The amplitude of a sample from a signal having frame-to-frame redundancy is transmitted only if the amplitude differs significantly from the amplitude of a corresponding sample having the same time position in a previous frame interval. An address word is assigned to each sample to indicate the position of that sample within a group of samples. Group synchronization is maintained between the transmitter and receiver by forcibly transmitting the last sample in each group whether or not that sample represents a significant change. The value of each address word indicates the length of the interval between the location of its respective sample and the end of the group. Only those bits of an address word which are necessary to locate the sample in the interval between the last-transmitted sample and the end of the group are transmitted to the receiving location. A control word whose bit values indicate which bits of the next address word must be transmitted is developed from each address word and is stored until the next selected address word in response to an indication that its corresponding address word has been selected for transmission.