We propose a programming model dedicated to real-time videostreaming applications for embedded media devices, including highdefinition TVs. This model is built on the synchronous programming model extended with 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. Such properties are tedious to analyze by hand, due to the combinatorics of video filters, multiple data rates and formats. We show how to extend a core synchronous data-flow language with a notion of periodic clocks, and to 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.