Pattern matching apparatus for comparing signal samples with a pattern to find signal samples substantially matching said pattern, the apparatus comprising: a projector for projecting said signal samples over a tree of projection vectors of successively increasing frequency at nodes of said tree to successively provide projection values, a distance measurer, associated with an output of said projector for determining a distance between a projection value and a corresponding projection value of said pattern, said distance being an expression of a minimum bound of a true distance between said sample and said pattern, and a thresholder, associated with an output of said distance measurer for applying a threshold to said minimum bound to successively reject samples at respective nodes.