Statistics compiled by GfK, the consumer electronics industry's research body, show nearly 10m people are tuning in to DAB radios in the UK and DAB set penetration now stands at 18% of UK households - just short of the DRDB's target figure of 20.6% by the end of the year.
The number of sets sold has increased by three million in less than two years.
Dickens' departure is no surprise to the DRDB board, which knew of his intention to step down "several months ago", and he now plans to sail round the world.
Ralph Bernard, DRDB chairman, said: "Ian has been a superb champion of DAB digital radio and is rightly credited with helping to pull the industry together to make it happen. Today's announcement of five million sales underlines how successful he has been."
The DRDB is now "actively seeking" a new chief executive
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