58 BELL SYSTEM TECHNICAL JOURNAL 



13 db below the transmitting toll switchboard which is the common 

 reference point. The output from the system is at a level 4 db above 

 that point. Equipment for coupling the system to both two-wire and 

 four-wire circuits has been designed. The circuits employed in each 

 case may be seen in Fig. 6. 



The modulator circuit, shown in Fig. 7, uses copper-oxide varistors * 

 for converting the voice frequencies to the higher line frequencies. 

 The high degree of balance obtained in the copper-oxide varistors 

 has the important advantage of making carrier leak a practically 

 negligible factor. This is of particular importance in the case of that 

 channel to which the pilot current is adjacent in the frequency spec- 

 trum. The modulator circuit is also designed to limit the peaks of 

 very loud talkers which would otherwise overload the common ampli- 

 fiers. The effect of this limitation on the quality of the speech 

 transmitted is not noticeable. 



The oscillator which supplies the carrier to the modulator is designed 

 to be stable in both output and frequency. When it is once adjusted 

 with the oscillator at the distant end, departures from synchronism 

 will be relatively small. Part of this stability is due to a new circuit 

 design employing coil and condenser elements having opposite temper- 

 ature coefficients so that changes in one will be compensated for by 

 changes in the other. 



The band filters use coils wound on magnetic core material, having 

 improved modulation characteristics, instead of the solenoidal air 

 core coils previously used. This results in a considerable reduction 

 in the space which they occupy. 



The transmitting and receiving filters associated with each channel 

 are identical as to band width. They are further characterized by a 

 more abrupt increase in discrimination immediately below and above 

 the pass-band frequencies than was realized in the channel filters for 

 the previous Type C systems and also by less distortion across the 

 pass band. Most of this distortion is in the form of higher loss in 

 the vicinity of the band limiting frequencies. It was deliberately 

 included in the design of the filters for the purpose of masking delay 

 distortion effects on overall transmission quality which might otherwise 

 become noticeable when four or five type C carrier telephone systems 

 are connected in tandem. 



The uniformity and symmetry of the various filters are shown by 

 Fig. 8 which gives the characteristics of those in the upper frequency 

 group. This symmetry is required in this group in order to make 

 the CS allocation convertible into the CU by moving the carrier from 

 one end of the band to the other. 



