In this paper, the implementation of a digital signal processor (DSP) based H.264 decoder for a multi-format set-top box is described. Baseline and main profiles are supported. Using several software optimization techniques, the decoder has been fitted into a low-cost DSP. The decoder alone has been tested in simulation, achieving real-time performance with a 600 MHz system clock. Moreover, it has been integrated in a multi-format IP set-top box allowing the implementation of actual environment tests with excellent results. Finally, the decoder has been ported to a latest generation DSP.