Electrical – How to convert Pulse Position Modulation (PPM) to duty cycle

arduinomicrocontrollerModulationpwm

I can convert PWM to duty cycle since each frame starts with a rising edge. Then I count the on time, off time and take the ratio to find duty cycle.

However, for PPM signal, there is no start of frame or end of frame so I am stuck. That is even with the assumption that frame length is known.

There are some modules that claim they can convert PPM to Analog. One of them is PT01A WingXine which can convert PPM to Analog AND vice verse, there is only 1 single IC on there so I was wondering how it functions.

Best Answer

According to Tech-FAQ you are correct.

Figure 1. Signal encoded with pulse amplitude, pulse density, pulse position and pulse code modulation.

From the Tech-FAQ article:

How Pulse Position Modulation Works

Pulse position modulation works by sending electrical, electromagnetic, or optical pulses to a computer or other device in order to communicate simple data. It requires both devices to be synchronized to the same clock so that when a series of pulses is sent, the device decodes the information based on when the pulses were broadcasted. Alternately, another form of pulse position modulation known as differential pulse position modulation, allows all signals to be encoded based on the difference between broadcast times. This means that a receiving device only has to observe the difference in arrival times in order to decode a transmission.

Further down the article they continue:

Disadvantages

Pulse position modulation requires that both devices are synchronized or differential pulse position modulation is used. Also, pulse position modulation is highly sensitive to multi-pathway interference, such as echoing, that can disrupt a transmission by altering the difference in arrival times of each signal.

The Wikipedia article Pulse position modulation gives food for though (but not much else) when it states

Synchronization

One of the key difficulties of implementing this technique is that the receiver must be properly synchronized to align the local clock with the beginning of each symbol. Therefore, it is often implemented differentially as differential pulse-position modulation, whereby each pulse position is encoded relative to the previous, such that the receiver must only measure the difference in the arrival time of successive pulses. It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of affecting all successive measurements.

An article on PCB Heaven explains several different PPM schemes. The last one looks the simplest for analog signal transmission without a clock.

Figure 2. This simple analog PPM scheme can reconstruct the signal without the original clock but notice that the number of samples decreases for high voltages.

That complete article is worth a read.

Figure 3. The board and close-up of the setting jumpers.

The board does seem to support PPM but without a datasheet it's not certain.

Related Solutions

Electrical – Arduino – Pulses get unstable when updating PWM duty-cycle

Could be because your variable 'val' does get different values. Because usually when you have a closed loop system the LSB's toggle even though the final value stabilizes. Even if yours is not a closed loop system I think by just logging 'val' first and checking the values must give a clue. Because if 'val' changes then its correct that your duty also changes with it.

Because in your second case you have the same code but it does not jitter which means that the arithmetic operations are not causing the jitter. I therefore think if you log the data (print the data continuously and see the value of 'val') it would be helpful or at least directing you in finding the right solution.

Electronic – Setting a simple PWM on atmega328 at certain frequency

For full 16 bit resolution, you'll want Waveform Generation Mode 8, i.e. WGM13:0 = 1000b.

In this mode, you define the resolution (=max counter value) through the value of the Input Capture Register, so you need ICR1 = 0xffff.

Depending on the polarity you want you set the Compare Output Mode COM1A1:0 to 10b or 11b.

TCCR1B = 0; // (If timer may already be running:) Make sure the timer is stopped during (re-)configuration

TCNT1 = 0x0000; // (If timer may have already been used since last reset:) Set counter value to begin of PWM cycle to prevent a potential glitch in the first cycle

ICR1 = 0xffff; // "TOP" = 0xffff -> full range of 16 bit counter

OCR1A = <initial duty cycle>; // 0x0000...0xffff

TCCR1A =
    // Compare output mode (10b):
    (1 << COM1A1) | (0 << COM1A0) | 

    // lower 2 WGM bits (00b):
    (0 << WGM11) | (0 << WGM10);

TCCR1B =
    // upper 2 WGM bits (10b):
    (1 << WGM13) | (0 << WGM12) |
    
    // Set prescaler/start timer (001b):
    (0 << CS12) | (0 << CS11 ) | (1 << CS10);

(Haven't tried this.)

Now the timer is running up and down toggling the OC1A pin every time it passes OCR1A. All you have to do is set a new OCR1A value when you want to change the duty cycle; the rest is done by the hardware.

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Related Solutions

Electrical – Arduino – Pulses get unstable when updating PWM duty-cycle

Electronic – Setting a simple PWM on atmega328 at certain frequency

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