Matrix decoding is an audio technology where a small number of discrete audio channels (e.g., 2) are decoded into a larger number of channels on play back (e.g., 5).
Many matrix encoding methods have been developed: The earliest and simpler form of decoding is the Hafler circuit, deriving back channels out of normal stereo recording (2:2:4).
The Dynaquad matrix introduced in 1969 was based on the Hafler circuit, but also used for a specific encoding of 4 sound channels in some albums (4:2:4).
[1] The Stereo-4 matrix was invented by Leonard Feldman and Jon Fixler, introduced in 1970, and sold by Electro-Voice and Radio Shack.
phase-shift The basic SQ matrix had mono/stereo anomalies as well as encoding/decoding problems, heavily criticized by Michael Gerzon and others.
[4] An attempt to improve the system lead to the use of other encoders or sound capture techniques, yet the decoding matrix remained unchanged.
[5] With the Ghent Microphone, SQ was transformed from a Matrix into a Kernel and an additional signal could be derived to provide N:3:4 performance.
In 1976, Ben Bauer integrated matrix and discrete systems into USQ, or Universal SQ.
For a USQ FM broadcast, the additional "T" modulation was placed at 38 kHz in quadrature to the standard stereo difference signal and the "Q" modulation was placed on a carrier at 76 kHz.
For standard 2-channel SQ Matrix broadcasts, CBS recommended that an optional pilot-tone be placed at 19 kHz in quadrature to the regular pilot-tone to indicate SQ encoded signals and activate the listeners Logic decoder.
CBS argued that the SQ system should be selected as the standard for quadraphonic FM because, in FCC listening tests of the various four channel broadcast proposals, the 4:2:4 SQ system, decoded with a CBS Paramatrix decoder, outperformed 4:3:4 (without logic) as well as all other 4:2:4 (with logic) systems tested, approaching the performance of a discrete master tape within a very slight margin.
It first matrixed the four outputs of the SQ decoder to derive additional signals, then compared their envelopes to detect the predominant direction and degree of dominance.
A processor section, implemented outside of the Tate IC chips, applied variable attack/decay timing to the control signals and determined the coefficients of the "B" (Blend) matrices needed to enhance the directionality.
Dolby used the Tate DES IC's in their theater processors until around 1986, when they developed the Pro Logic system.
Unfortunately, delays and problems kept the Tate DES IC's from the market until the late-1970s and only two consumer decoders were ever made that employed them, the Audionics Space & Image Composer and the Fosgate Tate II 101A.
The Fosgate used a faster, updated version of the IC, called the Tate II, and additional circuitry that provided for separation enhancement around the full 360 soundfield.
Unlike the earlier Full Wave-matching Logic decoders for SQ, that varied the output levels to enhance directionality, the Tate DES cancelled SQ signal crosstalk as a function of the predominant directionality, keeping non-dominant sounds and reverberation in its proper spatial locations at their correct level.
The Dolby Stereo Matrix is straightforward: the four original channels: Left (L), Center (C), Right (R), and Surround (S), are combined into two, known as Left-total (LT) and Right-total (RT) by this formula: where j = 90° phase-shift The center channel information is carried by both LT and RT in phase, and surround channel information by both LT and RT but out of phase.
The surround channel is a single limited frequency-range (7 kHz low-pass filtered[8]) mono rear channel, dynamically compressed and placed with a lower volume than the rest.
To overcome this problem the cinema decoder uses so-called "logic" circuitry to improve the separation.
In addition to the logic circuitry the surround channel is also fed via a delay, adjustable up to 100 ms to suit auditoria of differing sizes, to ensure that any leakage of program material intended for left or right speakers into the surround channel is always heard first from the intended speaker.
This exploits the "Precedence effect" to localize the sound to the intended direction.
phase-shift The Pro Logic II matrix provides for stereo full frequency back channels.
Normally a sub-woofer channel is driven by simply filtering and redirecting the existing bass frequencies of the original stereo track.