Because there is no need to pay licensing fees in order to build the player, today many software MP3 players are free. Some companies claim to have patents covering the MP3 format, but the validity of these patents is questionable because the format specification was published more than 20 years ago, meaning that any relevant patents would have expired. The most famous MP3 players are "Winamp", "Sonique", "iTunes" and "MusicMatch". The main feature of an MP3 player is to turn the MP3 files into standard audio form and then send them to the soundcard of the computer. The soundcard then outputs the files into speaker so the user can hear them. Every MP3 player interprets music differently, though each one of them uses the same code for playing the MP3 files. There are also stand-alone (without a computer) MP3 players. These may be portable, or they may be built-into a stereo system or car stereo.