1. An apparatus, comprising: a transformer to convert at least a portion of a block of audio into a frequency domain representation including a plurality of frequency components; a decision metric processor to: define a band of the frequency components; determine a difference in energy between a first convolution of a first complex vector with a first group of frequency bins in the band and a second convolution of a second complex vector with a second group of frequency bins in the band; and determine a decision metric for the band based on the difference; a signature determiner to determine a signature based on a value of the decision metric; and a processor to implement at least one of the transformer, the decision metric processor, or the signature determiner.