An improvement in simulated three-channel stereo

Some two-channel stereo systems have employed a third full-range speaker system in the center, reproducing an equal in-phase mixture of the signals in the two channels. Advantages of this arrangement over the usual two-speaker array are better reproduction of the location and size of central sound sources. A disadvantage is the sizeable reduction in the apparent spread, or distance between flanking sources. The reasons for these effects are discussed, and it is indicated that this disadvantage can be largely overcome by electrically reducing the ratio of sum to difference of the two channels, which ratio was effectively increased by the addition of the center speaker. It is shown that the signals to the three speakers may be regarded as three, independent channels with certain signal-to-crosstalk ratios, which are derived as a function of the level of the center speaker and the amount of electrical reduction of the sum-to-difference ratio. The choice of optimum parameter values and appropriate circuits is discussed.