Nielsen Entertainment's Broadcast Data Systems (BDS) will begin tracking tethered digital music downloads, in reponse to the growing business of on-demand music subscription services.

Rob Sisco, president of Nielsen Music says the company is adding tethered download monitoring "as a companion feature to our tracking of digital download sales in Nielsen SoundScan."

Tethered music downloads are digital songs available on an unlimited basis through online music subscription services. The tethered music information will identify and display the number of times U.S.consumers listen to specific song titles in the music libraries of all of the subscription services.

At launch, BDS will track tethered downloads from MusicNet (a supplier to AOL, Yahoo!, Virgin Digital, and Cdigix), Napster and RealNetworks' Rhapsody.

The data will help flesh out Nielsen BDS' tracking of activity in the subscription space. The company already monitors programmed and on-demand streaming activity from AOL Music and Yahoo! Music. In conjunction with the launch of tethered download data, Nielsen BDS will also be adding streaming data from Napster, RealNetworks' Rhapsody and MusicNet.

Nielsen BDS projects that it will be tracking more than 100 million streams a week and estimates that it will track more than 5 million tethered plays a week by the end of 2005. Nielsen BDS currently monitors music performances from more than 1,400 radio stations, all major music video outlets in the U.S and Canada and satellite radio from XM and Sirius.