Synchronizing Periodic Clocks in Kahn Networks

We propose a programming model and language dedicated to high-performance streaming applications. In particular, we study real-time video-streaming for embedded media devices, including high-definition TVs. This language builds on the synchronous programming model and on domain-specific knowledge --- periodic evolution of streams --- to allow correct-by-construction properties of the application to be proven by the compiler. These properties include buffer requirements and delays between input and output streams. Correctness of the implementation is difficult to assess with traditional (asynchronous) approaches. Such properties are tedious to analyze by hand, due to the combinatorics of video filters, multiple data rates and formats. For example, the design of communicating buffers between filtering processes (image scaling, quality enhancement, etc.) whose clocks do not strictly match is tedious and error-prone. Two communicating periodic processes are defined as n-synchronous if they can be implemented in the ordinary (0-)synchronous model with a FIFO buffer of size n. We extend a core synchronous data-flow language with a notion of periodic clocks, and design a relaxed clock calculus (a type system for clocks) to allow non strictly synchronous processes to be composed. This relaxation is associated with a subtyping rule in the clock calculus. Delay, buffer insertion and control code for these buffers are automatically inferred from the clock types through a systematic program transformation