In 1970 RCA introduced quadraphonic 8 track tapes. This new system provided discrete 4 channel sound, similar to today’s surround sound. These tapes required special systems capable of sensing the 4 channels of sound and delivering it to 4 speakers, typically one in each corner of the room. Music being delivered from all corners of the room gave the experience of a concert hall.
This equipment had to be capable of playing both quadraphonic and stereo tapes. To accomplish this the cartridges had to be the same with one exception. The quadraphonic cartridges had a notch in the upper left hand corner. When a stereo tape was inserted into one of these players, the cartridge would push on a switch and let the electronics know that a stereo tape had been inserted. When a quadraphonic tape was inserted the switch was not engaged because of the notch in the cartridge. This let the player know that a quadraphonic tape had been inserted.
Blank recording tapes were also available. These tapes had a removable plug in the notch. If you wanted to make a stereo recording, you left the plug in the cartridge. If you wanted to make a quadraphonic recording, you removed the plug.
The quadraphonic scene was short lived, due mostly to the fact that most people did not want to invest in the new specialized equipment. Due to its short life span, quadraphonic equipment and tapes are more rare than stereo equipment and stereo tapes. As such, quadraphonic equipment and tapes are much more valuable in the collector’s market.
In the photo below the tape on the left is a stereo tape (no notch), the center tape is a quadraphonic tape (notch in the upper left corner) and the tape on the right is a blank tape for home recording (plug in the notch).
