sentences:  All long haul "channel groups" used the single-sideband/suppressed carrier heterodyne 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 bandwidth and routing compatibility, was carried through to the short haul carriers, as well, as they started developing to eliminate voice band open wire trunk lines in the 1950s.The Bell System vacuum-tube driven N-1 Carrier of the early 1950s was the most used twelve channel carrier system, using double sideband/unsuppressed carrier operation which didn't need network timebase synchronization to maintain frequency accuracy.