A processing system, such as typically a CPU, is used for converting a digital signal organized in pixels, such as a video signal, between a first format and a second multiple-description format. The system comprises at least one input register and at least one output register, and is configured via instructions, that can be constituted at least in part by instructions of a SIMD type, so as to: order the pixels of the signal to be converted in a set of input registers; and take selectively the pixels from the aforesaid set of input registers and place them in an orderly way in at least one output register.