All About Quadrophonics
Quadraphonic systems allow the recording and reproduction of four distinct channels of sound (as opposed to the two channels of standard stereo). The system the Floyd used for their three Quad albums (_Atom Heart Mother_, DSotM, and WYWH) was also known as “the SQ system.” The following is an explanation of the system taken from a quad LP, and some additional notes, both courtesy Aaro Koskinen.
“How the SQ system works”
The SQ systems enables four channels of sound to be reproduced from the left and right hand walls of the groove of an LP record.
To attain this it has been necessary to develop two devices — the Encoder to convert four signals from the quadraphonic master tape to two signals and the Decoder to decipher these complex signals and convert them back again to four separate signals for reproduction from four loudspeakers.
With the two additional channels of the quadraphonic tape, the Encoder produces two additional modulation components.
The SQ disc, then, carries in its grooves four encoded signals.
To decipher these four signals and recreate the quadraphonic sources, the SQ Decoder is needed. The Decoder senses the four basic modulations and produces four signals containing predominantly the sounds of the corresponding original four quadraphonic tape channels.
For the SQ system, these are:
Front left – major left w/minor feeds to rear left and front right
Front right – major right w/minor feeds to front left and rear right
Rear left – major rear left w/minor feeds to front left and rear right
Rear right – major rear right w/minor feeds to rear left and front right
These signals are then directed to power amplifiers and four loudspeakers placed in the respective corners of the listening area, resulting in a realistic reproduction of the original quadraphonic master tape.
However, no special pickup is needed. Stereo pickup cartridges are capable of receiving all of these modulations.
“What do I need to play Quad albums?”
The only special equipment you need is a “SQ decoder”, which you place on the turntable (cartridge) output leads. Then you need of course two amps and four speakers.
Apparently the decoders are no longer being manufactured.
BTW, if you play Quad LPs without decoder, the result is just normal stereo (except the mix is different. e.g. the version of Money in the quad mix features a few seconds of Dave Gilmour scatting at the end of the song that are not on the regular stereo mixes).
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