Back in November 2004, Apple filed a patent for an "audio user interface for computing devices" such as "an MP3 player, a mobile phone, or a personal digital assistant." The patent application was just published today by the U.S. Patent and Trademark Office.

The patent filing adds weight to speculation offered by a number of bloggers and journalists that Apple is working on a mobile phone, and it suggests that the iPod will eventually become a phone--an eminently sensible endgame for the device, assuming ease of use can be maintained.In the abstract and noncommittal language of patent filings, Apple summarizes the invention as "an audio user interface that generates audio prompts that help a user navigate through the features of a computing device. The audio prompts provide audio indicators that allow a user to focus his or her visual attention upon other tasks such as driving an automobile, exercising, or crossing a street."

The patent filing describes the device alternately as a media player, such as a portable audio device, or as a hand-held device with a scaled-down computing architecture for the sake of portability.

It's describing a voice interface for the iPod.

But there's more to it. A closer reading of the patent suggests that the invention is intended to leverage the link between Apple's iTunes software and the iPod. The patent details how the computing resources of a host computer--the Mac or PC running iTunes--can be used to do text-to-speech conversion to create audio files that then get transferred to the portable device when synced.

These audio files are intended to provide spoken feedback for vocal commands issued by the user. So a user might say, "Play next song," and the iPod might respond, "Now playing London Calling."

However, a voice interface doesn't make a lot of sense for a music player that lives inside pockets or on belts. True, the iPod Nano may get worn around the neck, where it would be better off accepting voice input. But I'm skeptical there's really that much demand for voice-driven menu navigation on the iPod. And I'm willing to bet that people wouldn't want to order their iPods around in public because they'd feel foolish doing so. There's no social precedent for chatting with an iPod.

A voice interface makes sense for a device designed for verbal input--for a phone. In fact, the use case cited in the patent filing addresses real usage patterns of cell phones in vehicles. A hands-free cell phone fits into a known product niche.

A number of bloggers, pundits, and media types have predicted Apple is planning to roll out a phone. From the look of this patent filing, I'd say the chances look better than ever.

Apple has to move into phones. That's where the growth is. The desktop PC market isn't exactly exploding anymore, and iPod sales can't continue to expand forever. Some day soon, Apple is likely to want a product that can tempt its massive user base to upgrade. And what better for that than an iPhone, complete with voice interface, not to mention a monthly fee.