176 BELL SYSTEM TECHNICAL JOURNAL 



cessive interference into the carrier system due to talking at the way 

 station, and into the way stations due to talking on the carrier system, 

 is likely to occur unless suitable filters are provided at each way station. 

 A simple filter for this purpose has been developed for use with the 501 

 type subscribers set, and work is proceeding on a similar filter for use 

 with other types of subscriber sets. 



The line filters and the filters for use at way telephone stations each 

 introduce a loss of about 0.15 db to the through voice-frequency 

 transmission. Where the voice-frequency circuit is equipped with 

 repeaters and return loss conditions p«-mit, these additional losses 

 may be taken care of by readjusting the voice-frequency repeater 

 gains. In extreme cases, particularly where a considerable number of 

 filters are to be added, it may be necessary to resort to other means of 

 improving the transmission on the voice-frequency circuit, such as 

 loading of incidental cables or the addition of a voice-frequency 

 repeater. 



On circuits equipped with way stations, selective signaling by means 

 of selectors is sometimes used. Such signaling systems are generally 

 arranged to apply an "answer back" tone to the line when a station 

 has been called to indicate to the calling party that the selector has 

 operated. This tone contains a considerable amount of high-fre- 

 quency currents so that it is necessary to modify the selector circuit to 

 filter out the high frequencies. The modification is a simple one and 

 makes the answer back tone inaudible on the carrier circuit. 



Design P'eatures 



In the development of the Type H system advantage was taken of 

 many new devices which have been perfected in recent years, adapting 

 them to the particular conditions of this application. A discussion 

 of the more interesting features relating to the design of the various 

 parts of the system is given below. 



Modulators 



The modulator and demodulator used in the Type H system are of 

 the double-balanced copper-oxide type. Each modulator or de- 

 modulator consists of an input transformer, an output transformer, a 

 copper-oxide "varistor" and a carrier supply. Although the modu- 

 lators are bilateral, in the present application they are used in one 

 direction only. The varistor consists of 48 copper-oxide discs 

 assembled on a single bolt and connected as shown in Fig. 13. 



The principal advantage of this type of modulator or demodulator 

 is that in the ideal case (and to a lesser degree in the practical case) 



