Apple Computer (search) is trying to stop RealNetworks from taking a bite out of the iPod (search) market.

Apple is blocking the technology used by RealNetworks Harmony music downloading service.

Software upgrades on Apple's newest iPods prevent songs purchased from RealNetworks (search) from playing on iPods.

RealNetworks introduced Harmony in July.

The Seattle-based RealNetworks claims it reverse-engineered Apple's copy-protection code. With Harmony, RealNetworks said songs purchased from non-Apple online outlets could be playable on Apple's best-selling digital music player.