Simultaneous Telephone and Telegraph Operation

Simplex Telegraphy. The method of obtaining a simplex circuit over the four wires of a phantom group is illustrated in Fig. 20. Thus, with four line or cable wires, three telephone and one telegraph channel can be operated simultaneously. If the theory of the phantom circuit presented on page 224 is studied, no discussion of the theory of the simplex circuit will be required. A simplex circuit may be obtained from, and is often used on, one pair of telephone cable wires.

Composite Telegraphy, The composite arrangement is extensively used because a telegraph channel can be obtained from each of the four telephone line wires of the phantom group of Fig. 21. With a composite circuit,⁴ the upper telegraph frequencies transmitted are about 80 cycles per second. The inductor and capacitor combination in the telegraph circuits comprises a low-pass filter permitting the low-frequency telegraph currents to pass through to the telegraph instruments. Similarly, the inductors and capacitors in the telephone

* Methods of switching are receiving much attention; see Western Union Technical Review, January and July, 1948, Vol. 2, Nos. 1 and 3, and also Elec, Eng., July, 1948, Vol. 67, No. 7, circuits constitute a high-pass filter permitting the relatively high-frequency voice currents to pass through.

Figure 20. A simplex telegraph circuit on a phantom group providing simultaneous telephone and telegraph service. A simplex circuit may also be provided on a pair of wires.

Figure 21. Simplified circuit of terminal equipment at one end of a composited phantom group. This provides four telegraph channels, in addition to the three telephone channels, and is particularly applicable to open-wire lines.