Personal CD players were first introduced in the 1980s, but weren't predominant until the 1990s when anti-skip players were sold. At first, most people thought of personal CD players to be no more portable than a cassette player. Also, since it had no anti-skip feature, they couldn't be used in a portable manner. But in the early 1990s, companies introduced shock absorption to minimize the skipping, which while making them useless for heavy activities, allowed them to be portable. After that refinement, most people used CD players rather than cassette players. Then in 1997, Electronic Skip Protection was used on portable CD players to make it possible for heavy activity use.
