Digital rights management (DRM) software is a system forced onto distributors that protects only a handful of the world's music, according to Apple chief executive Steve Jobs. 

In an open letter posted on Apple's website, Jobs outlined the reasons behind the company's decisions to close its FairPlay DRM system to other developers.

Jobs claimed that Apple was being held hostage by the "big four" record labels, which threaten to pull their entire catalogues from the iTunes store if FairPlay is breached.

"If the big four music companies licensed their music to Apple without the requirement that it be protected with DRM, we would switch to selling only DRM-free music on iTunes. Every iPod ever made will play this DRM-free music," said Jobs.

The Apple chief lashed out at the labels for requiring Apple, Sony and other companies to encode music files with DRM software to prevent copying while not applying the technology to CDs, which comprise the overwhelming majority of music sales.

Previous attempts at putting DRM in CDs have proved disastrous.  

"If the music companies are selling over 90 per cent of their music DRM-free, what benefits do they get from selling the remaining small percentage of their music encumbered with a DRM system? There appear to be none," said Jobs.

Much of the three page letter is devoted to explaining why Apple had refused to open its FairPlay system to other vendors and allow for songs purchased from iTunes to be used with third-party devices.

Currently, the songs will play only on iPods and computers running iTunes software.

The reason for this, according to Jobs, is that when someone cracks FairPlay's copy protection, Apple only has a "small number of weeks" to update the software before labels have the right to demand that the music be pulled from the iTunes store.

"Such an undertaking is very difficult when just one company controls all of the pieces," said Jobs.

"But it is near impossible if multiple companies control separate pieces of the puzzle, and all of them must act quickly in concert to repair the damage from a leak."

The problem with the DRM system lies with the record companies, not Apple, said Jobs. As such, he suggested that unhappy consumers and government agencies redirect their anger towards the "big four". 

"Much of the concern over DRM systems has arisen in European countries. For Europeans, two and a half of the big four music companies are located right in their backyard," he said.

"The largest, Universal, is 100 per cent owned by Vivendi, a French company. EMI is a British company, and Sony BMG is 50 per cent owned by Bertelsmann, a German company. 

"Convincing them to license their music to Apple and others DRM-free will create a truly interoperable music marketplace."