Voice-Frequency Telegraphy

telegraphy over telegraph communications channels using the frequency range of 300–3400 hertz (Hz). Voice-frequency telegraphy provides low-speed channels for the transmission of telegrams (50–200 bauds) and, since the 1960’s, for data transmission. Methods of voice-frequency telegraphy are used in multiplexing standard high-frequency telephone channels and radio channels. From six to 48 telegraph transmissions may be carried simultaneously by one such channel, and the entire frequency bandwidth of the channel is usually used. Voice-frequency telegraphy is also used for simultaneous telephone and telegraph communication over a common telephone channel; one to five telegraph transmissions are carried out simultaneously with the telephone conversation. Voice-frequency telegraphy appeared in the 1930’s, and by the 1960’s it had become the principal method of telegraphy. The total length of voice-frequency telegraph channels in the USSR and abroad exceeds the equivalent of 10 billion km.

Voice-frequency telegraphy may use amplitude, frequency, or phase modulation. Frequency-modulation is the most widely used method because of the equipment’s high immunity to noise. With frequency modulation, the incoming signals from the information source are converted at the transmitting station to AC pulses of various frequencies. A reverse conversion takes place at the receiving station.

The division of a telephone channel into several voice-frequency telegraph channels may be achieved by means of frequency division, time division, or frequency-time division. Time division is used for radio channels. Frequency division and frequency-time division are used for operation over lines because they make separation of a part of a voice-frequency telegraph channel at intermediate points fairly simple.

With frequency division, the bandwidth of a standard telephone channel (300–3400 Hz) is divided into 24 equal voice-frequency channels, each of which has a bandwidth of 80 Hz and a deviation (departure of the modulated frequency from its nominal value) of ±30 Hz. With frequency-time division, the frequency spectrum is divided by filters into four channels, each with a bandwidth of 700 Hz and a deviation of ±200 Hz. Each of these channels is then multiplexed for a 12-fold time division, which provides for successive transmission of information from 12 sources.