Quadraphonic sound

Modern 4 channels Quadraphonic label

Quadraphonic (or Quadrophonic and sometimes Quadrasonic) sound – equivalent to what is now called 4.0 surround sound – uses four channels in which speakers are positioned at the four corners of the listening space, reproducing signals that are (wholly or in part) independent of one another. Quadraphonic audio was the earliest consumer product in surround sound and thousands of quadraphonic recordings were made during the 1970s.

It was a commercial failure due to many technical problems and format incompatibilities. Quadraphonic audio formats were more expensive to produce than standard two-channel stereo. Playback required additional speakers and specially designed decoders and amplifiers.

Operation

Quadraphonic audio reproduction on vinyl records was problematic. Some systems used a demodulator to decode discrete sound channels. This allowed for full channel separation, although such systems were prone to reduced record life. Other systems used matrix decoding to recover 4 channels from the 2 channels cut on the record. Matrix systems do not have full channel separation and some information can be lost between the encoding and decoding processes. Both discrete and matrix quadrophonic recordings could be played in two channels on conventional stereo record players.

There were also less sophisticated "derived" solutions that only provided back ambience channels, not a defined placement of individual instruments. [1]

Quadraphonic systems based on tape were also introduced, based on new equipment capable of playing four discrete channels. These recordings were released in reel-to-reel and 8-track cartridge formats.

A full, four-channel (quadraphonic) system will reproduce the Left Front, Left Back, Right Front, and Right Back audio signals in each of four separate speakers. Regardless of discrete or matrix formats, in four-channel stereo the rear speakers should be of the same or almost-same size or quality and have the same or almost-same frequency range as the front speakers.

Discrete (4-4-4) formats

Discrete reproduction is the only true Quadraphonic system. As its name suggests, with discrete formats the original four audio channels are passed through a four-channel transmission medium and presented to a four-channel reproduction system and fed to four speakers. This is defined as a 4–4–4 system.

Matrix (4-2-4) formats

With Matrix formats, the four channels are converted (encoded) down to two channels. These are then passed through a two-channel transmission medium (usually an LP record) before being decoded back to four channels and presented to four speakers. To transmit 4 individual audio signals in a stereo-compatible manner, there must be four simultaneous linear equations to reproduce the original 4 audio signals at the output. The term "compatible" indicates that:

  1. A single-channel (mono) system will reproduce all four audio signals in its one speaker.
  2. A two-channel (stereo) system will reproduce the Left Front & Left Back audio signals in the Left speaker and the Right Front & Right Back signals in the Right Speaker.

The original systems (DY & EV-4) were basic and suffered from low front L/R separation (around 12db) and a poor rear L/R separation of 2db. The decoders were designed more to give an effect rather than accurate decoding, which was mainly due to limitations in both systems, although as both systems were very closely related mathematically, users only needed one decoder of either system to play back albums of both systems.

The aboves' poor decode performance was the main reason for their disappearance once the improved matrix systems arrived based on the work by Peter Scheiber. His basic formula utilized 90-degree phase-shift circuitry to enable enhanced 4-2-4 matrix systems to be developed, of which the two main leaders were Columbia's SQ and Sansui's QS Systems. With Scheiber and Martin Willcocks, Jim Fosgate developed the Tate II 101 SQ decoder, which produced a very accurate sound field by using gain riding and the Haas effect to mask decoding artifacts. It used custom, hand-assembled and -calibrated circuitry with components sorted to 1%, for exact performance. Sansui's QSD- series decoders and QRX- series receivers were very good, even synthesizing L—R stereo into a ⋂ horseshoe topology. However, all these came too late in the game and were too expensive or difficult to procure for public purchase, to rescue matrix quad.

The differences between the original systems and the new were so large that it made it impossible to decode DY/EV-4 with either SQ or QS decoders with any accuracy, the results being just a form of artificial quad.

[2] [3] This 4:2:4 process could not be accomplished without some information loss. That is to say, the four channels produced at the final stage were not truly identical to those with which the process had begun.

Derived (2-2-4) formats

Derived (2-2-4) formats were inexpensive electronic solutions that provided back ambiance channels from regular stereo records. There was no deliberate placement of individual instruments on the back channels. [1]

Other Languages
asturianu: Cuadrafónicu
dansk: Kvadrofoni
Deutsch: Quadrofonie
español: Cuadrafónico
français: Quadriphonie
italiano: Quadrifonia
polski: Kwadrofonia
português: Quadrifônico