At mid-year 2012, album sales were down 3 percent from 2011 at 150.5 million units sold over the last six months. Digital song sales, however, notched a 6 percent rise with 698 million tracks downloaded since January.

Overall music figures for physical and digital album and single sales showed a 4 percent increase, with 853.2 million sold so far in 2012.

"While retail sales continue to face challenges related to broader market trends, digital sales gains are expected to achieve another record-breaking year," said David Bakula, senior vice president of client development at Nielsen, in a statement.

"The strength in other monetized digital formats, including music streaming services, is equally encouraging," he added.

British singer Adele continued her reign on U.S. music charts, as her Grammy-winning "21" remained the biggest-selling album at the year's halfway mark with 3.69 million copies sold.

Australian singer Gotye had the biggest digital single with "Somebody That I Used To Know" featuring New Zealand singer Kimbra, selling 5.5 million downloads.

This is the first time a single has passed the 5 million download milestone at the year's halfway mark, topping Katy Perry's "E.T." that set a record for 4.12 million digital downloads in the first six months of 2011.

Adele's "21" is the only set to have sold more than 1 million copies this year, and is yet to drop out of the top 10 in the Billboard 200 album chart since its release in February 2011, when it debuted at No. 1.

At the halfway point in 2011, "21" had sold 2.5 million copies and went on to become the year's top-selling album in 2011 with sales of 5.8 million. Adele's 2008 debut album, "19," is the tenth best-selling album this year.

Lionel Richie's country album "Tuskegee" is the second biggest-selling album in 2012, with 912,000 copies sold since its release in March, and British boy band One Direction notched the third best-seller with their debut set, "Up All Night," selling 899,000 copies, also released in March.

New York indie-pop band Fun. scored the second best-selling single in 2012's first six months with "We Are Young," featuring Janelle Monae, which has sold 5.1 million digital copies.

Elsewhere, on this week's Billboard 200 album chart, alt-rock band Linkin Park debuted at No. 1, edging out fellow southern Californian pop rockers Maroon 5 by just over 1,000 copies for the top spot.

Linkin Park's fifth studio album, "Living Things," sold 223,000 copies last week to score their fifth No. 1 album, the only band since 2000 to notch five chart-topping records on the Billboard album list.

Maroon 5 generated sales of 222,000 for their fourth studio album "Overexposed," placing them at No. 2.

The album chart also featured new entries from artists at Maybach Music label, scoring No. 4 with "Maybach Music Group Presents: Self Made 2," while R&B singer R. Kelly debuted at No. 5 with his 11th studio album, "Write Me Back."

(Reporting by Piya Sinha-Roy; Editing by Bob Tourtellotte and Lisa Shumaker)

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