A new patent application published by the U.S. Patent and Trademark Office this week and discovered by AppleInsider today seems to provide some details about Apple’s rumored cloud-based music service, and how iTunes on the desktop and mobile devices like an iPhone could manage to improve streaming quality and speed by locally syncing snippets of songs. The patent, entitled ”Local Storage of a Portion of Streamed Media Items”, notes how most streaming services allow users to access an online library of music and cache contents locally on device to enable playback when an Internet connection is not available. This happens in popular service Spotify, which enables users to keep a local cache of albums and songs so they won’t be forced to always be connected to the Internet to listen to music. Whilst Spotify’s cache action has to be triggered manually by the user, other cloud music services automatically cache a song in the background once a user starts listening to it. Apple’s proposed solution is different, and it involves a new menu in iTunes (for the desktop) that syncs via USB “partial music” to an iOS device – that is, small snippets of a song, with the remaining parts available online.

Unlike cache, partial local sync has the advantage of letting users start listening to music immediately without waiting times. Because cache in other services is still obtained with an Internet connection, users have to wait for the caching process to finish before they can start playback. And if the song is not cached, users have to wait for the remote server to begin streaming – when quickly jumping between artists and songs (as most users do on iPods and iPhones), having to wait a few seconds for streaming to begin can be annoying. Assuming a user is syncing music that’s available both locally (on a Mac or PC) and in the cloud (the music service), Apple’s proposed solution skips cache and waiting times entirely by saving locally a first snippet of a song, and then fetching the rest remotely. This way, iTunes doesn’t have to sync full content (thus cutting syncing times) and a mobile device doesn’t have to stream the first seconds of a song. The benefit for users is that playback will start immediately regardless of how fast they’ll switch between songs; the obvious downside is that something will still have to be synced locally. AppleInsider also posts more details from the patent:

The application suggests that the remotely stored content could be a user’s own library, which they have streamed from a home computer or a remote server. Or, it could be streamed from a large “content source,” such as the iTunes Music Store. The described system would also utilize authentication methods, such as with an iTunes account username and password, to ensure that the user has purchased the items and has the right to stream them.

If this patent is of any indication, it could lend some credence to the reports that claimed Apple’s cloud music service was being set up in a way that users could both stream their own music libraries (by uploading them first) and music they didn’t purchase, with a subscription model similar to Spotify. The method described above would clearly require an update to the iTunes application, and it’s unclear whether Apple could also provide a way to “always stream”, avoiding partial sync and relying exclusively on a remote connection. Apple is expected to unveil iOS 5 with new cloud-based features at the WWDC, which kicks off in San Francisco on June 6.