We propose a method for reducing test data volume of integrated circuits or cores in a System-on-Chip. This method is intended to reduce the required number of Automatic Test Equipment (ATE) output channels compared to the number of scan-in input pins in a classical multi-chain implementation (horizontal compression). Compression and decompression are based on arithmetic operations and structures which present a very low area overhead. The proposed compression scheme does not impact the fault coverage achieved by the original test sequence before compression.