Emerson String Quartet, Queen Elizabeth Hall, London

The classical recording industry has belatedly woken up to the idea of promoting concerts to sell CDs. True, the two nights of the complete string quartets of Mendelssohn came under the umbrella of the South Bank Centre's current Mendelssohn festival. But one hopes that Deutsche Grammophon - and the Emerson String Quartet, whose release of all these works on DG coincided with these concerts - made the South Bank an offer they couldn't refuse.

There are, however, dilemmas. Those sitting in the Queen Elizabeth Hall who afterwards shelled out £40 for the four-CD box (a bonus disc includes the Emerson playing "with themselves" to make up the forces for the Octet, plus an explanation as to how they did it) will be pretty surprised at how different it sounds. Clever twiddling of the knobs does not disguise weaknesses. Certainly, it all sounds much louder than in the hall - but with microphones up the nose, what wouldn't?

But intensity is not volume, and what was sorely missing in the hall is also missing in the recordings. Slow vibrato as manifested by Eugene Drucker - sometimes first fiddle, sometimes second - doesn't get speeded up, either. (The Emerson, an American quartet, take democracy very seriously, so leading a work is fairly divided between the two violinists). Balance can be fixed in the recording booth, but in the concert hall it must be fixed instantly and constantly. And making studio recordings is anti-risk: any blemishes or blunders are removed. But concerts are not like that. Risk means spontaneity, improvisation, the sacrifice of detail in the interests of the whole.

Mendelssohn's music is mainly glorious. The six major quartets straddle his short (he died at 38) lifetime. Here, the Emersons played them in chronological order of opus numbering, which muddles up the actual order of writing: Mendelssohn's staggering Op 13 in A minor is, in fact, written before Op 12 in E flat, and the three Op 44s are not written in the order of numbering as published. The CDs, however, play chronologically, as written.

Mendelssohn's style is, of course, driven, passionate, urgent, the music of a prodigiously talented young man. His works reflect Beethoven - particularly the A minor aping Beethoven's Op 132, and the F minor Op 80 his Op 95 - and, though they never lived in the same city, the aesthetic of Schubert. Restless, syncopated accompaniments support miraculous melody, wistful and melancholic, never far from the minor mode. And then there are those amazing scherzos. But only Lawrence Dutton, viola, could be counted on always to capture the moment, beautifully shaping often-repetitive material, showing how varied the patterning can be. And only Dutton, in his body language, really swung. The second night was better than the first. For Mendelssohn's sake, buy the discs.