A gap is a word budget, and it is smaller than it looks.

Audio description goes in the silences between dialogue, and what a silence is worth is not its length. It is what is left after a handle at each end, times the delivery rate, rounded down — because half a word is no words. At three words a second a two-second silence holds four words, and a half-second silence holds none at all.

The engine is free and needs no account. The handles come off every gap regardless of its length, so a short silence is not a small budget — it is no budget, and twenty half-second gaps carry nothing between them. Which is why a programme can look full of pauses and have almost nothing describable in it. And what needs describing happens while somebody is talking: the action, the entrance, the caption all land during dialogue, and the first place anybody is allowed to speak is afterwards, so description is structurally late and the only question is by how much. The consequence is the one nobody expects — a film can have 2.6 times the room it needs and still deliver five of its seven descriptions late — two of them by more than a minute — because all the silence is in the tail. A surplus in the wrong place is not a surplus. Every lane ships a worked example that costs nothing. Writing the report needs credits.