Signaturing methods and apparatus as described. An example method includes applying a first function to a portion of digital data to produce a first windowed block, applying a second function different from the first function to the same portion of the digital data to produce a second windowed block, determining a first characteristic of a band of frequencies in the first windowed block, determining a second characteristic of the band of frequencies in the second windowed block, comparing the first characteristic to the second characteristic, and assigning a signature bit representative of the portion of the digital data based on the comparison of the first characteristic and the second characteristic.