Quadraphonic sound refers to four-channel audio recording and reproduction. It is also called quad stereo or four-channel stereo.

A conventional stereophonic circuit splits sound into two channels, the left channel and the right channel. In a quad stereo system, both of these channels are separated into front and rear components. In a true quadraphonic system, each of the four channels is independent of the other three.

This is not the case in all so-called quadraphonic systems. Although true quad sound needs four tracks on a recording, some systems use two tracks and combine the left and right channels in phase quadrature (90 degrees out of phase) to obtain the front and rear signals.

The speakers should be level with the listener, equidistant from the listener, and separated by angles of 90 degrees from the listener’s point of view. If the listener is facing north, the left front speaker is to the northwest, the right front speaker is to the northeast, the left rear speaker is to the southwest, and the right rear speaker is to the southeast.

This provides optimum balance, and also facilitates the greatest possible left-to-right and front-to-rear contrast in the perceived sounds.

It is possible to add two more channels to quad sound, placing a fifth channel above the listener and a sixth channel below. These might be called “zenith” and “nadir” channels, respectively, and the system called hexaphonic sound.

The main problem with this scheme is that the optimum placement of the listener is critical and rather bizarre. The ideal arrangement would require that the user sit or lie suspended in the exact spatial center of a perfectly cubical room. However, some audiophiles might put up with that to enjoy the effects.

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