Overall U.S. music album sales dropped slightly in the first six months of the year, but digital album sales are up more than 13 percent over a year ago.

Tracking firm Nielsen SoundScan said Thur that 57 million digital albums have been sold so far in 2012, compared to 50 million during the same period last year. Digital album sales comprise 38 percent of all album purchases for the first half of the year.

Adele's Grammy-winning "21" is the top selling CD, both digitally and physically.

It claimed the biggest one-week sales of 2012, with 730,000 copies sold in the week following the British singer's Grammy Awards sweep.