In analog telephony, a telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the airing of telephone callers, the broadcast-quality telephone hybrid is known as a broadcast telephone hybrid or telephone balance unit. The need for hybrids comes from the nature of analog plain old telephone service (POTS) home or small business telephone lines, where the two audio directions are combined on a single two-wire pair. Within the telephone network, switching and transmission are almost always four-wire circuits with the two signals being separated. Hybrids perform the necessary conversion. In older analog networks, conversion to four-wire was required so that repeater amplifiers could be inserted in long-distance links. In today's digital systems, each speech direction must be processed and transported independently. The line cards in a telephone central office switch that are interfaced to analog lines include hybrids that adapt the four-wire network to the two-wire circuits that connect most subscribers. 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.
Technology The fundamental principle is that of impedance matching. The incoming signal is applied to both the telephone line and a "balancing network" that is designed to have the same impedance as the line. The outgoing signal is derived by subtracting the two, thus canceling the incoming signal from the outgoing signal. Early hybrids were made with transformers configured as hybrid coils that had an extra winding that could be connected out of phase. The name hybrid comes from these special mixed-winding transformers. An effective hybrid would have high trans-hybrid loss, which means that relatively little of the incoming audio would appear on the outgoing port. Too much leakage can cause echoes when there is a delay in the transmission path, as there is with satellite, mobile phone, and VoIP links. This is a result of a talker's voice traversing to the far-end hybrid and returning to his own receiver with insufficient attenuation. ITU-T Recommendation G.131 describes the relationship of echo delay vs. amplitude to listener annoyance. At 100ms, 45 dB return loss is required for less than 1% of test subjects to express dissatisfaction. Good cancellation depends upon the balancing network having a frequency-vs.-impedance characteristic that accurately matches the line. 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. These may reach greater than 30 dB trans-hybrid loss, measured with white noise as the send signal. DSP hybrids are also called "line echo cancellers" (LECs). (The phrase "echo canceller" in this context is misleading since there is no cancellation of echo per se but rather of leakage from the analog line interface with very short time delay more accurately characterized as phase-shift. However, any uncancelled hybrid leakage will cause echo when the associated transmission path has delay, so the effect on the system is reduction of echo.) Hybrids and cancellers are sometimes combined with echo suppressors. These work on the assumption that usually only one of the two parties to a conversation is speaking at a given time. 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, 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. End-end VoIP needs no hybrids unless adaptation to a two-wire line is required.
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