Pulse Position Modulationor Pulse Code Modulation?The million dollar question... Pulse Position Modulation (PPM) and Pulse Code Modulation (PCM) are different RC radio languages that transmitters and receivers use to communicate. 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 became available people continued to refer to PPM as "FM". The new Pulse Code Modulation language was referred to as "PCM". It is still common to refer to these two languages as "FM" and "PCM". This is confusing because an FM radio system can be either Pulse Code Modulation or Pulse Position Modulation. 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.
A servo receives the command or pulse from the receiver every 20ms. The servo must hold that position for the next 20ms until it gets the next pulse. The servo must hold this position even if the forces from the control surface are trying to make it move. If you try to move the elevator of your airplane with the radio on and with the sticks in neutral, the servo will fight you to keep the elevator in neutral position. How does it do this? It does it exactly the same way cruise control maintains the same speed going up hill or down hill. Both servos and cruise control use feedback. With cruise control the speedometer provides the feedback. The electronic circuitry looks at the speedometer and compares it to the set speed. If the value of the speedometer is greater than the set speed, the circuitry sends a signal to let off the gas. If the value of the speedometer is less than the set speed, the circuitry sends a signal to give more gas. It does this several times a second where in most cases it is so smooth you don’t even notice it. Instead of a speedometer, a servo uses a potentiometer (pot) for feedback. The potentiometer is a resistor that changes resistance and it rotates. It is mechanically coupled to the motor and gears so it rotates with the servo motor. The circuitry in the servo knows the exact position of the servo from looking at the voltage coming from the pot (resistance goes up, voltage goes down). The circuitry compares the position dictated by the receiver with the position indicated by the pot. The circuitry then tells the servo motor to move in the appropriate direction to minimize the difference.
Below is the amount of time during the designated 2ms time frame that the pulse will be on for the three most extreme positions(far left, far right, neutral). There are an infinite amount of positions in between these values and amount of time the pulse is ON changes in proportion. This is what makes Pulse Position Modulation analogue. Servo #1:
When controlling multiple servos each consecutive 2ms time frame corresponds to a servo. In other words, the first 2ms controls servo one, the second 2ms controls servo two, the 3rd 2ms time frame control servo three, and so on. The receiver sorts all this out and sends the correct pulse to each servo. There is a long pause at the end of this sequence where there is no pulse for at least 2ms. This lets the servo know that the sequence is starting over and the next pulse is meant for servo #1. The 20ms cycle is long enough to represent up to 9 servo positions. If there are only three servos like the example above, the remainder of the 20ms is simply coded as pulse off. Since this system uses the "pulses" to dictate the position of the servos it is called Pulse Position Modulation.
So what is PCM language? It stands for Pulse Code Modulation. Instead of using the amount of time that the pulse is on to dictate a position, 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 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. 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. 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!
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 which 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 location 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 which 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.
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