Blatant self-promotion or linking to info?. You be the judge!!
www.geocities.com/quadaudio/
Look around inside and at the links page. A little bit of info about quadcasting.
Some attempts used two stations and others used one station broadcasting a matrix record. Supposedly any station broadcasting in FM could air quad if a SQ or QS record was played, though that is just what I read at a couple Web sites when I dd research awhile back.
The FCC had a long-running effort at setting a standard that wasn't decided upon until after quad had "died."
Various stations across the land purposefully broadcast in quad though their efforts were limited due to the sparseness of material i.e. some records only available in CD-4, not all music released in quad, etc.
The King Bisquet Flour Hour had some quad live performance releases that are highly sought after today.
Sadly, many of the sites I retrieved info from are no longer on the Web. I wished I would have gathered more info. Some of those sites included info written by those that had been involved with quadcasting. I also corresponded with a retired DJ a few years ago who had been working when quad was being aired. He had some interesting stories that I should have kept. Sigh.......
Anyway.... try my site and the links along with a thorough Web search. Ensure you try various search terms since different results are obtained when word and sentence structures are altered.
Happy Quadding!!!!!!!