He explained: "A Two Ronnies sketch might spark a completely irrelevant memory, such as 'My father wore an overall like that in the garden...'
"At its best, it's like there's no dementia: the person is talking away normally, and it's quite wonderful. Even a son or a daughter who knows them very well might hear a story they've not heard before."
BBC news and entertainment footage dating from the 1930s to the 2000s has been assembled - alongside scenes and sounds from ordinary life - into what's been called a Reminiscence Archive. But it does not include full shows that dementia patients would not be able to follow.
Dr Alm stressed the material, accessed with a tablet computer, only helped unlock existing memories, and did not enable dementia patients to recover lost brain function.
But he said the project did help lift the mood of sufferers.
"Imagine your memory being wiped clean every ten seconds," he said. "It's horrific because you can't read a newspaper, watch television or hold a conversation with anyone.
"But long-term memories are stored in a different way and are still in the brain, if you can only get to them."
Peter Rippon, the ex-Newsnight editor who heads the BBC's online archive, said the Corporation's interest in dementia was "not altruistic".
In a blog he wrote: "The number of people living with the condition is already approaching a million and rising fast. If you add in the numbers of carers, family members and friends who are close to people with dementia, it is a sizeable chunk of our audience.
"It is our heartland audience, too. The age groups most affected are some of the heaviest consumers of what we make."