SPP PPM problem

[Lindsay]

Using XP and a Futaba with PCM I have had a good stable input for FMS but I can't get a stable input under PPM with either the Futaba or my new DX7. There is a signal showing with winscope and I have tried all input 'volumes'. The symptoms are a very twitchy/jumpy control signal which makes the SPP calibrating bars jump up and down at random.

Looking at the winscope signal, should I be able to get it to lock? I can't find a trigger that locks the signal. There is also ringing showing at the top and bottom of each pulse. Is this normal?

Any ideas on how to get a stable PPM signal welcome.

Lindsay.

[shaul_eiz]

PCM is much more stable since it is a digital format while PPM is analogue (on the temporal axis). The problem you describe might be a real one, this can be tested with a sensitive receiver and a very fast servo motor.
However, if the signal is totally random, that is, much more than mere fluctuations around a correct value, then the problem is a different one.
Let's go back to your Futaba transmitter. If it works fine in PCM but stops working (after re-calibration) for PPM then we know that the problem is not an electrical one but related to the decoding. Did you remember to switch SPP to PPM? Did you try both types (Negative/Positive)?
Winscope cannot lock on a PPM signal because it is not 100% repeatative. You will have to lock it manually.
The ringing on the top might be some noise caught by the un-shielded cable. If it is not in the order of magnitude of the signal itself it is probably unimportant.
Please let us know how it goes

[Lindsay]

Thanks for the reply, Shaul. Yes, I switched SPP to Generic PPM and tried both negative and positive (it is negative and doesn't work at all on positive). The ringing is of minor amplitude compared to the signal pulse so is unlikely to be the cause of the problem.

With extensive calibration it seems to be improving but it is still pulsing. I can see the plane twitching and I can hear the sound pulsing. It pulses 4 (sometimes 5?) times then pauses before repeating the pattern. The 4 pulses take about 2 secs.

The sound card is a Creative Audigy SE but I am sure it did this with my older aureal card, too.

[shaul_eiz]

Since you did not change anything but the modulation - it is probably the fault of the signal source, the transmitter. But since the problem occurs in both transmitters, I tend to blame the cable. Is it very long?
Or, perhaps you have too high expectations from PPM? It is less accurate than PCM.

[Lindsay]

I use two cables, one for each Tx. They are both 1.5m long.

I don't know about my expectations being too high but it is just not a good training tool as it is. In no way is it realistic. Coming in to land with engine surging the plane glides in, surges up again, touches down, jumps back into the air etc. Not to mention the jerking of all the other control surfaces! Keeps you on your toes but I'd rather have a stable signal.

I thought to sell my 8UAP now I've bought the DX7 but maybe I have to keep it for its PCM ability as I do want to keep up my practice on the fsim. Or perhaps look at another interface. USB? I want to permanently switch to Vista very soon, too.

Cheers and thanks.

Lindsay.

[shaul_eiz]

1.5 meters is is not too long.
PPM should not be as bad as you describe.
I'll be sorry to lose a user, but you are right to be unsatisfied.

[Lindsay]

Shaul, I have just tried both Tx's and cables on my wife's XP machine via the mobo AC97 sound setup. They both work as expected! There is no perceptible instability and I can clearly see the difference in signal stability in the calibration screen.

So about the only difference is the sound card in my main machine. Damn. I might try feeding it via a resistor to damp it a bit to see if it makes any difference. In any event, I at least have another option to use the other machine.

Lindsay.

PS You haven't lost me yet

[Lindsay]

Success, Shaul I simply fed the signal through a 27K resistor and it calmed everything down superbly. So I guess we have to put the problem down to an overloading of the sound card front end. I guess I should have thought of this earlier.

Do you have any plans for trying to get winmm.dll working under Vista? I suppose it is pretty difficult if you haven't got it on your machine.

Great program - thanks for everything,

Lindsay.

[shaul_eiz]

I do not plan to move to Vista soon. I'm short of time and means at this point.
However, I toy with the idea of moving to DirectX and eliminate the need for PPJoy (which is used for the Generic version of SPP) and create a unified piece of software that will run above DirectX regardless to the OS. But this also will need a lot of time.
Glad you solved the problem.
Shaul