What is a receiver or tuner “FM Quadradial Output” jack used for?
Quadradial jacks are basically a direct take-off of the FM detector circuit prior to the filters that remove the stereo subcarrier signal components at 19 and 38 kHz. The idea at the time was that FM stations would begin broadcasting in a 4 channel mode (some did, in fact, do so) and that Marantz (or a third party) would develop a device that would extract the 4-channel information from the “raw” FM signal available at the Quadradial jack in much the same manner that the stereo decoding circuitry in tuners had been extracting stereo since the late 50’s/early 60’s. There were a number of quad encoding schemes tried; but all in all, there are very few, if any, external FM decoding units out there now, and of course there are no quad FM signals to decode, either… That direct take off can be used to decode other specially encoded signals, though. There are a number of things “stuffed” into the FM signal such as SCA audio (“Muzak”-type stuff, usually), the new character station ID’s and s
