It isn't only the astonishing durability of Luciano Pavarotti's voice that has defied all expectations. When he was first making his name not a few critics and other experts in the music profession diagnosed the voice as a fundamentally lyrical instrument, and predicted trouble if he attempted heavier roles. He himself used to say that although he was a big man he did not have a big voice, and there were numerous parts which for a long time he refused. At La Scala (as also at Covent Garden) he made his debut as a last minute replacement for another tenor, in both cases singing Rodolfo in La bohème. When, for his official Scala debut the house made him the flattering offer of Arnold in a new production of Guillaume Tell, he refused, and when the same theatre mounted Bellini's I Capuleti e i Montecchi, making the controversial decision to cast Romeo as a tenor, he was evidently not offended to be given the less demanding role of Tebaldo. Opera houses had to wait until he was ready before he would consider Manrico, Ernani or Radames.

These heavier roles, when he took them on, did not greatly darken his voice or cause its upper register to retreat. Indeed the tenor who was later marketed as 'the King of the high Cs' actually found the highest notes much more awkward in the early part of his career than later: in a live recording of his Scala Capuleti of 1966 he is audibly taking a risk with some of them and even in 1969, in a live recording of 'A te, o cara' from I Puritani, the notorious high C sharp is terribly precarious and evidently a great strain. Some people hearing him then may have shaken their heads; singing teachers among them might not have predicted a long career.

It has lasted 40 years (and counting); we must admit - and how it hurts a critic to say this! – that Pavarotti knew best. In retrospect it's pretty obvious that he did from his 1969 recording of Mascagni's L'amico Fritz, which has ringing tone in Fritz's despairing Act 3 monologue, but also a bel canto limpidity (and a beautifully floated head voice) in the early scenes with Mirella Freni's Suzel. Vocal self-knowledge is the secret of vocal longevity, but to have achieved for so many years an effortless upper register after such evident early difficulties, and to have cautiously taken on heavier roles in the teeth of pleas not to, argues not only uncommon self-knowledge but considerable determination and hard work. The first and perhaps the greatest pleasure of Pavarotti's finest performances and recordings has been that of hearing a voice in flawless condition and of remarkable quality , produced with absolute security throughout its range.

Among the other qualities that distinguish him, which make his sound instantly recognisable, the fact that his is an Italian voice stands out. Many of the other tenors of his period have had their own qualities, and several have been more versatile, but with the exception of Carlo Bergonzi none of his distinction have been so evidently made for Italian opera. Another and related characteristic that singles him out is his sense of unbroken, cantabile line, despite the fact that his diction is flawless (he would deny the 'despite', probably: for a well-schooled Italian singer diction and cantabile are not in conflict). In his best performances he has been an effective vocal actor, and it no doubt says something about his character that several of those performances have been in roles that require a degree of boyishness, of good humour, even of exuberance - and this despite the fact that for a good many years now he has been an awkward figure on stage.

One of my most enduring and endearing memories of him is of an Elisir d'amore in which, because of his bulk and bad knee problems, he rather seldom left the stage, even in scenes where Nemorino should have been absent. Making himself as unobtrusive as possible (which is to say not very unobtrusive, of course), in effect becoming a member of the chorus and entering into the spirit of what they were doing with good-natured enthusiasm, he was in fact the most accomplished actor on that stage. And, by the way, does any singer currently before the public have a more expressive face?

If one had to admit to a flaw in his singing it would be that he has quite often been too generous with his tone. As the Duke in Rigoletto (I refer to his 1993 recording; there are of course others) he misses something by singing 'Parmi veder le lagrime' so amply; it is a splendid sound, but when he reaches the cabaletta, 'Possente amor mi chiama', he can do no more: at that volume he cannot find another vocal colour, so the two sections of the aria sound rather similar. In that same performance, however, it is his good nature and sheer enjoyment of what he is doing that gives an edge of amused derision, in 'Questa o quella', to the Duke's dismissal of constancy as a 'cruel malady'. It has to be said, of course, that by being prepared to use less voice, Caruso in 1904 found more light and shade in 'La donna è mobile' and Gigli in 1934 a playful, almost feline wit.

Lofty comparisons, but he deserves them and often emerges from them with distinction. In his earlier accounts of Un ballo in maschera, of which perhaps the best is the first, with Tebaldi in 1970, the absolute ease and smoothness of the singing throughout is superb - it is one of his very finest recordings. He does, though, sing 'E scherzo od è follia' in the conventional way, amused and with a few (not too many) interpolated laughs. It is worth seeking out a much later (1991) video recording of a production at the Metropolitan Opera, albeit with the rest of the cast not on his level, where he now sings the aria (perhaps at the suggestion of the stage director Piero Faggioni?) as a vividly acted cameo of disquiet and foreboding hidden under lightness of tone.

One even more enlightening comparison can be made between a quite early (1969) recording of the aria 'La mia letizia infondere' from Verdi's I Lombardi and Pavarotti's quite recent (1996) account of the complete opera.

Even the dates are significant: of how many other tenors could one compare recordings made 27 years apart without making allowances for immaturity on the one hand or for declining vocal resources on the other? In 1993 he had selected I Lombardi to mark the 25th anniversary of his Met debut. A shrewd choice for an ageing tenor with mobility problems on stage: Oronte is Verdi's shortest starring tenor role, making his first brief appearance in Act 2 and dying in Act 3 (he reappears in Act 4, as a vision, for precisely two minutes). But this apparently slight role is a demanding one: that entrance aria in fact requires suave line and elegant flexibility, and it is followed by a cabaletta with few fireworks (save a high C) but which calls for even smoother singing and if possible even sweeter tone. A very useful role, in short, for a young lyric tenor feeling his way into Verdi, a distinctly risky one for a singer in the latter stage of his career. Both performances are beautiful, but it is the 61-year-old Pavarotti who takes the additional risk of singing that cabaletta distinctly more slowly than he had done before, smoothing its cantabile still further by not taking a breath at the end of the first line, thus registering that while the first part of the aria speaks of love, the second expresses religious ardour.

Some have said, and there is some justice in it, that he makes much the same sound in whatever role he sings. This is perhaps especially noticeable in the concerts of operatic arias and lighter material that have become his principal activity in recent years. In the context of an operatic performance, however, especially of an opera that he obviously cares for greatly, or where he is working with colleagues whom he likes (notably Dame Joan Sutherland, of course, but also Mirella Freni, a friend since their school years) he can be more subtle than that. Not by totally transforming his sound, of course, but by using the text to colour that sound. You may not always agree with the result - I personally find that the vehemence he adopts in Edgardo's 'Fra poco a me ricovero' in Donizetti's Lucia di Lammermoor disturbs his line - but there is no doubt of his intention: to express the character's predicament, and the sound here cannot for one moment be confused with that which he adopts for the lovelorn Tonio in the same composer's La fille du régiment, the courtliness of his appeal in 'Pour me rapprocher de Marie' charmingly suggested by elegant portamento. And Fernando in La favorita is another sort of soldier entirely.

One could wish that his repertory had been larger, of course. Only four Bellini operas, and very little of the French repertory (very little indeed in French: when he eventually did get round to Guillaume Tell it was in Italian). And with very few exceptions it was standard Italian repertory. We have perhaps no right to complain - it was what his voice was intended for, after all - but his few excursions outside the standard repertory L 'amico Fritz, Donizetti's Beatrice di Tenda – make one regret that there were not more of them, especially since to some of those 'standard' pieces he brought no special insights. His Turiddu in Cavalleria rusticana and his Canio in Pagliacci are richly and glamorously sung, but one only has to turn to Gigli or Martinelli to hear how much more expression can be drawn from the former's 'Mamma, quel vino è generoso' or to the young Caruso's aching pathos in the latter's 'Vesti la giubba' to wish that Pavarotti had found more roles which his particular qualities – above all his engaging humour and the sudden tenderness that illuminates his Rodolfo - would have enriched.

But the word 'generoso' having been mentioned one should pay tribute to that quality in him. In both senses: how considerately he partners Joan Sutherland in the duet 'Ah morir potessi adesso' in Ernani: it was their last recording together, she was close to retirement and could not have responded to trumpet tones from him. But also in the sense in which Turiddu uses it. The word is variously translated: 'heady', 'powerful', 'ardent'; my Italian dictionary adds (and so would I) 'liberal', 'open-handed', 'high-spirited' and 'noble'.