The MX613 is a wide-band, `N-Tone' non-predictive tone decoder that measures telephone system call progress tones in PABX, Pay/Feature-Phone, Fax and Modem systems. Adhering to Must/Must-Not Decode limits and able to measure inband frequencies in outband modulation, this decoder measures the frequency of input signals in the range 300 to 2,150Hz. The result of each measurement is presented to a system Processor as a 6-bit serial word. The decode frequency range, which covers the world's call progress application spectrum, is processed internally as two bands: LO = 300 to 660Hz and HI = 900 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.