1. A method of upmixing a two-channel audio signal to a three-channel audio signal, the method comprising: computing in a computing device a sum magnitude by calculating the magnitude of a sum of a left input vector and a right input vector, wherein the left and right input vectors are related to a left audio channel and a right audio channel, respectively, of the two-channel audio signal; computing a difference magnitude by calculating the magnitude of a difference of the left input vector and the right input vector; using the sum magnitude and the difference magnitude to obtain an estimated center output magnitude; calculating in the computing device a center output vector using the estimated center output magnitude; computing in the computing device a left output vector; and computing in the computing device a right output vector, wherein the left, center, and right vectors are related to left, center, and right audio channels, respectively, of the three-channel audio signal.