Transoceanic Low-Frequency Radio-Telephone Systems

Figure 17. Terminal arrangement for the long-wave transatlantic radio-telephone system. Instruments are shown at A, B, and C for monitoring purposes. (Reference 17.)

A schematic of the low-frequency, long-wave (60,000 cycles, 5000 meters) transatlantic radio-telephone circuit is shown in Fig. 17, and its operation¹⁷ is as follows: The audio-frequency telephone signals produced by the speaker's voice pass over the connecting telephone line, through the toll switchboard, and into the hybrid coil. The signal voltage is impressed on the transmitting repeater where it is amplified, and then it passes through the Vodas, the compressor, the privacy equipment, and then over a wire line to the radio transmitting equipment. Since the radio transmitting and receiving equipment may be located some distance apart to prevent interference, the privacy equipment, the Vodas, and the other associated apparatus are terminal equipment located at one point.

The Vodas¹⁸ (voice-operated device, anti-singing) is a device which is automatically operated by the voice waves and which in effect short-circuits the receiving circuit at the sending station during speaking and then immediately returns it to the normal condition for receiving the answer from the distant station. In this way, feedback from the transmitting antenna of one channel to the receiving antenna of the same terminal is prevented, and two-way conversation is possible over the same band of radio frequencies.

The compressor is a vacuum-tube circuit for compressing the volume range of the speech signals at the sending end before transmission by radio to the distant station. At the receiving station the signal is passed through an expander which returns the signal to its original relations. These two units comprise the Compandor, which reduces the interference due to static.¹⁷ It is apparent that, if the voice could be transmitted without any variations in signal volume, then the signal level could be maintained (most of the time) far above the static level During portions of the speech where the signal is weak, static interference is bad because the signal-to-noise ratio is low. Thus, if it is possible to compress the speech and transmit it in this compressed form to the distant station where it is expanded to its original relations, less interference from static will result.

The privacy equipment ''scrambles" the speech, rendering it unintelligible unless special "unscrambling" equipment is used. Several such systems are possible, among which is a frequency inverting method. Thus if a 50,000-cycle carrier is modulated by a 3000-cycle wave and the lower sideband only is selected with a filter, an output of 47,000 cycles will result. If this wave is now modulated by a 42,000-cycle wave and the lower sideband again selected with a filter, a 5000-cycle wave will result. If the original modulating wave had been 200 cycles, then the output of the second filter would be a 7800-cycle impulse. Thus, if the voice-frequency band used in commercial telephony varies from 200 to 3000 cycles and if this complex wave were impressed on the filter, the lower frequencies would appear as high-frequency components, and vice versa, in the output of this scrambling or inverting process. Also, the frequencies can be shifted from time to time to render "decoding" more difficult.

Single-sideband transmission is used in the transoceanic long-wave system. This requires a smaller frequency band and saves power and power-handling capacity. The power radiated in the sideband is about 200 kilowatts, or some 100,000,000 times that in the original speech signal. ^19,20 The so-called multiple-tuned antenna is used for transmission, and the wave antenna (page 479) is used for reception. Typical characteristics of the transatlantic path are shown in Fig. 18.

Figure 18. Long-wave curve and corresponding short-wave curves of received field strength. (Courtesy Bell Telephone System.)