Abstract: The present invention addresses a timing synchronization problem which arises in tandem modem operation when a single codec strobed by a common sample clock is used for A/D and D/A signal conversion in a modem. Tandem modem operation is defined as a system application when two modems have their RS-232 interfaces directly connected to one another. In accordance with the present invention, a digital interpolator connected to the A/D converter in the modem effectively alters the timing phase of the A/D converter in response to first and second control signals. The first control signal is generated in response to an externally supplied clock and an internal modem clock and is representative of any perturbations in the externally supplied clock with respect to the internal modem clock.

Abstract: Primary and secondary data are communicated from a transmitter to a receiver in a communication system. The value of an individual primary data word is communicated by transmitting an individual one of first plurality of channel symbols associated with that value while the value of a combined data word comprising an individual primary data word and an individual secondary data word is communicated by transmitting an individual one of second plurality of channel symbols associated with that value. In the receiver, both the primary and secondary data word values are recovered from the channel symbols thus transmitted. The approach of alternating two sets of channel symbols for transmission depending on the data word size can further apply to data communications at various fallback speeds.