The QS system worked on phase frequency differences to get the channel separation, plus Sansui used some slight EQ in the rear channels. The weakness of the SQ/Tate/Sony system was to get the channel separation, it worked on volume level and would mute down the other channels to make 1 louder than the others but tended to have a pumping action. They did improve some things with their full-logic ICs but still it tended to pump. The CD4 system essentially was a FM stereo x 2 system developed by Matsushita(Panasonic), it had good channel separation but lacked signal to noise. But required a special phono cartridge and shibata shaped stylus capable of going up to 50kHz, and the turntable needed to have low-capacitance cables. The quad 8 tapes ran @ 3 3/4" had good channel separation but like the CD4 system lacked the higher signal to noise that the matrix systems had. Quad systems did set the stage that later advanced into the Dolby surround and DTS digital systems especially effective for surround audio for movies, marketed strongly for laser and DVD discs...

Just my $0.02....