Analog video + digital commands and telemetry over a single pair tether - is it possible?

General Discussion
1

My last 2 ROVs were fully digital, with Raspberry Pi and PLC module acting as a converter to allow signals to travel over a single pair tether instead of the usual 4 pairs that are used for ethernet connections (link to how to do it).

Unfortunately, after spending months at it, I was never able to bring video latency below 150ms, and that is too much. I know that analog video cameras can have far lower latency, and analog video signal only requires a single pair cable, but the problem is that I also want to send digital signals (telemetry from ROV to laptop, and commands from laptop to ROV) over the same cable. I don’t need to transmit power, just data.

I know this should be possible, back in analog TV days they used to send digital data over the video cables in higher frequencies (for subtitles, etc.), but I can’t find much info on it.

Right now, my idea is to connect analog video camera on the ROV to that single pair cable, and then splice in the PLC module on both ends, but I’m not sure if it would work.

Here is a diagram of how I imagine it:

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Like I said, ROV is battery-powered, so it needs no power through that pair, just the data.

Is this possible? Or is there a better way to have analog video and digital data over the single pair cable? Perhaps I can wire things this way, but need to use some other type of device instead of PLC?

The reason I’m asking is because I already have 100 meters of very expensive single pair neutral-buoyancy tether, and I want to reuse it.

I know that back in the analog TV days, closed captions (digital data) were transmitted over the same analog video wires, either during the small time period during vertical frame retrace, on in a higher frequency that does not mess with the video quality. However, all these old devices are massive, and I can’t find any modern chips that would do the same thing, nor any tutorials on how to set the whole thing up.

2

Hi @laukejas,

Interesting idea. If the signals are carried on sufficiently different frequency ranges then I believe it should be possible, as long as all connected devices are able to withstand the voltage range on the cable, and all receivers have appropriate signal filtering to remove the noise from the other signal.

If they don’t have signal filtering built in then you might need to look into low-pass and high-pass filter circuits / modules, to cut out the undesirable frequency ranges.

This kind of approach requires either having a global controller that determines when each signal gets sent, or having the line monitored for space to send in (usually with some kind of prioritisation / re-transmission protocol for handling conflicts).

Power line communications modules already monitor the line so each end knows when it can talk, but I’m not certain how conflicts are handled, so if the video signal is in a conflicting frequency range and forces itself onto the line then that may cause some issues (or at least reduce the achievable data rates).

3

Look for Ethernet over coax adapters on Amazon. They will allow you to send 100mbps over 4k ft of RG59 coax. I use a security system dvr onboard to send 8 analog camera feeds topside while still having full control.

4

Appreciate your comment. I don’t have much to go on here though, I guess this kind of a solution isn’t documented anywhere. I don’t even know where to start. For now I will just try hooking up a pair of PLC modules to the analog video camera wires and see what I get.

5

Well, I’m happy to help. The (stripped-down, inside the ROV’s watertight chamber) DVR digitizes the analog video feeds and connects to a router. Your Raspberry Pi connects to the router. The router connects to the Ethernet over coax adapter. Coax runs to the surface. Connects to the topside adapter, then to your computer. You view the multiplexed video feeds in a browser window, and control the ROV through the computer as you normally would.
Use separate battery packs for the control electronics and the motors, to keep the control processor from rebooting if you run the motors hard.

6

You can get tiny cheap microcams for drone building off Amazon, Runcam or Eachine, put one in a PVC endcap, pot in clear casting epoxy from a craft store. If bubbles form in the epoxy pop them by waving a propane torch over the surface. This will allow the cameras to be mounted externally and take the water pressure. External servos can be filled with mineral oil to keep water out. Pot LED’s in epoxy the same way. You can get 624 lumen LED’s from Amazon 20 for $25.

7

Thanks for the suggestion, but I’m actually trying to avoid digitizing video feed because that introduces a lot of delay. Also, I am trying to avoid using Raspberry Pi on my next ROV because it would be too large for my enclosure, and it is generally an overkill anyway. Aside from that, your other suggestions will likely work. Since you’re suggesting coax - any cheap neutrally buoyant coax cable? I got my current neutrally buoyant (single pair) cable for 1€/meter, so I should either use that one, or find something that is comparable in price.

8

Sorry, I don’t do small ROV’s. My main WTC is 7" diameter and 3’ long.