Design Implementation The Semantic Time Framework (STF) is a software library for Mac OS X for designing and constructing interactive systems that modify the timebase of multimedia. STF is unique in that it supports a more general timing model tied to the semantics of the media data (“beats of music”), rather than engineering constructs (“samples of audio”). The software architecture is based on a system of nodes assembled into pipelines, with additional constructs for supporting time and synchronization. Media flows through the pipelines like fluid flows through a pipe; playback rate can be interactively adjusted using rate-changer nodes, much like a valve controls the rate of fluid flowing through a pipe. STF builds on a number of Mac OS X technologies, including QuickTime, Core Audio, and Core Image/Video. The library is written in Objective-C, which makes it simple to include in Cocoa-based applications. STF has been successfully deployed in a number of audio editing and interactive orchestral conducting applications. It is an open source project