The call-waiting feature is supported on voice-and-data modems (22,25) by causing voice-and-data modems to go into voice-only mode upon receipt of the modem clear-down signal (352-354). A first voice-and-data modem engaged in an initial voice-and-data call with a second voice-and-data modem responds to receipt (202) of a call-waiting signal by generating (204) a replica of the call-waiting signal on the user's audio channel. The first modem responds to a user request (300) to pick up the waiting call by sending (306) the modem clear-down signal to the second voice-and-data modem, going (312) into voice-only mode, and transmitting (314) a waiting-call acceptance signal. The initial call that is placed (332) on hold is thus a voice-only call. While in a voice-plus-data picked-up waiting call, the first modem responds to a user request (400) to reinstate the initial call by again sending (406) the modem clear-down signal, going (412) into voice-only mode, and transmitting (422) a call-resumption signal. The first modem is thus in voice-only-mode when it is reconnected (454) to the initial, voice-only, call. The re-connected initial call may now be re-converted into a voice-plus-data call in the conventional manner (424-488).

The power differential between balanced signals is measured and the resultant level is used to control the phase between the signals. In one embodiment, I and Q inputs to vector modulator is adjusted to control the phase.

A communications system simultaneously transmits both a primary data signal and a secondary voice signal in such a way that the dynamic range of the secondary voice signal is increased even though deliberate errors are introduced into the transmitted data portion of the signal. In particular, the communications system includes an error correcting technique like channel encoding and the constellation signal space is divided into a number of regions, where at least one of the regions overlaps with another region. The primary signal is channel encoded to select a particular one of the number of regions, the region being represented by a reference signal point value. The voice signal is encoded to provide a signal point vector, which is added to the reference signal point in such a way that the resultant signal point is located in the overlapping region thereby deliberately introducing an error into the resulting transmitted signal point stream. Notwithstanding the deliberately introduced error, the channel encoding of the primary signal allows a receiver to recover the originally selected reference signal point value, which when subtracted from the received signal point value approximates the voice signal.

A simultaneous voice and data modem performs "voice-activated" data rate changes to improve the transmission quality of the voice signal. In particular, when the simultaneous voice and data modem detects that a local telephone set has gone "off-hook," the simultaneous voice and data modem selects a signal space with a lower symbol density, which, although resulting in fewer bits per symbol, provides for a higher quality voice transmission.

A method and apparatus for concurrent communication of analog information and digital information. In general terms, when the communication channel is viewed as a multi-dimensional space, the digital information signal is divided into symbols, and the symbols are mapped onto the signal space with a preset distance between them. The analog signal, generally limited in magnitude to less than half the distance separating the symbols, is converted to component signals and added (i.e., vector addition) to the symbols. The sum signal is then transmitted to the receiver where the symbols are detected and subtracted from the received signal to yield the analog signal components. The transmitted analog signal is recreated from those components. Both half-duplex and full-duplex operation is available in accordance with this invention.