[thegeek]

The original Invisidot inserted OP47 data in the VANC of the SDI stream, using off-the-shelf kit from Axon. The encoders were loaned out to studios and OB facilities providing live programmes, but after several years of being shipped around the country were starting to get a bit flaky and I think the pool of working units was getting down to single figures.

I think EVS still make an OP47 inserter card, but ITV have gone down the route of commissioning Invisidot 2. It's a custom bit of kit that generates an audio tone that sounds not dissimilar to the Audio Description signalling tone or LTC (in fact it wouldn't surprise me if it was the same kind of encoding) and is embedded on one of the higher tracks of the distribution feed.

Here's a little bit of spiel from the manufacturer:
bendentech.co.uk