1. An apparatus for on line welding assessment, comprising: first sampling means to sample the welding voltage or current to provide a sequence of values for a first signal, second sampling means to sample the welding current or voltage to provide a sequence of a values of a second signal, a signal generating means to generate one or more sequences of values for one or more artificial third signals from the first signal and second signals where the artificial signals depend upon values of the first and second signals through generalised discrete point convolution operations, tripling means to identify corresponding values of the first, second and third signals as triplets, collection means to collect triplets of values which are useful for quality monitoring into groups or regions.