668 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



relays, is almost ideally suited to the needs of the smaller central offices 

 and TWX stations. 



FREQUENCY ALLOCATIONS 



The 43 A 1 system provides two groups of ehannel-fre([uency alloca- 

 tions, as follows: 



a — A three-channel high-frequency allocation, using frequencies be- 

 tween the upper edge of the voice-frequency band and the lower edge of 

 the type-C carrier telephone band. This allocation is primarily for opera- 

 tion on open-wire lines but can also be operated on cable circuits where 

 the loading provides a suitably high cut-off. 



b — ^A voice-frequency allocation capable of providing six channels 

 on two-wire circuits or twelve channels on four-wire circuits. The chan- 

 nels of this allocation are for operation over telephone speech channels 

 on any of the standard facilities, including broad-band carrier and cable 

 or open- wire physical circuits. 



The present frequency allocations are shown in Fig. 1. The voice- 

 frequency s.ystem is based on twelve nominal midband frequencies 

 spaced 170 cycles apart from 595 cj^cles to 2635 cycles, omitting 1615 

 cycles. The carrier frequency is shifted ±35 cycles about midband, and 

 either the higher or the lower frequency may be used for marking sig- 



4-WIRE 



VOICE FREQUENCY 



± 35'^SHIFT 



CHANNEL NUMBER 



p e e e 



SMSMSMSMSMSM 

 2 3 4 5 6 7 



P P 



3MSMSMSMSMSM 

 9 10 II 12 13 14 



2-WIRE 



VOICE FREQUENCY 



±35'^ SHIFT 



CHANNEL NUMBER 



c' e 



" 



SMSMSMSMSMSM 

 2 3 4 5 6 7 



P P P <? ^ ^ 



_ ii' PJ 



SM SM SM SM SM SM 

 2 3 4 5 6 7 



HIGH FREQUENCY 



i. P t 



CHANNEL NUMBER 



» T T I 



MSMSMS SMSMSM 

 3 2 1 12 3 

 ±40'^ +45'V +50"^^ 

 SHIFT SHIFT SHIFT 



Fig. 1 — Frequency allocations. 



