The hybrid

When communicating in full duplex over a 2-wire circuit, the transmitted and received signals travel over the same wire pair. A circuit called a hybrid is needed at each end of the line to separate the transmitted and received signal from each other. Another important function of the hybrid is to provide galvanic isolation to ensure the electrical safety of the user.

A proper hybrid requires either active circuitry, which would be cumbersome to power, or a signal transformer with more than two windings, which was not available when building the AuDSL prototype. Instead, the hybrid used in the AuDSL prototype is a simple passive mixing circuit which does not even attempt to cancel out the outgoing signal from the incoming one like a proper hybrid would.

This means that the AuDSL modem will in practice hear its own transmitted signal signal several times louder than the received signal, but its software echo canceller is able to cancel it out sufficiently well that things work. I do intend to build a real hybrid as soon as I can get suitable components.

The transformers currently used in the prototype were scavenged from a Nokia SAB 19.2 kbps baseband modem. The only other component needed is a 150 ohm resistor.

Simplified hybrid circuit used in AuDSL prototype

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