| 摘要 |
<p>17 "Digital duplex communication system". A digital duplex communication system includes an echo canceller of the look-up table type. The echo canceller comprises a random access memory (RAM) (41), a digital to analogue converter (DAC) (45), a subtractor (5), adaption means (47), and an address generator (40). The echo canceller stores echo replicas for combinations of data bits transmitted by the transmitter (1) and subtracts these echo replicas from a received signal on line (9) before application to the receiver (2). The capacity required for the RAM (41) is reduced by storing only one echo replica for each of two complementary sets of data. To achieve this the address generator (40) is split into a primary address generator (73) and a complementary address generator (74) so that complementary sets of data present the same address to the RAM (41). The primary address generator (73) produces a signal to indicate which of the two complementary data sequences is being transmitted and appropriately sets the adaption means (47) over line (71) and a complementing circuit (70) over line (72). The combination of the signal on line (71) and the output of the subtractor (5) on line (12) causes the echo replica to be updated by the adaption means (47) which may be an adder circuit. The complementing circuit (70) operates on the output of the RAM (41) via the latch circuit (43) to determine the sign of the echo replica applied to the subtractor (5) via the DAC (45).</p> |
