US 3,944,753 A · Apparatus for distinguishing voice and other noise signals from legitimate multi-frequency tone signals present on telephone or similar communication lines

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.

Claims12

Dependent claims

An apparatus for detecting when signals appearing on a communications line do not contain voice or other noise signals, comprising:

a. detector means coupled to the communications line for providing a first output signal which is initiated whenever signals appear on the communications line and which is terminated only when the signals on the communications line rapidly undergo a variation in frequency exceeding a predetermined amount,
b. timing means coupled to said detector means for providing a second output signal at a predetermined time after said first output signal is initiated by said detector means, and
c. gating means coupled to said detector means and to said timing means and providing a third output signal upon the concurrence of said first and second output signals.

A circuit which discriminates against voice and other noise signals appearing on a pair of telephone lines, said circuit having an input for connection to the pair of telephone lines and an output, and including

a. a first frequency change detection means having an input connected to said circuit input and an output, and operative to provide a first signal on its output which is initiated whenever signals are present on the pair of telephone lines and operative to terminate said first signal whenever the signals rapidly undergo a variation in frequency exceeding a predetermined amount,
b. timing means having an input connected to the output of said first frequency change detection means and an output, said timing means being operative to provide a second signal on its output at a predetermined time after said first signal is initiated by said first frequency change detection means, and
c. first gating means having first and second inputs connected to the outputs of said frequency change detection means and said timing means, respectively, and an output connected to said circuit output, and operative to provide a third signal on its output at the concurrence of said first and said second signals.

Dependencies are inferred from the claim text.

Apparatus for distinguishing voice and other noise signals from legitimate multi-frequency tone signals present on telephone or similar communication lines

Life of this patent

Apparatus for distinguishing voice and other noise signals from legitimate multi-frequency tone signals present on telephone or similar communication lines

Life of this patent

Abstract

Claims12