The genius of Louis Armstrong, the jazz-loving movie star Tallulah Bankhead wrote in 1952, "reaches across land frontiers and oceans. I believe in one world. I don't use the term politically. I am thinking of the oneness of humanity. Louis is the epitome of one world."
Sixteen years later, Bankhead's assessment of "Satchmo" received an unexpectedly cheesy confirmation. A genially rasping number called Wonderful World, a song of such guileless sentimentality as to be all but unperformable by anybody but Armstrong, was a huge international hit in 1968 - the year that marked the high tide for street protest and counter-culture all over the globe.
Maybe it was an allergic reaction. But if Wonderful World was an attempt to drown out the Vietnam war with schmaltzy string arrangements, it was nonetheless invested with a kind of affable dignity by Armstrong's extraordinary singing voice, a sound like someone trying to tell a joke through a gravel-filled trombone. Wonderful World - for all its eyes-wide-shut take on the times it was living in - somehow embodied an indestructible optimism about the oneness of humanity that Armstrong had been singing about, infectiously talking about, laughing and crying about, and blowing stories about through the bell of his horn since he was a ghetto kid in New Or leans in his and the 20th century's teens.
The downside was that, with Hello Dolly and Mack the Knife, Wonderful World's partners in Armstrong's late-career trophy cabinet of vocal pop hits, he came to be seen as a charmingly eccentric old crooner. Those chart hits meant that when Armstrong died in 1971, millions of those who mourned his passing had no idea that for years he was an instrumental virtuoso for whom singing was initially almost a sideline.
Armstrong had been a big star since around 1929, already able to work 365 nights a year if he wanted, demonstrating an extraordinary fertility on both trumpet and vocals, fronting his own orchestras yet holding down every show on his own. Night after night through the 1930s, he loosed off more fearsomely demanding trumpet virtuosity throughout full-length concerts than the creative spirit - let alone the musculature of the human lip - should have been able to handle; and he did it all with openness and generosity. The short, stocky, broad-grinning, sweat-mopping man in effect became jazz music's first concerto recitalist during those years - but with his materials in his head, not on a score sheet, and in a process of constant spontaneous transformation.
Armstrong's genius could set almost any music alight, however clapped out or corny. His sound was much copied but inimitable, even on the most unlikely materials. The writer James Baldwin, listening to Armstrong play the American national anthem at the 1958 Newport jazz festival, remarked: "That's the first time I've liked that song."
If it had been left to Armstrong, we would already have celebrated the centenary of his birth. For years he insisted that he was born on Independence Day 1900, a birth-date easy to remember and honourable to bear. The world now knows with reasonable certainty that he was born Louis Daniel Armstrong on August 4 1901, in cosmopolitan New Orleans's black ghetto, to a labourer called Willie Armstrong and a domestic and part-time prostitute called Mary Ann, or Mayann. His father left home shortly afterwards, leaving the boy to be raised at first by his grandmother Josephine and later by his affectionate but inattentive mother in the red- light district of Storyville.
It was a tough childhood, with the next meal a chance occurrence. To raise money for Mayann and himself, Louis delivered coal and sang on the streets in barbershop groups, an early experience in harmonising on the fly. Proto-jazz was in his ears all the time, though the word "jazz" was not yet pinned on it. In the streets strutted the chug-chug of parade-band marches (already beginning to gain sly rhythmic twists and soulful melodic slurs); from the doorways of dance halls and clubs blew the blues and the clattering vivacity of ragtime.
The story runs that Armstrong enthusiastically fired a gun in the air on July 4 1913 and was dispatched to the city's Colored Waifs Home to be straightened out. The truth may have been that the sentence was a rap on the knuckles for a spirited street-kid left too much to his own devices. But it was a turning point. They gave Armstrong an alto horn to play in the brass band. He couldn't read the music, so he figured out the harmony line as he went. He moved from the horn to the bugle, then to the brighter, faster, more expressive cornet, and they made him the bandleader.
Armstrong's career was on a roll. His virtuosity and unerring musical ear got him jobs all over the New Orleans ghetto, and then entertaining the high-rollers on the floating casinos of the Mississippi steamboats - jobs demanding a bigger repertoire and some awareness of what the dots on a score sheet meant. By the time he was 18, Armstrong was good enough to replace New Orleans's best cornettist, Joe "King" Oliver, in trombonist Kid Ory's band. He also entered the first of four marriages, a tempestuous, short-lived relationship with a New Orleans prostitute named Daisy. Armstrong followed the fast-rising Oliver to Chicago in 1922 to play second trumpet at the Lincoln Gardens. This historic residency changed the lives of countless embryonic jazz performers, from the South Side blacks to ecstatic white high-school kids, including Bix Beiderbecke and Benny Goodman.
The tough, worldly Oliver was a big influence on the younger man, although he did not always play it straight with his protégé's wages. Later, Armstrong would turn over business decisions and career choices to his second wife Lil, then, from 1935, to his hard-nosed and crassly commercial manager Joe Glaser. Some pop psychologists have suggested that his unstable childhood led him to put his life in the hands of powerful figures and prevented him from realising his full potential.
When Armstrong married Lillian Hardin in 1924, she was King Oliver's pianist and a schooled musician who helped the self-taught trumpeter to understand some of the formal mechanics of music. She also invited him to be more ambitious, and Armstrong began working with Fletcher Henderson's dance orchestra in New York that year. He not only transformed the band's solo strengths but began to change the way arrangers phrased the written parts with the power of his improvisations. Then, in November 1925, Armstrong began an extraordinary series of recordings that came to be known as the Hot Fives and Sevens.
Over three years these bands cut the most memorable music of the first phase of jazz. They revealed an awesome power and forcefulness in Louis Armstrong's sound, particularly as he switched from the cornet to the more penetrating trumpet. He played with as much conviction and clarity in the upper register as in the middle, and he possessed a skilled composer's sense of form, giving definition and unity to the often ragged narrative of the improvised jazz solo.
More profoundly, Armstrong's intuitions were releasing jazz soloing from the dictatorship of the underlying beat. As a syncopated music, jazz had naturally toyed with the beat from the beginning. But Armstrong was not just mischievously placing one or two notes at odds to the pulse here and there; he was skidding away from it for whole bars at a time. This audacity did much to give jazz its characteristic rhythmic ambiguity and vigour, and the canny hipness of appearing to be saying one thing and meaning something else. Armstrong's effortless, vaulting ideas have since influenced dance, song, instrumental techniques and even the rhythms of everyday speech.
But it was in Armstrong's singing that his canny managers spotted a skill that could make their property even hotter. Record director Tommy Rockwell began Armstrong's new career as a "popular entertainer" (which meant reaching beyond the black audience's "race records" to middle-class whites) when he brought his charge to play and sing in Fats Waller's 1929 Broadway musical Hot Chocolates. Glaser, who succeeded Rockwell as manager, kept Armstrong on the same track for the rest of his career.
This broadening of his appeal made Armstrong a huge star in the 1930s, though his marriage to Lil Hardin was on the rocks following onstage altercations. After her came a brief marriage to Alpha Smith, and eventually a stable relationship with showgirl Lucille Wilson. He toured constantly, and began a series of visits to Britain in 1932. Armstrong's trip brought him new fans everywhere he went, though some jazz buffs were beginning to question his use of vocals over the trumpet and the expanding role in his shows for Broadway show music, and he had to contend with some excruciating reviews (the Daily Herald noted that "he might have come from some African jungle, and then after being taken to the slop tailors for a ready-made suit, been put straight on the stage and told to 'sing' "). He had also acquired a typewriter and become a tireless and immensely entertaining correspondent with friends he made on tour. The British critic Max Jones sometimes received the bowel-preoccupied trumpeter's surreal correspondence typed on toilet paper, and as ever he signed himself "red beans and ricely yours".
If this kind of popular magnetism alienated a section of Armstrong's jazz following, and some African-American observers took issue with a hamminess that seemed to oil white preconceptions of black artists, Armstrong's achievements as a black entertainer paved the way for generations to follow him. He was the first black artist to appear regularly in feature films (he had been in over 50 by the end of his life) and to have his own sponsored radio show. And if generation, background, self-education and lifelong modesty made militancy alien to him, he was publicly outraged by the race riots in Little Rock in the late 1950s.
The collapse of the big-band era of the 1940s led Armstrong - who never liked swing's arty offspring, bebop, and sometimes hilariously battered the point home on stage - to continue his touring life into the 1950s under Glaser's direction. His various lively small bands were dubbed the All-Stars, and they really were all stars at the outset, with the likes of Jack Teagarden and Earl Hines on board, but Armstrong later reflected that "too many stars make bad friends". Increasingly, the ensembles became vehicles for the leader, but Armstrong proved he could rise spectacularly to unique occasions, such as the 1953 and 1955 tributes to legends WC Handy and Fats Waller.
His punishing schedule led to a heart attack in 1959, but he continued throughout the following decade to be one of America's best-loved musical ambassadors. Armstrong died peacefully in bed on July 6 1971. He always called his career "a hustle", a throwback to his early days on the New Orleans streets playing for dimes. But it was a hustle that turned the world's music upside down.
• Humphrey Lyttelton presents Louis Armstrong: the Centenary Concert at the Queens Hall, Edinburgh (0131-668 2019) on August 4.