Steady growth in sales of ringback tunes and mobile-based full-track downloads helped power a 6% rise in digital-music sales in Japan to 120.8 million units in the first quarter.

In value terms, digital sales -- including mobile- and PC-based downloads -- rose 28% to ¥22.5 billion ($215.2 million) in the January-March period, according to data released May 29 by the Recording Industry Assn. of Japan (RIAJ).

While mobile-based master-ringtone sales by the RIAJ's 46 member companies in the quarter fell 16% from the corresponding period of 2007 to 51.1 million units for a value of ¥5.9 billion ($56.3 million), down 13%, ringback-tune sales rose 18% to 22.6 million units, for a value of ¥1.9 billion, up 47%

Sales of mobile-based full-track downloads, meanwhile, rose 48% to 34.4 million units, for a value of ¥11.4 billion ($109.2 million), up 58%.

Overall, mobile-based downloads rose 4% to 110.9 million units, for a value of ¥20 billion ($191.4 million), up 41%.

PC-based music downloads in Japan in the first quarter were up 34% to 9.9 million units, for a value of ¥2.1 billion ($19.9 million), up 50%.

The RIAJ defines the PC-downloads category as comprising singles, albums, mini-albums and "other related content" sold over wired networks, with albums and mini-albums counting as single units regardless of the number of tracks they contain. The association says that although the data doesn't differentiate between sales of single tracks and albums, singles account for the overwhelming majority of download sales.

The mobile category comprises sales over wireless networks of full single tracks, ring tones, master ring tones and other related content.