Hey Reed:
There were several publications that came out in the quad era but they would be hard to find now. I can give you a quick rundown and keep it simple but I warn you, it does get technical.
There were two types of quad equipment. one was tape and the other was LP based systems. The tape systems were:
Reel to Reel- Used 1/4 inch tape 4 track tape. (note that 4 track does not mean the same as 4 channel. The track number referred to the number of stripes of recording that was on the tape. This is important because stereo tapes are also four track. Reel to Reel machines were discrete, which meant that there was no intermixing of channels. This is a desireable quality as you will see later.
The other tape system was eight track. Eight track had a continuos loop 1/4 inch tape in a cartridge that was inserted in the machine. Unlike reel to reel, eight track could be used in an automobile, and there were many automotive quad units built. Eight track did not have a high end that was as good as reel to reel, not that it couldn't be done, but the high speed duplication techniques that the record companies used didn't lend itself to good sound. Still the sound wasn't bad, and the eight tracks were one of the more popular forms of quad and are easy to find. They were discrete and gave good separation. Eight tracks suffered well known maladies. Sometimes the machine would get hungry and eat the tape, destroying it. Other times, the capstan would slip and the tape would drag. With quad tapes there was twice as much tape, amplifying this flaw.
The other main type of quad system was the LP record based system, and was popular because the LP was the highest quality music delivery medium at the time. Some say it still is, and if you ever heard a good LP system with a new record on it you would understand why. Of the LP systems there were two types. Discrete and matrix. There were two dominant matrix systems and only one discrete system. There was also a strange hybrid (UD-4) of the two but it was never dominant, so we will not discuss it here.
Matrix LP systems used an encoder in the recording process. The encoder mixed the four channels down to two, using a fixed special phase relationship that allowed a decoder to decode the original four channels during playback. The encoded signal was recorded on a regular stereo record. The problem with the matrix systems is that it is difficult to extract the original four channels without a lot of crosstalk. the early decoders couldn't do it. Later gain logic circuits were added that improved the situation. The dominant systems were Columbia records' SQ system and Sansui Electronic's QS system. The pinnacle of development for the SQ system were the Tate SQ decoders made by Audionics and Fosgate. Sansui's Variomatrix system was the QS ultimate. Both were very good, but came late in the game, and were still not as good as discrete quad. There were also rans such as Dynaco, Electro Voice and et al.
The other LP based system, CD-4, was considered discrete, and used a modulator during the recording process. There was on the record not only the audio which contained the front two channels mixed in phase with the rear two channels respectively. it also had two supersonic subcarriers that had the front channels mixed with the rear channels out of phase. The left channels were kept separate from the right channels assuring good left to right separation. The supersonic subcarriers were FM modulated with the out of phase audio. JVC's ANRS noise reduction was used on the subcarrier's audio. It was similar to Dolby B noise reduction, and was double ended. A CD-4 demodulator is required for playback, and since the frequency range of the record extends up to 45 KHZ, a special linear contact stylus and a high frequency cartridge is employed. The turntable wiring has to be low capacitance, to avoid killing down the high frequency subcarrier. CD-4 systems have good separation because there are actually four channels recorded on the disc. The demodulator separates the four original channels from the sum and difference signals. This method is the same as is employed in FM stereo radio and TV stereo audio. A demodulator actually is two demodulators one for left side and one for the right. The problems with CD-4 are that the stylus tracking is very finicky, and if it isn't right, it will let you know in very grating terms. But when it is good, it is very good. RCA promoted CD-4 as well as others and a lot of material was produced. CD-4 records were difficult to make, employing half speed mastering which was created for it, because the cutting heads of the time couldn't follow the supersonic subcarriers.
Many pieces of equipment were built, some employing several of these formats, the one exception was reel to reel which was almost always a separate piece of gear. It was common for a radio receiver to have a matrix decoder and a CD-4 demodulator in the same box, and Sansui, especially the latter of, were some of the best. But the best gear were the separate components. Some of the cheaper systems would have an eight track and a turntable and a matrix decoder in the same box, These are to be avoided, for the decoder was usually very poor, and the turntable was usually cheap and hard on records. By buying separate components, you were able to select the best of the best providing you had the cash. Some of this gear can be had for a modest price on Ebay, and some like the Audionics Space and Image Composer (SQ decoder) can sell for a couple of grand. But don't let that discourage you. Armed with this knowledge, you should be able to get started for a modest amount. Happy hunting!
The Quadfather