Digital music player sales seem set to explode once again this year, according to researchers at iSuppli.

Sales of these devices doubled last year and it appears likely they will grow by 57 per cent this year.

MP3 music player shipments are expected to expand to 132 million units in 2009 from 36.8 million last year. Shipments rose 116 per cent last year.

Sales of hard drive-based music players seem set to reach 56.2 million units in 2009 from 9.8 million last year.

The analyst firm warned manufacturers not to focus on adding extra features to their products: "The so-called 'Swiss Army Knife' approach has not succeeded in the MP3 market. Simple, elegant products that perform a few functions with easy-to-use interfaces have sold well in the marketplace, while the do-everything approach has failed", iSuppli said.