These two DVDs are the stuff of legends. They derive from Austrian TV broadcasts of concerts at the Vienna Festivals in the respective years. Knappertsbusch had given up conducting at the State Opera and at the Salzburg Festival when Karajan was put in charge of both organisations in 1957, but he continued to conduct the VPO. In his mid-seventies at the time of these concerts, he remained indomitable and uncompromising. His rangy, slightly unkempt looks tell of his refusal to become a personality or indeed have much truck with audience applause: in every case here, after one bow, he leaves the soloists to take the limelight.

He is at his most pawky in Leonore 3, the reading slow and undramatic. Once joined by the 78-year-old Backhaus, he is transformed. Backhaus gives an interpretation of the G major Concerto that is a distillation of all his years in the composer’s service. The reading is illumined by his grasp of colour, his delicate execution and his sheer joy in purveying the composer’s ideas as if he had alighted upon them for the first time. In the finale the years fall away as the two veterans make the music fleet and dance-like. Backhaus rightly receives a spontaneous ovation.

For all that, the extracts of Wagner are the invaluable documents. In each case, the con-ductor confirms his mastery in creating long paragraphs, persuading an orchestra that obviously respects him, to produce a concentrated, saturated sound and creating an organic whole. The results are achieved with the minimum of gesture: a firm lead here, a nod there and miracles are performed. Both in the Liebestod and at the end of the Walküre love duet, you sense that incandescence that should be, but seldom is, a sine qua non in these passages.

Nilsson’s voice emerges out of the stillness of the Tristan Prelude in glorious form, steady and gleaming in Isolde’s transfiguration. Then at the height of her powers, she discloses her amazing facility as regards technique and a control of nuance, both virtually unheard-of today.

In 1963, it is another soprano, the American Claire Watson, then active in Munich, who impresses so much as Sieglinde. This is a wonderful souvenir of a role she took at Covent Garden at around the same time with just the unassuming musicality and beauty of tone she shows here. Her phrasing, her feeling for and her understanding of every facet of the role make this a luminous, unforced interpretation. Fritz Uhl, her Siegmund, is another artist who exhibits innate musicianship, and an idiomatic command of the text, but in his case a slightly gritty timbre is a drawback. Josef Greindl’s commanding tone and ideal enunciation reveal why he was such a prized Wagnerian bass at the time.

The camerawork on both occasions is rudimentary, the sound a shade restricted, but who cares about that or the black-and-white format when the music-making is god-like.