Security identification system using pulse code modulation

• US 4,289,931 A
• Filed: 11/28/1979
• Issued: 09/15/1981
• Est. Priority Date: 11/28/1979
• Status: Expired due to Term

• Current Assignee: Micro-Tek Incorporated
• Original Assignee: Micro-Tek Incorporated
• Inventors: Baker, Donald G.
• Primary Examiner(s): Popek, Joseph A.
• Application Number:
  US06/095,004
• Time in Patent Office: 657 Days
• Field of Search: 179/2 DP, 179/5.5, 179/18 FH, 179/2 CA, 340/147 R, 370/110
• US Class Current: 379/247
• CPC Class Codes: H04M 1/573 Line monitoring circuits for detecting caller identification

Abstract

A security identification system for telephone, teleprinter, computer, and similar communication applications comprising a plurality of transmitter station identification units, one for each transmitter station and each including an identification signal generator for generating a pulse code modulated identification signal, unique to the transmitter station, that comprises a repeating sequence of uncoded signal segments each including N pulse intervals of given duration alternating with coded signal segments each including M pulse intervals of like duration, coding being effected by the presence or absence of pulses in the coded signal segment intervals. Each pulse is transmitted as a burst of cycles of a carrier signal preferably having a carrier frequency above 3 Khz. The system further comprises a receiver identification unit at a receiver station, including a demodulator, an identification control circuit, and a visual display; the control circuit counts the pulse intervals in a demodulated uncoded segment of the identification signal to aid in identifying the beginning of each coded segment and decodes the coded segments, on a redundant basis, to actuate the display and afford a visual identification of the transmitter station.

Claims

1. A security identification system for a telephone transmission system including a plurality of individual transmitter stations at varying locations connectable through the transmission system to a receiver station, the security identification system comprising:

a plurality of transmitter identification units, each connected to one of the transmitter stations, each transmitter identification unit including;

identification signal generator means for generating and applying to the transmitter station telephone line a pulse code modulated identification signal unique to the transmitter station to which the identification unit is connected, the identification signal including a repeating sequence of uncoded and coded signal segments, each uncoded signal segment comprising a series of N pulse intervals of fixed duration D, each coded signal segment comprising a series of M pulse intervals of fixed duration D with coding effected by the presence or absence of pulses in those intervals, each code pulse comprising a burst of cycles of a carrier signal of frequency F, with F>

>

1/D,

the identification signal generator means comprising a carrier signal generator, including a stabilized oscillator, for developing a continuous carrier signal at the carrier frequency F, an encoding circuit, utilizing the carrier signal as a synchronizing signal, for developing a pulse code signal corresponding to the uncoded and coded segments of the identification signal, and a modulator for modulating the carrier signal with the pulse code signal to generate the identification signal,

and filter means, interposed in series in the transmitter station telephone line, to preclude transmission of the identification signal to the transmitter station;

a visual display associated with the receiver station;

and a receiver identification unit, associated with the receiver station, comprising;

demodulator means, coupled to the receiver station telephone line, for demodulating the identification signal to develop a pulse code signal corresponding to the identification signal but with the carrier effectively eliminated,

control means for utilizing the pulse code signal to actuate the display to afford a visual representation identifying the particular transmitter station from which that identification signal originates, the control means including means for identifying the beginning of a coded signal segment in the pulse code signal by counting successive pulse intervals in an uncoded signal segment to assure identification of the beginning of a coded signal segment,

and filter means, interposed in series in the receiver telephone line, to preclude transmission of the identification signal to the receiver station.

2. A security identification system according to claim 1 in which the control means in the receiver identification unit includes means for verifying at least one repetition of the coded signal segment without change as a precondition to actuation of the display.

3. A security identification system according to claim 1 or claim 2 in which the carrier frequency F is at least about 3 Khz.

4. A security identification system according to claim 3 in which the filter means in the transmitter identification unit is a notch filter tuned to the carrier frequency F.

5. A security identification system according to claim 1 or claim 2 in which the identification signal generator means further comprises an integrator circuit interposed between the modulator and the transmitter station telephone line.

6. A security identification system according to claim 1 or claim 2 in which the encoding circuit comprises:

a frequency divider, actuated by the carrier signal, for developing a pulse signal having a repetition frequency 1/D at a sub-multiple of the carrier frequency F;

and at least one coded shift register, actuated by the pulse signal, for developing the pulse code signal, with N=M.

7. A transmitter identification unit for connection to the telephone line of an individual transmitter station in a telephone transmission system, as a part of a security identification system for the telephone transmission system, comprising:

a carrier signal generator, including a stabilized oscillator, for developing a carrier signal at a frequency F;

an encoding circuit, utilizing the carrier signal as a synchronizing signal, for developing a pulse code signal including a repeating sequence of uncoded and coded signal segments, each uncoded signal segment comprising a series of N pulse intervals of fixed duration D, each coded signal segment comprising a series of M pulse intervals of fixed duration D, with coding effected by the presence or absence of pulses in those intervals, with F>

>

1/D,

a modulator for modulating the carrier signal with the pulse code signal to generate an identification signal corresponding to the pulse code signal but with each code pulse comprising a burst of cycles of the carrier signal;

coupling means for applying the identification signal to the telephone line of a transmitter station;

and filter means for interposition in the telephone line to preclude transmission of the identification signal to the transmitter station.

8. A transmitter identification unit for a telephone security identification system according to claim 7, in which the encoding circuit comprises:

a frequency divider, actuated by the carrier signal, for developing a pulse signal having a repetition frequency 1/D at a sub-multiple of the carrier frequency F;

and at least one coded shift register, actuated by the pulse signal, for developing the pulse code signal, with N=M.

9. A transmitter identification unit for a telephone security identification system according to claim 7 or claim 8 in which the coupling means comprises an integrator circuit.

10. A transmitter identification unit for a telephone security identification system according to claim 7 or claim 8 in which the carrier frequency F is at least about 3 Khz.