Multiple Channels: As the 1990s, movie theatres have upgraded their sound systems to surround sound systems which carry more than two channels. The most well liked illustrations are Advanced Audio Coding that is AAC such is used through Apple's iTunes and Dolby Digital, also known as AC-3. Both codecs are copyrighted and encoders or decoders cannot be offered without paying a license fee. Less general are Vorbis and the current MP3-Surround codec. The most well-liked multi-channel format is termed as 5.1, along with 5 normal channels as front left, or middle, front right and back left, back right and a subwoofer channel to take low frequencies merely.