1. Field of the Invention
The present invention relates to communications systems and, in particular, to a system improving modem transmission by adjusting the modem's echo cancellation scheme based on the connection type.
2. Description of the Related Art
Digital communications systems take analog signals and digitize them, pass them through the system, and convert them back into analog form. The digital-to-analog (D/A) and analog-to-digital (A/D) conversions can cause discontinuities in the communications link known as “echo.” Such echoes represent an undesired or interfering signal to the receiver in the modem. Echoes are typically classified as near-end or far-end echoes. Near-end echoes are reflections which normally occur relatively close in distance and not delayed too long in time relative to the transmitting modem. Far-end echoes are reflections which are remote, usually in distance and time, to the transmitting modem and typically occur at the remote modem. In particular, a digital-to-analog conversion at the transmitting modem can cause a near-end echo (relative to the transmitting modem), and an analog-to-digital conversion at the receiving modem can cause a far-end echo.
Conventional echo cancellation schemes assume that there is only one D/A and A/D conversion in each connection, that is, there is only one switching device performing such conversions in the link between the transmitting and receiving modems. Such a scenario is illustrated in FIG. 1. As shown, a telecommunications system 100 includes a modem 102a and a modem 102b. Modem 102a and modem 102b communicate via a switch 104. Assuming modem 102a is transmitting, an A/D conversion takes place between modem 102a and the switch 104, and a D/A conversion takes place between the switch 104 and the modem 102b. The A/D conversion can cause near-end echo, and the D/A conversion can cause far-end echo.
Conventional echo cancellation schemes look for echo under 50 milliseconds in near-end echo cancellation circuitry, and echo of 100–150 milliseconds for far-end echo cancellation. If additional echo is introduced intermediate to either of the modems and the switch, conventional echo cancellation schemes can fail. While echo cancellation schemes are known which attempt to converge on intermediate echo, such schemes do not anticipate multiple far-end echo sources. For example, an echo cancellation scheme described in U.S. Pat. No. 4,970,715 looks for a most-dominant line echo, which may be a far-end or an intermediate echo. The system described then cancels the detected echo. However, the system assumes that there is only one non-near-end echo source.