The move could render tunes purchased by many iPod owners unplayable on their music players. For the last four months, RealNetworks has marketed its music store as the only Apple rival compatible with the iPod, following the company's discovery of a way to let its customers play their downloaded tunes on Apple's MP3 player.
Apple criticized RealNetworks' workaround, dubbed Harmony, as the "tactics...of a hacker," and warned in July that RealNetworks-purchased songs would likely "cease to work with current and future iPods." Apple offered no further statement Tuesday, but confirmed that the software released with its iPod Photo will not play music purchased from RealNetworks' music store.
The high-tech feud may be as grounded in public relations as it is in genuine technology development, but it highlights what remains a serious issue in the digital music business. Unlike CDs, songs sold by competing online stores are often not directly compatible with different brands of MP3 players.
Songs purchased from Apple's iTunes store can only be played directly on an Apple iPod, while songs purchased from Napster or MSN Music can only be played directly on a device that supports Microsoft's audio format, for example.
Record label executives, as well as rival technology companies, have repeatedly urged Apple to open up its iPod to play songs purchased from other music stores, but the company has declined to do so. Executives from several labels had applauded RealNetworks' attempt to create compatibility between its store and the iPod, even lacking Apple's permission.
RealNeworks said in a statement that it remains "fully committed to providing consumers with the freedom to use the music libraries they purchase from us on different portable audio devices they acquire, both now and in the future--including the iPod Photo."
To promote the release of its Harmony software, RealNetworks held a half-price sale in its song store in late August, charging just 49 cents per song. At the close of the three-week promotion, the company said it had sold more than 3 million songs during the sale.
RealNetworks said it did not know how many of those customers were iPod owners, however.
The changes Apple made were to the iPod's "firmware," which is the low-level software that powers hardware such as MP3 players or cell phones. This kind of software is often, but not always, updatable, and companies often use changes to provide new features or capabilities for devices.
Apple released its last update to the iPod software in mid-November, offering new versions for the iPod Mini as well as all fourth-generation click-wheel iPods.
The notes that accompanied the release mentioned several enhancements, but did not comment on Harmony. It was not immediately clear whether iPods older than the photo edition had as a result also been rendered incompatible with RealNetworks' technology.
CNET News.com's Ina Fried contributed to this report.