Until now the only downloaded tracks that could be played on an iPod were those purchased from Apple's iTunes Music Store. But it seems that RealNetworks has reverse-engineered the iPod.
"Compatibility is key to bringing digital music to the masses," explained Rob Glaser, founder and CEO of RealNetworks. "Before Harmony, consumers buying digital music got locked into a specific kind of portable player. Harmony changes all that. Thanks to Harmony, consumers don't have to worry about technology when buying music. Now anyone can buy music, move it to their favourite portable device, and it will just work, just like the way DVD and CDs work."
It appears that Harmony interferes with the digital rights management (DRM) software contained in devices like the iPod, so that it is no longer limited to playing music only from sources permitted by that device. Instead it will also play music downloaded from RealNetworks' own music site.
The move "stunned" Apple, which released a statement on Thursday declaring that it was taking legal advice. RealNetworks, it said, had used "the tactics and ethics of a hacker to break into the iPod."
Apple was focusing in particular, said the statement, on whether RealNetworks had breached the controversial Digital Millennium Copyright Act (DMCA), which restricts the circumvention of copy-protection systems.
However, the DMCA has a specific exception for reverse engineering for "the sole purpose of identifying and analysing those elements of the program that are necessary to achieve interoperability of an independently created computer program with other programs," and this, argues RealNetworks, is all it has done.
Responding to Apple, RealNetworks said: "Harmony follows in a well-established tradition of fully legal, independently developed paths to achieve compatibility. There is ample and clear precedent for this activity, for instance, the first IBM compatible PCs from Compaq."
"Consumers, and not Apple, should be the ones choosing what music goes on their iPod," it added.
Apple has already warned consumers that Harmony may not be compatible with future updates of the iPod software.