Telephone hybrid

In older analog networks, conversion to four-wire was required so that repeater amplifiers could be inserted in long-distance links.

The search for better telephone hybrids and echo cancelers (a related technology) was an important motive for the development of DSP (digital signal processing) algorithms and hardware at Bell Labs, NEC, and other sites.

An effective hybrid would have high trans-hybrid loss, which means that relatively little of the incoming audio would appear on the outgoing port.

Since telephone line impedances vary depending upon many factors and the relationship is not always smooth, analog hybrids are able to achieve only a few dB of guaranteed isolation.

For this reason, modern hybrids use digital signal processing to implement an adaptive least mean squares filter that automatically detects the line's impedance across the voice frequency range and adjusts to it.

The suppressor switches a loss into the inactive speech path, thus enhancing the echo-cancelling effect of the hybrid at the expense of simultaneous two-way conversation.

Despite being inherently four-wire,[citation needed] VoIP systems require hybrids when they interface to two-wire lines.

A VoIP-to-Telco gateway used to interface a VoIP PBX (private branch exchange) to analog lines would contain hybrids to perform the required conversion.

These devices often include processing in addition to the hybrid function, such as dynamics control, filtering, and equalization (EQ).

Some have dynamic EQ that adjusts parameters automatically to maintain spectral consistency from widely varying source audio.

Distortion of the host's voice can result from the telephone line's changing the phase of the send audio before it returns, with varying shifts at different frequencies.

If the leakage is very high, operators will not be able to control the relative levels of the host audio and the caller since the console telephone fader will affect both signals.

The digital signal processing technology used in modern hybrids addresses the isolation requirement and implements ancillary functions.