This is how you start an argument with someone in the hobby, ask them: Is PPM Analogue or Digital Signal?

     

I posted this question on my Facebook group, and there was many different views on this. Here is a summary of the responses.


It's Digital!

Levi W:
Its Pulse Position Modulation, So its a standard logic pulse(3.3 or 5v), the data is just defined by the position of the pulse in reference to the clock cycle. Similar to PWM. Analog is where a signal varys in voltage, Digital only has two states, on and off, Generally 0v and 3.3 or 5v


Adriano D:
Digital, 0 and 1 levels only! It is generated digitally... But, as a "protocol" can be decoded both, digital or analogic way... Omho

Although is time dependent, it uses a "fixed time frame" to code/decode, larger pulses are multiple from this timeframe, so the double time can be interpreted as two high pulses and so on...This is a kind of protocol... But the signal amplitude does not carry any info...is pure digital, also you need a digital decoder or a DAC to use the info transmitted, so its a digital thing...
Basically, circuits operating on a saturation and cut levels (two levels only) are Digital.
I thing the fact that we can use a simple passive circuit as DAC, makes the confusion...


Martijn DK:
PPM is not a continuous transmission signal, but transmitted in a 27ms pulse interval, so i'd say its digital.


Sindre J:
PPM is digital. The signaling is either 0 or 1. (We all know this). I would state that the PROTOCOL is what makes it convey analog values. Ie. the protocol is time-based, where as SBUS uses fixed time base and serial protocol to convey analog values. Both are digital as they can only transmit ones and zeroes, but they have different protocols.


Norbert T:
SBUS and PPM are both digital but ppm is less efficient, because the information is based on how long the signal is on or off, while sbus is transferring the actual number in digital format that given channel has, also has error correction built into the protocol.

John L.:
Digital of course. Because it contains encoded bits.


Ole L.:
It's a digital encoded protocol where you measure digital steps based on a timer. The digital resolution is based on the timer frequency. Nothing analog here !


Forrest A:
PWM is technically a digital encoding, sort of. It was originally created to mimic analog signals, but it is indeed a digital signal. It just only encodes a single value. PPM is a way of conveying multiple PWM signals. It is also digital. It's a bit laggy though



It's Analogue Signal!


David Winstall Podcase (shared by Jack N):

around The 1:11:00 mark best explanation I've had!

Benedikt S:
PPM in this configuration is a series of PWM -analogue spaced pulses in a sequence... so the only digital component in the signal is the synchronisation "bit" that indicates the start of the serial data - but the actual width of the pulses is pure analogue right ?
question is - is the position of the servo encoded in a digital code or not when it goes out as a ppm signal, my answer is not - it's decoded by measuring the time of the pulse - the FC does that and converts it back to a number. time is analogue

if you think about it - this method is way older than the digital radios - this is from the 27Mhz AM time when we had very simple radios with no TTL chips, the pulse width was just made with a oscillator from the position of the gimbals with normal potentiometers - think we still use old pot's - no optical encoders yet?



Rick V:
Digital is not ever-present, relies on a clock or timing signal and does not vary its strength.


Bob R.
The signal of each channel is determined by the width of that channels pulse. All channels are sent together in a compressed pulse. At least that's how I understand it. There is some delay depending on the pulse width which I think is about 27ms for 8 channels. Cleanflight based FC's usually average three pulse before responding so the delay is 27x3. Serial links don't have this dilemma. Also from this link: "A pulse Width Modulation (PWM) Signal is a method for generating an analog signal using a digital source."


Andrew M: 
Just because it uses high and low wave patterns doesn't make it digital smile emoticon Digital would be if it read 1 and 0 and turned that back into something. It uses the width which is varying length so instead of a wave form going up and down the length of the on pulse gets longer and shorter. That is where the analog comes in.


It's Both!

Eric Andreas S
PPM use the means of analog transfer from point A to point B (as all do) where the signal is either 0 (voltage < 1V. Note: Going down to 0V is pretty much impossible due to the heat it would generate) or 1 (voltage > 3,3/5V depending on hardware). This is similar to the way a CPU works where the voltage is an "analog" curve but the interpretation of this curve is digital, i.e. 0 or 1
depending on the sampling of the curve and the value of the curve at that time.


Justin N:
From what i undrstood, Ppm itself isn't anolg or digital, its a means to carry compressed information. Ppm can be used with digital or analog signals so shouldn't it depend on the component generating the signals?


Michael M
Just to clear things up, it is a bit of *both*. PWM is digital in the sense that it is not using a continuous range of voltages to represent its value, but instead pulses between a low voltage and a high one (typically 5V in our receivers). In that regard, it is digital. But, the signal it is transmitting is transmitted as an analog signal, using the time between digital pulses to store the data. This part is definitely analog, because there is an infinite amount of continuous values that can be stored in between the two pulses.

---

The question began with an email I got from someone, and I was shown a screen grab from wikipedia:

   

I thought it might be interesting to see what other people think as i wasn't sure 100% myself
However here was what I replied.

The list you are referring to is modulation, which to my understanding, is how the signal is processed and sent out (conveyed).

Digital signals have characteristics such as discrete amplitude and discrete time.
PWM and PPM are both modulated in digital forms (square wave), but the signal is determined by the width of pulse (how long it's been up or down)... so it's not strictly a digital signal in that sense....

That's why I guess you can argue they are either digital or analog signal depends on where you are coming from
But when you are also comparing to signals like SBUS, then PPM is very much an analog signal to me

+ for analog ((:

PPM is a digital communications signal. It meets the definition of a digital signal. The arguments against it being a digital signal focus on the pulse widths, which are not quantized. I will explain that modern specifications of the pulse width are quantized, and I will further prove that even the original specifications with continuous (not quantized) pulse widths are in fact digital communication signals.

First let us define a digital signal. As per Wikipedia (I have the second reference listed on Wikipedia so I can confirm): "A digital signal is a signal that represents, a sequence of discrete values.[1][2]" Bingo! PPM is a sequence of 0's and 1's. Hence, it is a digital signal. Digital signals are designed with discrete values that are sufficiently spaced such that there is a very low probability of noise changing the quantization. This describes the PPM concept exactly.

In original PPM specifications that were implemented by analog electronics, this pulse width is not quantized. That is, the pulse width could take an infinite number of values between a maximum and minimum pulse width. In modern implementations this is no longer the case. The transmitter of the signal is a clocked micro-controller that is sending pulses that are multiples of X nanoseconds. The receiver is a also clocked micro-controller that is (typically) aware of the X nanoseconds rule on the transmitter. This is a digital signal even by the restrictive definition used in digital electronics. That definition requires a fixed width pulse train on the output side. If the input side is implemented using analog electronics (not the case for flight controllers), then it is performing an analog interpretation of the digital output signal.

The definition used by digital communications is not as restrictive. They do not specify fixed pulse widths or sampling intervals because the primary benefit of a digital communication signal is to reduce the effects of noise during transmission. As PPM is a communications signal and not a digital electronics signal, there is no restriction on the pulse widths. Therefore any PPM signal can be called a digital signal in the context of digital communications.

In conclusion, PPM is a digital signal in the context of digital communications. For most modern implementations, it is also a digital signal in the context of digital electronics. There are some analog implementations/specifications in which the PPM signal is not comprised of smaller fixed-width pulses. Although these analog implementations transmit a digital signal in the context of communications, they do not transmit a digital signal in terms of analog electronics. As far as I am concerned, PPM is a considered communication signal and it is always a digital signal.

I think PPM isn't analog or digital, or both analog and digital signal. It's neither analog nor digital But things are much complicated - digital signal is an analog signal that has some special features that make it "digital". So the question itself isn't correct But who cares? Let's answer it anyway!

Actually PPM or PWM as well are discrete analog signals because they have some features of analog signal and some features of digital. The main thing that makes it not an common digital signal that transmitted "values" can float when EM interference occur. Under some conditions in some circuits pulse's front and tail can distort with bursts or deeps yielding that transmitted values can float or even synchronization can "slips".

The PPM signaling protocol is analog, but the signal itself is digital.