A group of engineers in Lancaster has been using an ADSL chipset, not for its intended purpose of telecoms, but to move data and video over a cable for underwater surveying.
Subsea Networks is using the MTK-20150 ADSL chipset from Alcatel Microelectronics for the link between its sonar and instrumentation pod and the remotely operated vehicle (ROV) or survey ship.
The link between the main ship and an ROV can be up to 2.5km long, and this has traditionally limited the data rates to around 100Kbit/s, which is too low for video, says Chris Edwards, technical director at Subsea Networks. The company of 12 engineers was formed in April from the R&D department of Norwegian marine equipment maker Kongsberg Simrad.
The ADSL-based system runs ATM networking protocols through the chip's Utopia interface and can handle up to 8Mbit/s back from the ROV to the ship and 700 to 800Kbit/s down for control data.
While many large ROVs use a fibre optic link from the ship to a 'garage' that sits on the bottom of the sea, there is still the need for a copper cable link from the garage to the ROV itself.
Using a copper link with ADSL means that power can be sent down the same cable, says Edwards. But this can use voltages from 300 to 3000V to compensate for the voltage drop along the cable, so needs careful filtering before the physical interface of the ADSL chipset.
The system uses a wavelet-based ADV601 video compression chip from Analog Devices, rather than standard MPEG2 compression.
Edwards said: "The ADV601 offers the ability to dynamically change the compression ratio and compress on a frame-by-frame basis. So it works even if the link is slower because you don't necessarily need realtime updates.
"Some of the algorithms used in MPEG2 can't cope with a snowstorm underwater, which is what we can sometimes see."
Alcatel helped with the design and testing of the system, which is now going into pre-production.