A synthesiser to generate sounds often in a musical environment but also in a speech environment. Such synthesis of sounds is often accompanied by at least one mode of filtering or many modes. Hitherto, the application of such filtering has involved a control for each filter. This may be by way of a lever, toggle or rotatable knob. Thus, a user when applying the filters is required to adjust each control independently of the other. This enhances the complexity and a large amount of skill is involved in accurately selecting the required amount and types of filtering. Thus, the present invention is directed towards a user interface for controlling the filtering of a signal input (such as a sound), said user interface comprising a rotatable controller having two or more sectors, each sector of said controller corresponding to a respective filter, whereby in use said controller is rotated to align said sectors with respect to said signal input thereby controlling the application of the filters to the signal input.