After a bit of thinking about what else works at decent speed over a single wire pair, DSL came to mind.
Ethernet over VDSL2 isn't that new or uncommon, and can give >50Mbps at 500m cable lengths. So I figure at 50m through sea water (why don't spec sheets cover this scenario? ;-)) it shouldn't have a problem at all.
This does mean fitting a VDSL2 to ethernet bridge in to the OpenROV too, which is going to be rather tight.
So I'm going to try anyway and I'll let you know how it comes along. Steps (as I see them at the moment):
- Find the smallest VDSL2 CPE Bridge I can find that runs on <=12V DC (Netsys NV-202 or LevelOne VDS-0200 so far )
- Test it works over my tether on the bench
- Strip it down as small as possible (bye bye chassis and other sticky-outy-bits)
- Make sure it still runs in a small volume with no cooling air flow
- Measure if it runs cooler/lower current in CO or CPE mode if it's switchable
- Redesign the e-chassis so I can fit it all in the OpenROV, I see plenty of potential here
- Laser cutting with Ponoko (I don't have access to one, so 2 week turn arounds it is...)
Feel free to suggest other potential products or solutions. I'll keep you all updated on my progress.