Traveling-Wave Tubes

TR.4 VELING-WA VE TUBES 3 gence by telegraph, by telephone or by facsimile is directly proportional to bandwidth; and, with an increase in communication in all of these fields, more bandwidth is needed. Further, new services require much more bandwidth than old services. A bandwidth of 4,000 cycles suffices for a telephone conversation. A bandwidth of 15,000 cycles is required for a very-high-fidelity program circuit. A single black-and-white television channel occupies a bandwidth of about 4 mc, or approximately a thousand times the bandwidth required for telephony. Beyond these requirements for greater bandwidth to transmit greater amounts of intelligence and to provide new types of service, there is currently a third need for more bandwidth. In FM broadcasting, a radio frequency bandwidth of 150 kc is used in transmitting a 15 kc audio channel. This ten-fold increase in bandwidth does not represent a waste of frequency space, because by using the extra bandwidth a considerable immunity to noise and interference is achieved. Other attractive types of modulation, such as PCM (pulse code modulation) also make use of wide bandwidths in overcoming distortion, noise and interference. At present, the media of communication which have been used in the past are becoming increasingly crowded. With a bandwidth of about 3 mc, approximately 600 telephone channels can be transmitted on a single coaxial cable. It is very hard to make amplifiers which have the high quality necessary for single sideband transmission with bandwidths more than a few times broader than this.