CARRIER SYSTEMS ON TELEPHONE LINES 609 



system with two intermediate repeater stations would be in the 

 neighborhood of 24 amperes at 24 volts and 1.3 amperes at 130 volts, 

 amounting altogether to about 750 watts. This corresponds roughly 

 to the amount of power consumed by about 80 telephone repeaters, 

 so that the total power required for three such carrier systems would 

 be about equal to that required for a cable repeater station having 

 between 200 and 300 repeaters. Assuming 2 or 3 repeaters in a 

 typical voice circuit, the carrier systems are seen to require over ten 

 times as much power as voice circuits in providing the same facilities. 

 The best results are obtained with the carrier systems when very 

 close regulation of this power supply is maintained. About ± 1 volt 

 for the 24-volt supply and ± 5 volts for the 130-volt supply are 

 desirable limits of variation. Means for obtaining such regulation 

 are added as required to the existing power plant. In the larger 

 offices this may consist of a duplicate battery with full-floating 

 operation. In the smaller installations, a relay regulating circuit may 

 be added which controls the filament current as the voltage varies 

 from 20 to 28 volts. This consists of a sensitive voltmeter relay 

 arranged with accessory relays to cut resistance in and out of the 

 individual filament supply circuits as the voltage varies. 



Design of Carrier Apparatus 



Many will, no doubt, be interested in the further technical details 

 of some of the more important units of the carrier system. 



In the development of the apparatus considerable preliminary 

 work was necessary to determine the circuit requirements imposed 

 upon the individual units. For example, preliminary to the design 

 of the filters, laboratory studies were made to find what interfering 

 frequencies might be expected in a channel and what attenuation the 

 different filters must offer at various points in the frequency range, 

 in order that the system should provide speech of satisfactory quality 

 and freedom from interference. As a result of such work, it was 

 possible to make the requirements of the filters no more stringent than 

 absolutely necessary, thus keeping the cost down to a minimum while 

 insuring adequate performance. Preliminary studies were also made 

 on the other parts of the system such as modulator, demodulator, 

 oscillator, etc. In the descriptions which follow, no attempt has 

 been made to describe this preliminary work, the discussion being 

 limited to the requirements imposed, and the circuits devised to meet 

 these requirements. 



Modulator and Transmitting Oscillator. A circuit drawing of the 

 modulator is shown in Figure 30. It may be considered that the 



