The process by which Hector Berlioz’s image changed from that of a maverick on the margin of musical history to that of a great composer had various causes – but one crucial event. Historically, there were many reasons why he should have been misjudged.

Who was Berlioz?

He was a 19th-century composer whose lucid, linearly conceived orchestral music was impervious to the influence of that dominant 19th-century instrument, the piano and its all-important sustaining pedal.

An heir of Beethoven who wrote symphonies outside the Austro-German symphonic tradition, Berliozwas a composer who treated harmony expressively rather than functionally and whose style, against the current of the age, was based on extended melody and rhythmic irregularity; a revolutionary Romantic who had deep Classical roots and a passion for Gluck.

Such characteristics, along with the often formidable technical difficulty of his music, formed serious barriers to understanding him.

Berlioz remained for a long time, for most people, disconcertingly strange, outlandish. It was easier to pigeonhole him as a writer who dabbled in composition, author of brilliant criticism and of an amusing if also disconcerting autobiography – that odd mixture of wild enthusiasm and caustic irony which is the Berlioz Memoirs.

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As a composer, Berlioz had to wait for a time (as I wrote in my biography of him) when ‘all epochs are potentially equal, all styles admissible, one thing is no longer judged by another, and the only laws a piece of music must be true to are its own’. His music needed to become familiar, and it has done, thanks to much more frequent performance and the advent of the long-playing record and the CD.

That crucial event was the production of Les Troyens under Rafael Kubelík at Covent Garden in 1957. It was the first time that Berlioz’s culminating masterpiece had been seen anywhere as he conceived it, performed with only minor cuts on a single evening.

The effect was revelatory. As they had done 60 or 70 years before when they heard Wagner's Tristan und Isolde, people paced the streets for hours afterwards. Many had come to hear the young Canadian tenor Jon Vickers rather than the piece he was appearing in, but found they were hearing a work of a power and intensity that required the history books to be rewritten.

What made it all the more striking was that until then Les Troyens had been regarded as an operatic white elephant, the work of an ailing, disillusioned artist who had done his best work years before, in the 1830s and 1840s – the decades of the Symphonie fantastique, the Requiem and La damnation de Faust. Few people were prepared for a score of such energy.

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The result was a re-evaluation of Berlioz’s whole output. He had always had his admirers, not least in this country where his visits to London between 1847 and 1855 had left a legacy of interest and enthusiasm.

His music had been kept alive here by conductors with a good understanding of it (and it is conductors that composers depend on for survival). Berlioz’s friend Charles Hallé, founder of the Manchester orchestra that bears his name, brought many of the works into his repertoire.

After him came Hans Richter, who was active in London in the 1890s and early 1900s and who, according to Bernard Shaw, gave excellent performances of La damnation de Faust in particular. After Richter there was Hamilton Harty and there was Beecham. Yet it was still an uphill struggle until the Covent Garden Les Troyens generated a Berlioz revival.

Following Les Troyens, musicians in various fields took Berlioz up, including Hugh Macdonald, Nicholas Snowman, Julian Rushton and, not least, Colin Davis. Macdonald, Snowman and Rushton all took part in concert performances of Les Troyens given at Oxford and Cambridge under Davis in the early 1960s by the Chelsea Opera Group, the body founded in 1950 by Stephen Gray and me.

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It would not be long before even the Parisians would seem to be having second thoughts about the composer they cold-shouldered for so long, and leading conductors would embrace his music: not only Colin Davis, but also Roger Norrington, John Eliot Gardiner, Simon Rattle, and Valery Gergiev, to name a few.

Yet the sheer diversity of Berlioz’s oeuvre makes him particularly hard to classify. Each score inhabits its own imaginative world with its own colour and atmosphere, ranging from the chamber-music scoring of L’enfance du Christ, to the Requiem, with its multiple brass and timpani.

In the ‘opera semi-seria’, Benvenuto Cellini, Berlioz re-creates Renaissance Italy, and in the massive Te Deum of 1849, we encounter the apocalyptic ‘judex crederis’.

So what is the typical Berlioz work? Is it the concert opera La damnation de Faust, so vibrant with life yet at heart desolate? Or the intimate song cycle Les nuits d’été, at once Classical and profoundly Romantic? Is it Roméo et Juliette, whose orchestral love scene was Berlioz’s own favourite among his compositions? Or his most often played work, the Symphonie fantastique?

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Each of these works is typical of Berlioz, but in its separate, unrepeatable way. Each takes a new direction and a new shape. One of the many lessons that Berlioz learned from studying Beethoven’s symphonies was that form is not something fixed and laid down in advance, but created afresh each time, according to the poetic content of the particular work.

The endless variety of Berlioz’s formal designs is one reason why his music may be difficult to grasp when we first encounter it and why it has fallen foul of the more literal-minded critics. A further reason is the length of many of his melodies – the motto theme of the Symphonie fantastique, for example, runs to 30 bars.

We have to know his tunes before we can follow them. Melody is the key to Berlioz. Find it, and we unlock the door to a land of unimagined beauty and unending fascination.

We named Berlioz one of the best French composers ever.

David Cairns