signal is corrupt for an extended period of time a PCM receiver will enter “safe mode”. Safe mode moves all of the servos to a predetermined position. At this point Sir Isaac Newton is piloting your airplane! Since he’s been dead for a while this is not a good thing!

For a more in depth discussion of pulse code modulation vs. pulse position modulation, be sure to check out the radio language page.