Apparatus for distinguishing voice and other noise signals from

Abstract

In order to distinguish voice and other noise signals which may be present on telephone or similar communications lines from legitimate multi-frequency tone data and control signals, three detectors are disclosed. A frequency drop detector is provided which receives an input from the lines and which provides an output signal upon receipt of any signals appearing on the lines which terminates when the frequency thereof rapidly drops by a predetermined amount. Means consisting of a long-time interval detector are provided for investigating the status of the frequency drop detector's output for a predetermined time after the production of said output signal. If the output signal from said frequency drop detector is provided throughout said predetermined time period, the signal on the lines is defined as legitimate and a circuit output signal is produced. In all other cases, the circuit output signal is inhibited. In a telephone communications application where audible twin-tone signals are present along with voice and other noise signals, first and second frequency drop detectors are provided for the high and low frequency groups making up such twin-tone signals. Also, a frequency coincidence detector is provided which provides an output signal only when the components of the signals in both the high and the low groups do not have the same frequency. In this embodiment, the presence of all of the output signals so described for a period of time equal to or greater than that established causes an output to be produced which signifies that the signals are not likely voice or other noise signals.