Such a coincidence that you ask, I was literally wondering the same thing yesterday. I suppose it would have to first check for cd-4 carrier, then check for presence of 90 degree OOP modulations for SQ, otherwise just assumes RM, probably can be fooled with certain recordings.
The ad actually mentions "analog" computer circuitry, which was definitely alive and well during that period, and would have been well-suited for the type of detection circuits furui describes above.
Probably didn’t work too well as others would had came up with a similar approach you would think. Just would had called it something different. Still it would be interesting to read a review of the thing to find out. I am somewhat familiar with detection circuitry as some high end Shortwave radios used something similar but it didn’t always work that great.
If the system automatically indicate if a non-quad stereo recording has SQ-like elements, that itself is interesting/useful because it provide a suggestion of which recordings decode more actively in SQ than RM, which is less common than the other way around.
Since I first heard about this automatic switching I was suspicious that it would either identify a signal as CD-4 and pass it to a demodulator, or otherwise route it to a generic matrix decoder. What Soundfield has identified about the schematic backs up that suspicion. I can't find many pictures of this model, so can't see all of the indicators lit, or what the controls say, but did find this on Ebay and the design hints that it may indeed lump all matrix together based on the indicator I see lit. Probably turn a selector between stereo and four channel, and let the receiver "do" the rest... Put on a phasey stereo album while in four channel, and bam! it thinks it's matrix.
