A study conducted by the Netherlands Organization for Applied Scientific Research (TNO) has determined that the expected useful life of a CD is approximately 30 years. After this period, CDs cease to produce sound due to oxidation affecting the small bumps on the metal surface, which correspond to the digital information. On the other hand the same organization reveals that the useful life for the video tapes is much smaller. A survey of video tape files revealed that units older than 3 years of age already had a certain "discoloration" to blue due to loss of magnetization. (1994)