[thegeek]

I'm fairly sure this is a first - putting out live AD via the AD track. Live audio description has been done before, for example on BT/TNT for the FA Disability Cup or on the BBC for Strictly Come Dancing, but using a red button channel. I might be wrong but I can't think of a time it's been done this way before.

I think ITV used the AD track for the Coronation, though their approach there was to squeeze descriptions in between the existing commentary rather than a whole new mix. I guess if you left the AD control track set to fully fade out the main audio for the whole match, you could put a full mix + effects on the AD audio track it would work - though the AD audio track tends to be very low bandwidth, so perhaps they've managed to allocate some more to it. (Also last time I looked into it, there wasn't anything commercially available which could generate the AD control track live, but perhaps they've got something custom made.)

As you say, BT/TNT and the BBC's approach has been to use a Red Button channel with an entirely separate mix.

BT Sport had an Ofcom ruling against them in their very early days for not hitting their AD quotas on ESPN UK (under its previous ownership). They said they'd considered using radio commentary as an AD track, but had deemed it not suitable because they're trying to achieve different things.