Apple's ever-active research and development team continues to invest in new user interface technologies.

Recent patents suggest the company's usability boffins are exploring new technologies to develop future interfaces. The moves seem to suggest attempts to extend existing paradigms for computer interfaces, namely the mouse and keyboard controllers that defined personal computing with the release of the first Apple computer.

A new May 2005 patent filing was published by the US Patent and Trademark Office yesterday. The patent, which extends a previous patent for colour-changing iMacs, now seems to describe a multi-purpose touch interface which would let users control a machine through their choice of mechanical control systems, called "mechanical overlays".

These overlays could be audio equalisers, music keyboards or any other controller. The filing explains several examples of how the system could be applied in both notebook computers and handheld devices, conceivably iPods.

"The touch-sensing input device is capable of sensing the mechanical inputs provided by the mechanical overlay and causing the host computing device to respond to those inputs," Apple said. "The inputs of the mechanical overlay may be assignable or they may be configured for a particular application of the host computing device.

The filing explains that numerous input devices for controlling electronic systems now exist, but adds: "Unfortunately, these conventional approaches do not fully satisfy user needs. For example, the rudimentary mechanical controls tend to be fixed and inflexible (not easily adjusted or configured for a new task). Further, each one includes electronic hardware that increases the cost of the device. In large control panels, which include a vast number of mechanical controls, the costs can be exorbitantly high."

The filing also criticises existing touch-based systems for the lack of direct feedback users receive when they use them. "As such, the user does not know when the device has produced a touch input," it explains.

It then expains multiple ways such an interface could work: as an input controller, a touch sensitive device, a mechanical overlay to a touch sensitive device.