Stereo-4, also known as EV (from Electro-Voice) or EV-4, was a matrix 4-channel quadraphonic sound system developed in 1970 by Leonard Feldman and Jon Fixler.
The system was heavily promoted by RadioShack stores in the USA, and there were quite a few companies that released albums encoded in this format. It is said it was the first commercial quadraphonic matrix system in the market. In America it was the first commercial quadraphonic system.
EV used different sets of coefficients for encoding and decoding. Most other systems have decode coefficients that mirror the encode coefficients. Therefore the EV Stereo-4 matrix was something in between a 2-2-4 derived system and a 4-2-4 matrix system. EV decoders were sometimes used to produce pseudo 4-channel effects from 2-channel stereo recordings.
The original EV system was compatible with the Dynaquad DY system, and is closely related to Sansui's QS Regular Matrix system. The EV and QS records are very close to each other—it would take an expert to tell them apart by ear.
In 1973 Electro-Voice signed an agreement with Columbia/CBS Records to build a new universal decoder that could decode both SQ and EV records with good results. It could even decode QS records—again, with good results. EV later suggested the same coefficients for an encoder, but no records were ever produced with the so-called EV Version 2 System. EV more or less disappeared since the SQ Stereo Quadraphonic system was introduced by Columbia/CBS Records in America.
Records
Some records were made in EV.
The record label Ovation, who initially used the EV matrix, later changed to the Sansui QS matrix.