The CMB spectrum takes memory of physical processes and astrophysical phenomena occurred at various stages of the evolution of the Universe. After COBE/FIRAS results, that confirmed the very good agreement of the current CMB spectrum with a Planckian shape, the catch of small deviations from a blackbody is open at various frequency ranges. The recent ARCADE 2 observations suggest an excess of the extra-galactic sky temperature in particular at 3.3 GHz. Explanations can be searched in terms of CMB distortions or of foreground signal.