Help with choosing a control system for my ROV

[rvanscotter]

On the newer Futaba's like mine you can select either PCM or PPM so that shouln't be a problem. Is polarity a problem with the raw PPM signal? I will look a RX and try it out.

[bikerbones1968]

Well that answers that question. Keep us posted as you test, and what receivers you try.

[derelicte]

pcm is a signal encoding method used over the rf link.

the data coming out of the trainer port is a ppm stream. the only variable is it a positive or negative pulse stream.

for example:

http://forum.tsebi.com/viewtopic.php?f=4&t=4

[bikerbones1968]

derelicte, your a wealth of knowledge!! Thanks for clearing up the PCM/PPM differences. I was having a hard time getting my head around how they differed.

[derelicte]

anytime. glad I could help.

[bikerbones1968]

Hi derelicte, I PM'ed you this question also but perhaps its best suited for the forum in the event others may experience this. I think I know what’s going on but thought you might have the answer.

As I reported the other day I tested the PPM signal down two conductors on a four conductor alarm cable to over 900 feet. It works perfectly.

Tonight I decided to connect my video camera to the same cable on the other two remaining wires. Between the RCA connector on the camera and the RCA connector on the LCD monitor I used two RCA to cat5 video baluns. I know I am trying to send that video signal a long distance, way more than they are rated but wanted to try anyway.

The camera is powered from the onboard batteries and the LCD monitor on its own battery. I was surprised that the video signal actually worked on that long of a run. The image quality was crystal clear.

Now here comes the interesting part. The servo relay began to chatter turning the lights on and off, the four ESC's that control the brushless out runners began to pulse on and off irregularly starting the motors and the camera servo was twitching like a dog with fleas!

My thought is that somehow the video signal is generating EMF on the unshielded four conductor cable and in turn spilling signal over onto the PPM signal (Crosstalk) as I understand it?

Your thoughts?

[rvanscotter]

That's not good. Would a ferrite choke help?

I wanted to do basically the same thing, PPM on a twisted pair and Video via baluns on a CAT5e. Did the cross talk occur on the 900' tether or did you go back to a shorter tether?

I'm starting with a 100' tether and I was thinking that perhaps that would be short enough to avoid disruptive RF interference.

Most real ROV tether has a sheilded mini coax for the video feed. The balun solution, as good as it is may be, may not work well on longer tethers.

If you can try various shorter tethers, maybe you find the critical length were CT/RFI takes over and makes our ROV's go catywomphis.

[bikerbones1968]

It works fine on a standard Cat5e cable, twisted pair. I tested it yesterday on 400 feet long run, the longest run I had on hand. The 900 foot cable was low quality untwisted four conductor. My actual tether is going to be shielded twisted pair and thus no issues with crosstalk.

[rvanscotter]

Thats good to know.

I was looking at two tether suppliers Falmat & Outland Technologies for their specs. OT uses a mini COAX for video while Falmat just uses a TP-24awg so I figured a there has to be a way to make it work.

[bikerbones1968]

I am still going to throw a ferrite choke on the Video cable on the ROV/camera end just to be sure I never see any issues. I contacted Outland for prices on cable systems. It not cheap that's for sure. #3.70 USD per foot and they sell minimum runs of 500 feet.