I 



NEW SINGLE CHANNEL CARRIER TELEPHONE SYSTEM 181 



Filters 



Filters constitute an important part of the Type H carrier system. 

 They represent about 30 per cent of the cost of the terminal and 

 occupy about 25 per cent of the total space. 



The various filters required at a terminal are indicated in Fig. 16. 

 The transmission characteristic of each filter is given in miniature 

 above or below the block representing the filter. 



The high-pass and low-pass line filters separate the ordinary voice 

 telephone channel and the added carrier telephone channel made 

 available by this system. The low-pass line filter passes voice fre- 

 quencies and suppresses all other frequencies. The high-pass line 

 filter passes the carrier frequencies and suppresses the voice frequencies. 

 Each filter offers a high impedance to the frequencies passed by the 

 other, and bridges off only a very small part of the energy of these 

 frequencies. 



The remaining filters are associated with the carrier terminal proper, 

 where they serve to separate the transmitting and receiving paths and 

 suppress unwanted frequencies. The voice frequencies pass through 

 the hybrid coil and the transmitting low-pass filter to the modulator. 

 This filter limits the path between the hybrid coil and the modulator 

 to voice frequencies only. Modulation of the voice with the carrier 

 frequency of 7.15 kc. produces two sidebands extending from 4.15 to 

 6.90 kc. and from 7.40 to 10.15 kc. At an east terminal, the upper 

 sideband is transmitted, and the modulator output filter passes this 

 sideband and suppresses the lower sideband, together with other 

 unwanted modulation products. In this manner it limits the load on 

 the amplifier to the desired sideband. The transmitting directional 

 filter offers further suppression to frequencies lying outside this band. 

 The receiving directional filter will not pass this band but has a high 

 impedance to these frequencies. The high-pass line filter passes all 

 frequencies above roughly 3.5 kc. and, therefore, this band passes 

 through it readily and out onto the line for transmission to the distant 

 terminal. Transmission from a west terminal is identical in principle 

 but here the lower sideband is passed by the modulator output filter 

 and transmitting directional filter while the upper sideband is sup- 

 pressed. 



It is apparent from Fig. 16 that the received sideband coming in on 

 the line from the distant repeater or terminal is operated upon by the 

 filters in a reverse manner from that described above for the trans- 

 mitted sideband. The incoming frequencies are directed through the 

 receiving directional filter to the demodulator, where modulation with 

 the original carrier reproduces the voice frequencies together with 





