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United States Patent 
Chung et ah
[ii] 4,368,362  Jan. 11,1983
 AUTOMATIC TELEPHONE
LOADED/NONLOADED FACILITY TYPE
 Inventors: Li-Jin W. Chung, Burlington, N.C.;
Ernest P. Moore, Berkeley Heights,
N.J.; Glendon R. Porter, Denville,
N.J.; Joseph F. Rizzo, Lodi, N.J.
 Assignee: Bell Telephone Laboratories,
Incorporated, Murray Hill, N.J.
 Appl. No.: 173,020
 Filed: Jul. 28, 1980
 Int. C1.3 H04B3/20
 U.S. CI 179/170.2; 179/170 D
 Field of Search 179/170.2, 170.6, 170.8
179/175.3 R, 175.31 E, 81 A, 170 D, 175.2 R;
 References Cited
U.S. PATENT DOCUMENTS 3,982,080 9/1976 Ukeiley 179/170 D
4,096,362 6/1978 Crawford 179/170 D
4,224,483 9/1980 Neigh et al 179/175.2 R
4,276,450 6/1981 Chowaniec 179/170 D
Primary Examiner—Joseph A. Popek
Attorney, Agent, or Firm—Thomas Stafford
A bidirectional transmission path is typically coupled to unidirectional receive and transmit paths to effect amplification in repeaters or the like. A determination of whether the bidirectional path includes either loaded or nonloaded type 2-wire cable is made by inserting a test signal having a predetermined frequency and a predetermined amplitude into the receive path and measuring the peak amplitude of a transmit signal developed on the transmit path. If the transmit signal peak amplitude is greater than a predetermined threshold value, the 2wire cable is considered loaded type, and if the transmit signal peak amplitude is less than the threshold value, the 2-wire cable is considered nonloaded type.
5 Claims, 4 Drawing Figures
U.S. Patent Jan. ll, 1983 Sheet 1 of 2 4,368,362
U.S. Patent Jan. 11,1983 sheet 2 of 2 4,368,362
Accordingly, shown in FIG. 1 is coupling circuit 101 including transformer 102 having a first winding 103 and a second winding 104. Winding 103 is adapted to be connected via terminals T and R to a bidirectional transmission path or facility, for example, two-wire 5 loaded or nonloaded telephone cable. Included in winding 103 is the usual midpoint capacitor employed in well-known fashion to extract signaling information. Winding 104 is adapted to be connected to receive and transmit unidirectional transmission paths or facilities. 1° Although winding 104 is shown as being connected in single-ended configuration, it may equally be connected in a balanced configuration, as will be apparent to those skilled in the art. Transformer 102 may be any one of numerous coupling transformers known in the art and, ^ preferably, has a 1:1 turns ratio.
A first terminal of winding 104 is connected to a reference potential point, for example, ground potential, while a second terminal of winding 104 is connected to adjustable capacitor 105, one input of gain unit 106 and one terminal of resistor 107. The output of gain unit 106 is adapted to be connected via terminal 116 to a transmit unidirectional path or facility and is connected to detector 108. Similarly, one input of gain unit 109 is adapted to be connected via terminal 110 to a receive unidirectional path or facility. The output of gain unit 109 is connected to a second terminal of resistor 107 and to an input of canceler circuit 111. An output from canceler circuit 111 is connected to a second input of gain unit 3Q 106. An output of test signal source 112 is connected to a second input of gain unit 109.
Control unit 114 generates signals for controlling operation of the transmission network, in accordance with the invention, to automatically identify the type 35 bidirectional transmission path or facility connected to terminals T and R. To this end, control signals are extended to adjustable capacitor 105, gain unit 109, canceler 111, and signal source 112, and output L/NL from detector 108 and output TCLK from source 112 are 40 supplied to control unit 114. A signal for initiating operation of the circuit either automatically or manually is supplied via start terminal 115.
Control unit 114 includes a microcomputer arrangement, for example, an Intel 8748 unit commercially 45 available. For additional details of control unit 114, see our copending applications filed concurrently herewith, Ser. Nos. 173,011 and 173,014.
Signal source 112 under control of signals from control unit 114 supplies a test signal having a predeter- 50 mined frequency and amplitude to the second input of gain unit 109 and, hence, to the receive transmission path. Since the test signal is inserted into the receive path of the coupling circuit and, hence, supplied via coupling circuit 101 to the bidirectional path or facility, 55 the need for connecting a measurement circuit directly to the bidirectional path or facility is eliminated. Signal source 112 may be any of numerous arrangements known in the art capable of controllably supplying a desired test signal. In this example, not to be construed 60 as limiting the scope of the invention, the test signal supplied by source 112 has an amplitude of 1 volt peak and a frequency of 3400 Hz. The reason for selecting a test signal having a frequency of 3400 Hz is discussed below in relation to FIG. 3. Signal TCLK supplied 65 from source 112 is a pulse signal having a frequency equal to the test signal and is employed in control unit 114 for synchronization.
Canceler circuit 111 includes active impedance circuits having transmission characteristics to match loaded or nonloaded bidirectional cables connected via terminals T and R. Details of the canceler circuits are not necessary to an understanding of the operation of the present invention. However, it should be noted that canceler 11 includes arrangements which in response to control signals from unit 114 controllably connect or disconnect the desired impedance circuits between the output of gain unit 109 and the second input of gain unit 106. In the operation of this invention, the canceler path is open circuited via signals from control unit 114. Details of loaded and nonloaded canceler circuits are shown in our copending applications, Ser. Nos. 173,011 and 173,014, respectively, cited above.
Similarly, details of adjustable capacitor 105 are not needed for an understanding of the present invention. It is noted, however, that in operation of the invention, adjustable capacitor 105 is adjusted via signals from control unit 114 to a prescribed value. In an example from experimental practice, the capacitance value is set to 0.006 microfarads. Details of an adjustable capacitor are also shown in our copending applications Ser. Nos. 173,011 and 173,014 cited above.
Gain units 106 and 109 are substantially unidirectional amplifiers of the differential type commonly referred to as operational amplifiers now well known in the art. Gain unit 109 also includes a switching arrangement to disconnect the receive path during operation of the invention. This is effected via signal CIP from control unit 114.
FIG. 2 shows in simplified form details of detector 108. Accordingly, FIG. 2 shows buffer amplifier 201, full wave rectifier 202, peak detector 203 and comparator 204. Signals from the output of transmit gain unit 106 are supplied via buffer amplifier 201 to full wave rectifier 202. The rectified output from unit 202 is supplied to peak detector 203. In turn, the peak detector output signal is supplied to one input of comparator 204. A predetermined threshold level is established at a second input of comparator 204 via a voltage divider including resistors 205 and 206. The output from comparator 204 is a first signal, namely, a high state signal representative of a logical 1 when the output from peak detector 203 is greater than the threshold value, and the output from comparator 204 is a second signal, namely, zero, representative of a logical zero, when the output from peak detector 203 is less than the threshold value. Thus, by employing a predetermined threshold value which is in predetermined relationship to the amplitude of the test signal, the type 2-wire cable is determined. In an example from experimental practice, a threshold level of 0.53 volts d.c. is employed when a 1 volt peak test signal is utilized to satisfactorily distinguish between loaded and nonloaded cable. Further details of detector 108 are shown in our copending application Ser. No. 173,011 filed concurrently herewith.
FIG. 3 shows impedance vs. frequency characteristics for loaded and nonloaded transmission facilities, i.e., two-wire telephone cable in accordance with the relationship V(dB)=ZC/(R+ZC), where R is the resistance of resistor 107 and ZC is the impedance developed across winding 104 when a bidirectional facility is connected to T and R (FIG. 1). It is readily seen that at a frequency of approximately 3400 Hz, the impedance or attenuation characteristics of the loaded facility and nonloaded facility are quite different. As is well known, the facility impedance connected to terminals T and R