 When the other Beach Boys returned from touring, they moved to Columbia's Hollywood studio, which was equipped with the latest 8-track technology; by this time, Wilson and his engineers had 'reduced' the pre-recorded 4-track backing tracks to a mono mix, which was then dubbed onto one track of the 8-track master tape; Wilson then recorded the vocal tracks, assigning one individual track to each of the six vocalists (including soon-to-be permanent member Bruce Johnston), leaving the eighth track available for final 'sweetening' elements, such as additional vocal or instrumental touches, and lastly, all these elements were mixed down to the mono master tape.Nearly all of the Beach Boys' 4-track and 8-track masters from this period are preserved in Capitol's archive, allowing the label to release several expansive boxed sets of this music; "The Pet Sounds Sessions" (1997), includes nearly all the separate backing and vocal tracks from the album, as well as new stereo mixes of all the songs, while the 9-CD "The Smile Sessions" (2011) features a wide cross-section of the huge amount of instrumental and vocal material (totalling around 50 hours of recordings) that was recorded for the group's never-completed 1967 "magnum opus", "Smile".All of the Beatles classic mid-1960s recordings, including the albums "Revolver" and "Sgt Pepper's Lonely Hearts Club Band", were recorded in this way.