Abstract: A device for providing conferencing communications. The device comprises: a summing circuit for forming a signal sum equal to a sum of signals received from at least three information signal sources along with an echo compensation signal, the signal received from each information source including echoes and the signal sum including an aggregation of such echoes; an adaptive filter having an input and an output, the input being solely responsive to the signal sum and the output being only coupled to the summing circuit, the filter forming the echo compensation signal which is an estimate of the aggregation of echoes; and a low level training sequence generator having an amplitude less than a predetermined level.

Abstract: A modem that matches itself to a central office codec to which it is connected and incorporates encoding of the data. An echo canceller following the central office codec subtracts an estimate of the echo introduced in the central offices. More particularly, digital data applied to the modem is multi-level encoded and mapped onto a subset of the levels recognized by the central office codec. In an illustrative embodiment, the multi-level encoding encodes the two least significant bits in the binary representation of the levels delivered by the modem. The least significant bit is encoded with a code that is more robust than the code of the next-to-least significant bit, in recognition of the fact that errors in the least significant bit are much more likely. The corresponding echo canceller in the network first estimates the echo and subtracts it from the signal applied to the digital network by the central office. Thereafter, a decoder decodes the received signal in stages.

Abstract: A modem that operates reliably at a symbol rate that corresponds to twice its bandwidth even when it is coupled to a receiving A/D converter that operates under control of a clock is realized by synchronizing the modem's operation to the A/D's clock. The superior operation of this modem advantageously extends to A/D clock frequencies beyond the frequency of twice the modem's bandwidth. To minimize quantization noise, the modem's output is conditioned to minimize intersymbol interference by adjusting the modem's output to the A/D converter's sampling times and slicing levels. When the A/D's clock is higher than twice the bandwidth of the modem's output signal, some intersymbol interference cannot be avoided. In accordance with this invention, the position and value of this interference is computed at the receiver and subtracted from the received signal.