English: The Blumlein shuffler is an analog processor that extracts left-right phase differences and converts them to left-right level difference. It was suggested by Alan Blumlein in his landmark UK patent 394325 as a tool for preserving stereo information otherwise lost during playback through loudspeakers. There are four steps
(1) take the original L+R stereo signal
(2) convert L,R to M(id) and S(ide) signals through plain addition/subtraction. M and S are, by design, spaced by exactly 90 degrees
(3) shift S 90 degrees, so it becomes in phase with M. This step could only be approximated through all-pas filtering, but it somehow worked in Blumlein's hands
(4) add/subtract M(id) and shifted S"(ide) to create new L and R channels. The same principle is used in analog Dolby pro-logic circuitry to steer signal into central channel.