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Comment by anfractuosity | Hacker News Reader

Would I be right in thinking a tether would be necessary with a ROV for a high bandwidth live video link as ultrasound seems only usable up to 100s of kHz.

I guess the only high bandwidth alternative might be lasers?

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Really bright LEDs can be used for underwater optical communication up to 100 m[0]. I'd really like to see someone make this tech into an FPV system for RC subs.

[0]https://optics.org/news/11/9/36

Cool, that's very interesting! I also just found this using an array of ultrasound transducers https://group.ntt/en/newsrelease/2022/11/01/221101a.html for standard definition video.

It'd be kind of sketchy to have as a single point of failure in an underwater ROV that might not be able to emergency-surface straight up (e.g. wreck or overhang exploration).

There's a ton of gnarly stuff that can happen underwater acoustically without obvious warning (salinity, temperature, topography, etc etc).

>>LEDs can be used for underwater optical communication up to 100m

Maybe in the clear open sea of the Caribbean. Not all water is perfectly clear at any wavelength. I give it something more like 100mm through turbid river outflow.

Wireless signals can drop off very quickly even in clear water. Add dirt, algae, turbulence, etc to that and it quickly drops off in about a meter for reasonable devices. For our bot we had about 3 1080p video streams being sent back over the wire, which requires a decent amount of bandwidth (we used a small fiberoptic ethernet switch on the surface and inside the enclosure).

Apart from that, if your ROV is light, and the cable is durable enough, the cable can make for a good retrieval method when you brown out the bot haha.

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