Matrix decoder

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). The channels are generally, but not always, arranged for transmission or recording by an encoder, and decoded for playback by a decoder. The function is to allow multichannel audio, such as quadraphonic sound or surround sound to be encoded in a stereo signal, and thus played back as stereo on stereo equipment, and as surround on surround equipment – this is "compatible" multichannel audio.

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). The channels are generally, but not always, arranged for transmission or recording by an encoder, and decoded for playback by a decoder. The function is to allow multichannel audio, such as quadraphonic sound or surround sound to be encoded in a stereo signal, and thus played back as stereo on stereo equipment, and as surround on surround equipment – this is "compatible" multichannel audio. ===Decoding matrix=== ===Encoding matrix=== ===Decoding matrix=== ===Encoding matrix=== ===Decoding matrix=== j = +90^\circ phase-shift, k = -90^\circ phase-shift The basic SQ matrix had mono/stereo anomalies as well as encoding/decoding problems, heavily criticized by Michael Gerzon and others. An attempt to improve the system lead to the use of other encoders or sound capture techniques, yet the decoding matrix remained unchanged. j = +90^\circ phase-shift, k = -90^\circ phase-shift The Forward-Oriented encoder caused Center Back to be encoded as Center Front and was recommended for live broadcast use for maximum mono compatibility - it also encoded Center Left/Center Right and both diagonal splits in the optimal manner. Could be used to modify existing 2-channel stereo recordings and create 'synthesized SQ' that when played through a Full-Logic or Tate DES SQ decoder, exhibited a 180° or 270° synthesized quad effect. Many stereo FM radio stations broadcasting SQ in the 1970s used their Forward-Oriented SQ encoder for this. For SQ decoders, CBS designed a circuit that produced the 270° enhancement using the 90° phase shifters in the decoder. Sansui's QS Encoders and QS Vario-Matrix Decoders had a similar capability. j = +90^\circ phase-shift, k = -90^\circ phase-shift The Backwards-Oriented Encoder was the reverse of the Forward-Oriented Encoder - it allowed sounds to be placed optimally in the back half of the room, but mono-compatibility was sacrificed. When used with standard stereo recordings it created "extra wide" stereo with sounds outside the speakers. Some encoding mixers had channel strips switchable between forward-oriented and backwards-oriented encoding. j = +90^\circ phase-shift, k = -90^\circ phase-shift ==Matrix H (4:2:4)== j = 20° phase-shift k = 25° phase-shift l = 55° phase-shift m = 115° phase-shift == Ambisonic UHJ kernel (3:2:4 or more)== j = +90^\circ phase-shift, k = -90^\circ phase-shift 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 ) mono rear channel, dynamically compressed and placed with a lower volume than the rest. This allows for better separation of signals. This gives good compatibility with both mono playback, which reproduces L, C and R from the mono speaker with C at a level 3 dB higher than L or R, but surround information cancels out. It also gives good compatibility with two-channel stereo playback where C is reproduced from both left and right speakers to form a phantom center and surround is reproduced from both speakers but in a diffuse manner. A simple 4-channel decoder could simply send the sum signal (L+R) to the center speaker, and the difference signal (L-R) to the surrounds. But such a decoder would provide poor separation between adjacent speaker channels, thus anything intended for the center speaker would also reproduce from left and right speakers only 3 dB below the level in the center speaker. Similarly anything intended for the left speaker would be reproduced from both the center and surround speakers, again only 3 dB below the level in the left speaker. There is, however, complete separation between left and right, and between center and surround channels. To overcome this problem the cinema decoder uses so-called "logic" circuitry to improve the separation. The logic circuitry decides which speaker channel has the highest signal level and gives it priority, attenuating the signals fed to the adjacent channels. Because there already is complete separation between opposite channels there is no need to attenuate those, in effect the decoder switches between L and R priority and C and S priority. This places some limitations on mixing for Dolby Stereo and to ensure that sound mixers mixed soundtracks appropriately they would monitor the sound mix via a Dolby Stereo encoder and decoder in tandem. 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. j = +90^\circ phase-shift, k = -90^\circ 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. ==See also== Ambisonic UHJ format Dolby Digital Dolby Pro Logic Haas effect Quadraphonic sound Surround sound

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