330 BELL SYSTEM TECHNICAL JOURNAL 



electrons and the crystal trap density. Experiments are described which 

 lead to a hypothesis of space charge neutralization. A possible cause of 

 the current fluctuations observed at high crystal fields is discussed. 



The Philosophy of PCM.* B. M. Oliver, J. R. Pierce and C. E. Shan- 

 non. Recent papers describe experiments in transmitting speech by PCM 

 (pulse code modulation). This paper shows in a general way some of the 

 advantages of PCM, and distinguishes between what can be achieved with 

 PCM and with other broadband systems, such as large-index FM. The 

 intent is to explain the various points simply, rather than to elaborate them 

 in detail. The paper is for those who want to find out about PCM rather 

 than for those who want to design a system. Many important factors will 

 arise in the design of a system which are not considered in this paper. 



Objectives for Sound Portrayal.^ Ralph K. Potter. Translation of 

 sound into visible patterns is discussed in terms of broad objectives. It is 

 suggested that no single design can be optimum and that perhaps the most 

 useful standard of reference is the human ear. Special interests and com- 

 plexity generally affect final design requirements. 



A Waveguide Bridge for Measuring Gain at 4000 Mc.^ A. L. Samuel and 



C. F. Cr-'\ndell. a bridge has been constructed for measuring the gain 

 and phase delay of ampUfiers in the vicinity of 4000 Mc. The equipment 

 is described, and the methods employed to reduce the possible errors are 

 discussed. The general method may be adapted for use in any desired 

 frequency range. 



Video Distribution Facilities for Television Transmission.'' Ernest H. 

 ScHREiBER. This paper describes the Bell System's plans for furnishing 

 network and local video facilities. The Telephone Company is now using 

 broad-band coaxial cable and microwave radio systems to provide regular 

 message telephone service on a number of principal intercity routes through- 

 out the nation. These facilities can be used to provide television transmis- 

 sion channels when properly equipped. Video service between Washington, 



D. C, New York, and Boston over these two types of facilities has been 

 demonstrated. New facilities are rapidly being extended. Local video 

 channels for pickup and metropolitan-area networks are provided by 

 ordinary paper-insulated cable pairs, special shielded polyethylene-insulated 

 pairs, by microwave radio systems, or by combinations of these systems. 

 Amplifier and equalizing arrangements for providing wide-band transmission 

 over these facilities are described. Present Bell System views of the avail- 

 ability of microwave and coaxial cable facilities on the principal routes, 



< Proc. L R. E., November 1948. 



^ Jour. Acous. Soc. Amer., January 1949. 



* Proc. L R. E. — Waves and Electrons Section — November 1948. 



^ 5. M. P. E. Journal, December 1948. 



