A good remastering job involves finding the best quality original source tape from when the album was first made, and transfer that tape to digital files so it can be manipulated in detail. The mastering engineer will then fine tune the sound with computer software to balance out the audio levels and improve clarity of the instruments and vocals. There would also be some special outboard audio gear involved to run the sound through.

Generally, the original stereo or mono tape is used as a source, because it is the mix as the original artist intended for radio and vinyl records. It’s also possible to REMIX and remaster an album using the original multi tracks, but to do it proper it would need to duplicate how the original sounded. This is more difficult. For example, The Beatles Sgt. Pepper album was recorded on a four track machine, so a tape exists of all four original tracks of each song. Eight years later in 1975, Queen’s Bohemian Rhapsody was recorded on a 16 track machine, which enabled them to record all the separate parts each on their own track. Remixing how the original songs sounded from the multi tracks is a much more difficult task. That said, a new remix & remastered version of The Beatles “White album” just came out last month. It was produced by Giles Martin, the son of George Martin who was the Beatles producer. Giles used the original multi tracks and very closely duplicated how the original songs sounded. Because he used the multitrack sources, he was able to get a great deal of clarity.

I worked at a major mastering studio in the 1990s as an assistant to the mastering engineers.