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Heroes of Comedy (C4) is not just a celebration of comic genius, it is a kind of diagnostic device in itself. The test it applies is this: can the performer in question resuscitate your laughter after it has been subjected to the slightly stifling analysis that surrounds every clip? This isn't to argue that the analysis is bad per se - in last night's profile of Alastair Sim there were several contributions that brought his talent into a much sharper focus, and said wise things about comedy besides. But the sequences in which admirers dissect the work of the comedian in question call into play a rather undermining kind of vigilance - you watch the performances that follow for evidence towards an argument, and it is all too easy to forget that you are meant to be laughing. It was a mark of Sim's talent that he reminded you, even when the rhythm of his performance was interrupted by a chuckling running commentary.

The programme ended with pious reverence that would probably have had the subject himself squirming with embarrassment - but it was difficult to see how it could have been otherwise. It is not fashionable to make simple moral statements these days, but Sim was the kind of person who elicits them. A generous and good man with a tangible ethical solidity, he was sceptical about celebrity and never gave an autograph - not even, according to one account, when it had been requested by a sick girl. The anecdote should have prejudiced you against him, but it didn't. It simply seemed consistent with his other slightly oblique principles, such as his insistence that he received no money until the cost of a production had been covered, at which point his own sizeable percentage would kick in.

And even if you felt that admiration had got the better of proportion on some occasions (Stephen Fry's account of the actor's rectorial speech to the students of Edinburgh made it sound like a combination of the Gettysburg address and the Sermon on the Mount, with some great gags thrown in), you were left with a persuasive account of someone who deserved the slightly unfounded affection we feel for people who make us laugh. He was very funny, too - from the absolute control of the voice (it was hardly surprising that Fry should admire him so much, given that he is the most obvious inheritor of Sim's richly savoured articulacy) to his genius for losing control. Nobody specifically mentioned the trademark hoots of alarm which often punctuated his flurries of consternation, but there were two fine examples here, and they both forced an echo out of me.

The feelings of nostalgic regret aroused by Heroes of Comedy was amplified by the fact that it immediately followed Planet Showbiz (C4), an altogether more callow exploration of the world of entertainment. This started reasonably well, with what purported to be a breach of convention. "Tradition dictates that this is the first minute of the first show of the series and I should say exactly what it's all about," said Mark Lamarr from the wheel of a huge pink automobile, "But this is too brilliant! You've got to watch this". It cut to a dazzling performance by a young New York street magician, a display of ungarnished sleight of hand which actually managed to justify that entirely synthetic urgency (partly because it showed you native New Yorkers, who value their insouciance very highly, in a state of boggled astonishment).

What followed was a good deal less entrancing, though - a Eurotrash sensibility applied to the evanescent oddities of American trivia culture. There was an interview with the student inventors of "Six Degrees of Kevin Bacon", a game which requires an encyclopaedic knowledge of the cast-lists of Hollywood movies and a visit to The Go-Nuts, a surf band whose repertoire is aimed at increasing the consumption of junk food, and whose stage shows are enlivened with the Snack Cannon, an application of rapid-fire technology to the food fight. It was an appropriate subject given that the programme itself is a fast-food bucket for television bulimics.