Quadrophonic sound refers to any audio system that uses four discrete audio channels to create a surround sound experience. The four channels usually consist of front left, front right, back left, and back right. This provides more directionality and realism than regular two-channel stereo.
The concept of quadrophonic sound dates back to the late 1950s and early 1960s, when engineers experimented with using four discrete channels of audio along with manipulations of interchannel time and phase differences to achieve a sense of surround sound. However, quadrophonic systems did not become commercially viable until the 1970s.
In 1970, CBS Laboratories introduced the SQ Quadraphonic system, which encoded four audio channels into two channels that could be played back through a regular stereo system. This allowed quadrophonic audio to be available on vinyl records, FM radio, and tape. Other competing matrix systems like Sansui QS and Ambisonics also appeared around this time.
The 1970s saw a boom of interest in quadrophonic sound, with many rock albums being released in quadrophonic formats and consumer audio equipment incorporating quad decoding capabilities. However, by the late 1970s, enthusiasm had waned due to poor mixing quality control and a lack of available content.
After its initial popularity period, quadrophonic sound remained a niche interest. However, it helped pave the way for future surround sound developments like Dolby Stereo in the 80s and 5.1/7.1 surround sound for home theaters. Elements of quadrophonic audio mixing techniques are still sometimes used in modern movie and music production.