▁a ▁legacy ▁of ▁the ▁system . ▁ ▁The ▁twelve ▁channel ▁scheme ▁was ▁first ▁de vised ▁in ▁the ▁early ▁ 1 9 3 0 s ▁to ▁provide ▁a ▁line ▁spectrum ▁covering ▁ 6 0 ▁to ▁ 1 0 8   k Hz ▁for ▁the ▁Type ▁J ▁Car rier ▁Tele phone ▁System , ▁an ▁equivalent ▁four ▁wire ▁( on ▁two ▁wire ▁facilities ) ▁open ▁wire ▁car rier ▁that ▁was ▁used ▁almost ▁exclus ively ▁for ▁inter state ▁long ▁ha ul ▁to ll ▁tele phony . ▁ ▁This ▁became ▁the ▁basic ▁building ▁block , ▁the ▁" channel ▁group ", ▁for ▁all ▁succeed ing ▁long ▁ha ul ▁systems , ▁such ▁as ▁Type ▁K ▁and ▁all ▁the ▁Type ▁L ▁systems ▁into ▁the ▁late ▁ 1 9 7 0 s . ▁ ▁All ▁long ▁ha ul ▁" channel ▁groups " ▁used ▁the ▁single - side band / supp ress ed ▁car rier ▁heter ody ne ▁scheme ▁that ▁was ▁produced ▁by ▁a ▁Western ▁Electric ▁Type ▁A - 1 ▁through ▁A - 6 ▁channel ▁bank . ▁ ▁The ▁twelve ▁channel ▁scheme , ▁in ▁order ▁to ▁maintain ▁some ▁band width ▁and ▁routing ▁compatibility , ▁was ▁carried ▁through ▁to ▁the ▁short ▁ha ul ▁carri ers , ▁as ▁well , ▁as ▁they ▁started ▁developing ▁to ▁eliminate ▁voice ▁band ▁open ▁wire ▁tr unk ▁lines ▁in ▁the ▁ 1 9 5 0 s . ▁ ▁The ▁Bell ▁System ▁vac u um - t ube ▁driven ▁N - 1 ▁Car rier ▁of ▁the ▁early ▁ 1 9 5 0 s ▁was ▁the ▁most ▁used ▁twelve ▁channel ▁car rier