The varied technologies used in portable audio devices developed since the 1970s.
The 8-track player was first introduced in the 1970s as the first actual portable audio device. While it did have large success, it was heavily criticized not only for its bulk but its lack of convenience. It could only play eight-track tapes so most tapes had to be separated into two, or even sometimes three, parts.
The personal cassette player was introduced in 1979 and sold very well. While it was criticized—eight-track tapes weren't compatible with cassette tapes—it was still a great improvement over the eight-track system. It was much smaller, the tapes were easier to carry around, and the sound quality was quite good.
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 the 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.
In 1998, digital audio players (based on solid-state disk or hard disk storage) started to be available (The Rio PMP300 from Diamond Multimedia is widely considered as the first mass market DAP). But as the portable players before them, there were many issues. MP3 files generally don't have better sound quality: normal compact discs play at 1411kbps, whereas MP3 files can only play at 800kbps. The files are listenable, but don't produce the same sound when used with higher quality headphones having a good range. This is still an issue today. Also, people found it troublesome to transfer files, even through transfer programs. However, as versions of iTunes became simpler to deal with, it was less of a bother. More companies were able to introduce their MP3 players as well when RealPlayer released their MP3 compatible program in May of 2004. And then, Microsoft released their MP3 player compatible program Windows Media Player 10 in July of 2004.