HD Radio, some 13 years after its invention, is making slow but steady progress. Over 2,100 digital stations are now broadcasting, with each of the top 250 markets offering a minimum of one HD outlet. "I always laugh that a couple of years ago I got an invitation to celebrate 25 years of digital television," said Robert Struble, founder of iBiquity Digital, which developed the format. "These things take time. There's a massive install base of existing analog radios. There's a massive infrastructure of existing analog radio stations. [But] we have tremendous momentum and we're picking up steam."