Bob Cranston on how his HD camera captured the sound of diving with sharks

Secret Shark Pits
 Off the Fence/Talking Pictures

The challenge Getting pictures and synchronous underwater sound from a remote dive site in a shark pit off the coast of Mauritius.

One of the things that makes this programme unique is that we had zoologist Ryan Johnson and local Mauritian dive master Hugues Vitry talking underwater with full face masks using wireless communications.

We wanted to show Ryan and Hugues interacting with real sharks and talking in real time, because it made what they had to say more personal, emotional and engaging for the viewer.

Most underwater photography uses voiceover commentary because it's difficult to get good sync sound. It has been attempted before by using cables, or by getting a sound recordist to record the output of the communications box on the boat, but in this instance we couldn't do this because the boat was too far away. We had to swim a long way from the boat through channels in a reef to get to the shark pit, so wireless recording technology was the best option.

This is where my uniquely adapted Sony HDW-F900 with Fujinon lens and own underwater camera housing came in. It works just like a topside HD camera but it has a microphone in the divers' masks. I have a wireless receiver developed by Ocean Technology Systems designed for underwater use mounted on the outside of the camera housing, which I have adapted so that I can adjust the sound to different recording levels.

One of the biggest challenges with underwater sound is minimising bubble noise. There's a lot of exhalation that goes on the soundtrack, so you have to practise talking and breathing underwater to minimise noise. One of the things I do to prevent adding the bubble noise is to use a closed circuit re-breather system so that I am not exhaling gas into the water.

We are trying to get masks that are more open-face so that we are able to see more of the diver's expressions. Most masks currently available cover up the nose and mouth, which makes life more difficult for the sound recordist - it sounds as if you are talking into a box.

This is one of the most extensive uses of underwater wireless sound and it's something that I think will catch on.

The end result is that under-water wireless sound has allowed us to make the film more personal. Hugues, the local dive master, has a big emotional attachment to these sharks and it helped him get across what wonderful animals they are.

Secret Shark Pits airs on 11 December at 10pm on Nat Geo Wild.

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