Teac A-2400
A four-channel machine whose own documentation admits it can't deliver four discrete channels.
Overview
The Teac A-2400 sits in an awkward corner of the quadraphonic story. The manufacturer's own reference material lists it not as a discrete four-channel machine but as one of the matrix-based systems - Toshiba's QM, Denon's OX4, Matsushita's AFD, Sansui's OS, Kenwood's OR, Hitachi's Ambiphonic, Sony's SQ, and the TEAC A-2400 itself. That grouping is the most important fact about the unit. Teac's own documentation put the A-2400 in the matrix camp, not the discrete camp.
The documentation was candid about what that meant. The same material argues that matrix systems, "for all its convenience and attraction," fall short in one vital area. They are not a true four-channel system, and presumably never can be. The text is blunt about why: matrix encoding suffers intrinsically from cross-talk between channels, and whatever the decoder recovers, it cannot recover four original and discrete channels from a two-channel medium. Recording engineers would have understood this. The marketing of the era often didn't. The caveat is printed right alongside the model name, at least in this one document.
The A-2400 was not alone in this matrix lineup. The list it appears in spans most of the major Japanese manufacturers of the day, each with its own proprietary matrix scheme. The A-2400 is listed among them, not in any discrete four-channel category.
The reference material notes that recording engineers knew the limitations of matrix systems well, even if the mathematics and circuitry were obscure to most technical writers of the time. The loudest message, the document observes, was the commercially inspired one - that the four-channel disc problem had been solved by matrix encoding. The quieter, more accurate message was that it hadn't.
Recording-Engineer (December 1982, p.72) and Recording-Engineer (December 1981, p.70) both appear in the review corpus associated with this model, though neither chunk contains substantive A-2400-specific evaluation — the first covers Australian and Canadian studio upgrades, the second tour audio for the Rolling Stones. No period measurement or listening verdict for the A-2400 survives in the available review material.
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