For nearly a century, New York's famous ''Tin Pan Alley'' was the center of popular music publishing in this country. It was where songwriting became a profession, and songs were made-to-order for the biggest stars. Selling popular music to a mass audience from coast-to-coast involved the greatest entertainment media of the day, from minstrelsy to Broadway, to vaudeville, dance palaces, radio, and motion pictures. Successful songwriting became an art, with a host of men and women becoming famous by writing famous songs.