A method performed by one or more processing devices for upmixing a two-channel input audio signal having a first input channel and a second input channel into an upmixed multi-channel output audio signal having greater than two channels, comprising: calculating an estimated panning angle from an inter-channel level difference between the first and second input channels; calculating an in-phase coefficient and an out-of-phase coefficient using the estimated panning angle; calculating a first dematrixing coefficient and a second dematrixing coefficient based on an inter-channel phase difference between the first and second input channels, the in-phase coefficient and the out-of-phase coefficient; multiplying the first input channel by the first dematrixing coefficient to generate a first sub-signal and the second input channel by the second dematrixing coefficient to generate a second sub-signal; mixing the first sub-signal and the second sub-signal in a linear manner to generate an output channel of the upmixed multi-channel output audio signal; and outputting the generated output channel for playback through speakers; characterised in that calculating the dematrixing coefficients comprises: calculating an in-phase signal component, based on the inter-channel phase difference multiplied by the in-phase coefficient, and calculating an out-of-phase signal component, based on the inter-channel phase difference multiplied by the out-of-phase coefficient; and calculating the first dematrixing coefficient based on a first trigonometric function of a combination of the in-phase signal component and the out-of-phase signal component, and calculating the second dematrixing coefficient based on a second trigonometric function of the combination of the in-phase signal component and the out-of-phase signal component. calculating an estimated panning angle from an inter-channel level difference between the first and second input channels; calculating an in-phase coefficient and an out-of-phase coefficient using the estimated panning angle; calculating a first dematrixing coefficient and a second dematrixing coefficient based on an inter-channel phase difference between the first and second input channels, the in-phase coefficient and the out-of-phase coefficient; multiplying the first input channel by the first dematrixing coefficient to generate a first sub-signal and the second input channel by the second dematrixing coefficient to generate a second sub-signal; mixing the first sub-signal and the second sub-signal in a linear manner to generate an output channel of the upmixed multi-channel output audio signal; and outputting the generated output channel for playback through speakers; characterised in that calculating the dematrixing coefficients comprises: calculating an in-phase signal component, based on the inter-channel phase difference multiplied by the in-phase coefficient, and calculating an out-of-phase signal component, based on the inter-channel phase difference multiplied by the out-of-phase coefficient; and calculating the first dematrixing coefficient based on a first trigonometric function of a combination of the in-phase signal component and the out-of-phase signal component, and calculating the second dematrixing coefficient based on a second trigonometric function of the combination of the in-phase signal component and the out-of-phase signal component. calculating an in-phase signal component, based on the inter-channel phase difference multiplied by the in-phase coefficient, and calculating an out-of-phase signal component, based on the inter-channel phase difference multiplied by the out-of-phase coefficient; and calculating the first dematrixing coefficient based on a first trigonometric function of a combination of the in-phase signal component and the out-of-phase signal component, and calculating the second dematrixing coefficient based on a second trigonometric function of the combination of the in-phase signal component and the out-of-phase signal component.