Wideband echo suppression using an improved bilateral opto-isolator circuit

In the telephone network, an echo is generated at the hybrid junction that connects four-to two-wire lines. A conventional echo suppressor or canceller is designed on the assumption that the echo is actually induced. In contrast, in our method, using a bilateral opto-isolator circuit, the echo is suppressed at its origin. This bilateral opto-isolator circuit has not only the property of bilateral transmission, but also the same property of suppression of impedance fluctuation as is characteristic of a buffer amplifier. Experimental results show that the degradation of echo return loss can be kept within a few decibels, even under the severe condition that the line impedance changes from being very small to very large. However, the bilateral opto-isolator circuit in previous work had the shortcoming that the operating frequency range showing the effect of echo suppression was narrow (below 1 kHz). In this paper, a method of echo suppression using an improved bilateral opto-isolator circuit is proposed. The operating frequency range is widenend from below 1 kHZ to several tens of kiloHertz.