(updated March 2014. scroll to bottom for new addition to this post)
SQ was created by the CBS Technology Center and was made available to the public in April 1971. Rear channel information was placed on a vinyl record by adding “…double helical groove modulations… alongside . . .vectored modulations…” that produced the sound for the front channels. The SQ decoder sensed the different signals and did its best to send the appropriate sound to the correct speaker.