a digital data transmission rate of three bits per cycle of bandwidth is achieved in precoded partial - response band - limited communication channels by partitioning binary digits into groups of three two - level digits and translating these binary groups of three into pairs of three - level digits prior to transmission . correct pairwise association of received signals is accomplished by reserving a three - level digit pair for monitoring purposes . this reserved pair can validly occur only at a transition between allowable pairs . by monitoring the presence of the reserved pair , correct pairwise association of ternary digits is assured and binary digits are properly decoded without having to provide a special framing signal .