a method of converting a plurality of input signals on first and second converters , such that the first and second converters are both used when the plurality of signals comprises two signals , characterised in that said method comprises : selecting more than two input signals ; determining the type of each selected signal ; combining any signals having the same type to form a combined signal ; converting one type of signal with the first converter ; converting a second type of signal with the second converter wherein the first or second type signals is a combined signal .