In the 1980s, engineers transferred the Beatles' albums from their analogue master tapes to digital recordings so they could be put on the new compact-disc format. For many of us who had only heard those beloved songs on the radio, low-quality cassette tapes or on scratched and worn vinyl albums, the result was amazing.
Suddenly, we had crystal-clear recordings of some of the best rock music ever recorded, in a format that would never wear out or degrade and could even be played on portable devices. It was a revelation.
In 2009, the process happened all over again. Technology had improved so much that those crystal-clear CDs sounded more like a cassette tape by comparison to modern recordings.
But both of those had a major weakness: They were pulled from the original master tapes recorded and mixed in the 1960s. That sounds like a strength, not a weakness, until you understand how the recordings were made.
While today's technology can accommodate almost any number of tracks — one track for each instrument or microphone — in the recording studio in the late 1960s, a four-track tape recorder was state of the art. When the Beatles recorded "Sgt. Pepper's Lonely Hearts Club Band" in 1967, the process went something like this: They would record four tracks on one tape, then play that recording onto one track of another four-track machine. Then they would record four more and record those onto the second track of that second machine. They did this over and over, mixing down the tracks until they had built an amazing soundscape the likes of which had never been heard before.
Source: Dan Stockman/globalsistersreport.org