It still beats me how information for left surround and right surround is embedded in stereo analog channels.
I wonder how Pro-logic II and Pro-logic IIz work decoding
I have no idea how modern analog stereo to surround sound algorithms work. Yet your question reminded me of how it was done when 8-track tapes were big news.
I'm old enough to have survived the industry's brief experiment with analog "quadraphonic" sound. The quadraphonic wars were fought by JVC's discrete system vs. several matrix systems. JVC's CD-4 system (no relation to audio CDs) put four channels of sound on a vinyl LP via ultrasonic sub-carriers -- in essence, two FM stereo stations on a record! This provided four truly separated channels. The matrix systems shifted the phase of the rear channels before mixing down to stereo for the record. Columbia's SQ system used a 90 degree shift, resulting in helical grooves for the rear channels. Other matrix systems did a 180 degree shift. Both were very limited in front-rear separation, which in some cases was augmented by various "logic" schemes. Meanwhile, Dynaco built some of their amplifiers with a "Dynaquad" matrix built in. Primarily intended to get a surround sound out of ordinary stereo records, it worked by subtracting a portion of the mono (L+R) sound separately from the left and right channels before sending them to the rear. Their amplifiers' L and R speaker outputs were common grounded, so to provide these outputs all they did was add a second set of speaker jacks with their common ground set apart from the main common ground by a 20 ohm resistor. It worked quite well for the time, and this is how I -- er, enjoyed the sound of that era.
It was fun to play with the same idea today, with this circuit:
The rear-channel sound tends to suppress front-center soloists, while leaving (or even accentuating) their reverb. With this accomplished, one could add, I suppose, some level expansion and / or delays to boost the effect further.
Thanks again Bob for your time and sharing your experience.
I am sure that you must be enjoying the solder-gun-less experiments with SigmaDSP technology.
A lot of good and not so good things have happened to audio that I am trying to understand.
Dynamic range: pain or boon, Surround sound: more of a pain if proper attention to the ratio of Dialogues to Music to Effects is not maintained. I am struggling with surround sound as it is unable to maintain 'constant acoustic power'.
SigmaDSP is a nice platform to experiment around all this.
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