The decode frequency range, which covers the World's call progress application spectrum, is processed internally as two bands: LO = 300Hz to 660Hz and HI = 900Hz to 2150Hz. Frequency measurement is achieved by counting the number of cycles in a set time period (LO = 39.47ms or HI = 13.16ms). Bad signal/level quality or NOTONE results in a count-abort, timing-reset and no output from the decoder.