1. A method of upmixing a two-channel stereo signal to a three-channel signal, the method comprising: computing a sum magnitude by calculating the magnitude of a sum of a left input vector and a right input vector; 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 a center output vector using the estimated center output magnitude; computing a left output vector; and computing a right output vector.