PPM signal (SmartPropoPlus)

[Adui]

OK Im using the SmartPropoPlus for FMS, version 3.2.2. I plugged my JR Quatro into the line in on my PC audio via a factory Buddy cord.

It works good, more or less. I get some jittery feelings noticable on a few sport models, and my trimms dont stay set. (It rolls slightly left so I add right trim on the Xmitter, then the next aileron movement reverses the problem and I have to give it a click or two back left.)

I ran the PPM Thermometer, all looked good. I run Winscope and see two issues.
First, my signal is NOT clean like the example, Its a jagged spur.
Second, its reversed.. The example shows the flatline on bottom, my flatline is on top. (Since the thing works I assume this is a non-issue, and am asking out of curiosity)

Like I said the thing works fairly well, Im just trying to learn some methods for tweaking to make it better. Im going to look into a simple inductive filter on the trainer cord (Ham radio term for adding 8 to 10 loops to the cord near one plug or the other).

I dont want to cut into it if I dont have to since it is a factory JR buddy cable, but if buidling a true filter becomes necesary I imagine I could come up with the parts to do it as an add on between the cord and the PC, I just need to know what frequencies to make a bandpass for, then research it a LOT.

[shaul_eiz]

The 'reversed' PPM signal you see on the scope is a JR-type PPM (Some call it positive) while the example shows a Futaba-type PPM (Negative). Both the thermometer and SPP are blind (by default) to the difference.
I wonder if the jitter is felt with the thermometer too. If so, we probably have a hardware problem.
It is hard to tell how bad is your bad signal. The reason to the jittery behaviour may be due to this, but it can also be due to genuine unstable PPM signal coming from your transmitter.
I recommend that you try to reduce the transmitter output current and the sound card input voltage by using a simple voltage divider. Something like this:


I'll be interested to learn if it improved the simulation experience.
Shaul

[Adui]

I printed the drawing and will build it soon as I can, and try it. Thanks for the info, I will let you know if this (or my filter idera) fixes things.

[shaul_eiz]

The exact values of the resistors are really unimportant.
Just stick to the following two rules:
1. Make the large (10K) resistor at least 4K but not over 100K
2. Keep the ratio between the small/large resistor around 1/10 (1/5 to 1/20 are ok).
So take just any two resistors you have that adhere to these rules.

[Adui]

[Adui]

Well the attenuator didnt help at all.. Noise is still on the signal, its just a smaller signal now.. I wasnt overdriving to begin with so that didnt help.

I guess I need to read up on noise filters in the audio range and build one..

[Adui]

You know, in retrospect I believe what I am seeing is a normal signal from my transmitter.. I think the jitteriness is nothing more than the performance of the more sporty models.

The trims are perfect, and handling quite docile and predictable in the trainer and slowfly models, so I must conclude that the signal is good and the pilot needs practice, nothing more..

[phildc]

If you are still (slightly ? ) in doubt about the signal, take a 'snapshot' with the function present in the thermometer, and send it to Shaul. You may send a copy to me too, if you want.

Phil.

[phildc]

The soft to look at a snapshot:



More info in the "Diagnostic techniques and tools" topic:




Phil.

[shaul_eiz]

[phildc]

The two algos used in the snapshot viewer, giving the red and green lines are published here:


Jitter tolerance?
Audio sampling is usually at 44100 Hz, which makes a 22.6 usec sample time.
Depending on the code, jitter below this value (or half of it ?) may be hidden.
If a 'smoothing' algo is used (e.g. averaging with previous value(s), the jitter is also reduced.

Phil.

[Adui]

Well I need to figure out how to take a snapshot in the Win-scope program.

Your image there looks pretty much the same as mine, inverted. However, Where your peaks are basically flat lined mine are saw-toothed and slightly angled down.

I really think my hardware is working correctly and its just pilot error and not the interface since the more tame models fly well and its just the more sporty models that I'm having problems with..

If I get a snapshot taken I will upload it for you to examine, otherwise I'm not going to worry about it anymore. (OK so I will, I'm paranoid LOL!)

[phildc]

A 'snapshot' is taken with
Just press the 'camera' icon when the ppm signal is recognized by the thermometer. You get a xxx.aud file.

Shaul prefers a pure '.wav' recording.

The snapshot can be viewed by another software as already explained.


Phil.

[Adui]

OK now I feel dumb LOL! you both have tried to point that tidbit out at least twice and I missed it. I will just for the sake of being sure start everything up this evening, take the snapshot and post it here for you to see.

Thanks again for all the help!

[Adui]

Well I took the snapshots, but I cant figure out how to upload em here.. I will work on it and get back to you.

I will note that signal doesnt look near as dirty with that viewer as it did with Winscope. Its ALMOST the same as your examples..