The proposed apparatus or the proposed method permits the linear variation of the degree of correlation particularly of pseudo-stereophonic audio signals, and overall provides a comprehensive, albeit extremely simple, postprocessing option. The novel reactants are derivatives of diethylenetriaminepentaacetic acid, wherein a suitable group is linked to the DTPA molecule, thus allowing site specific introduction of the ligand of the derivatives to bioactive molecules on solid phase in an oligopeptide synthesizer. A particular suitable ceramic is partially stabilised zirconia. The result is returned to the mobile device via the pre-processing server or MMR gateway.