blues influenced white folk song and the black popular music of the Northern ghettos, blues and black pop influenced jazz, and so on. But the single most important process was the influence of black music on white. By the 1930s, African American musicians, such as Cab Calloway, Fletcher Henderson and Duke Ellington, were developing swing music, essentially jazz played for dancing, and in some areas such as New York City processes of social integration were taking place. According to Palmer, by the mid-1930s, elements of rock and roll could be found in every type of American folk and blues music. Some jazz bands, such as Count Basie's, increasingly played rhythmic music that was heavily based on blues riffs. In Chicago, blues performers formed into small groups, such as the Harlem Hamfats, and explored the use of amplification. In the Midwest, jump bands developed instrumental blues based on riffs, with saxophone solos and shouted vocals. In Nashville and elsewhere, country music played by white musicians such as Jimmie Rodgers incorporated blues styles, and in some cases was recorded with (uncredited) black musicians. In Texas and Oklahoma, Western swing bands, such as Bob Wills, Moon Mullican and Cliff Bruner combined elements of big band, blues and country music into a new style of dance music. As musicians from different areas and cultures heard each other's music, so styles merged and innovations spread. Increasingly, processes of active cross-fertilisation took place between the music played and heard by white people and the music predominantly played and heard by black people. These processes of exchange and mixing were fueled by the spread of radio, 78 rpm and later records and jukeboxes, and the expansion of the commercial popular music business. The music also benefited from the development of new amplification and electronic recording techniques from the 1930s onward, including the invention of the electric guitar, first recorded as a virtuoso instrument by Charlie Christian. In 1938, promoter and record producer John H. Hammond staged the first "From Spirituals to Swing" concert in New York City to highlight black musical styles. It featured pianist Pete Johnson and singer Big Joe Turner, whose recording of "Roll 'Em Pete" helped spark a craze across American society for "boogie woogie" music, mostly played by black musicians. In both musical and social terms, this helped pave the way for rock and roll music. Economic changes also made the earlier big