Telephone hybrid

Telephone hybrids are used within the Public Switched Telephone Network (PSTN) wherever an interface between two-wire and four-wire circuits is needed. A two-wire circuit has both speech directions on the same wire pair, as exemplified by the usual POTS home or small business telephone line. Within the telephone network, switching and transmission are almost always four-wire with the two sides being separated. In older analog networks, this was required so that repeater amplifiers could be inserted in long-distance links. In today’s digital systems, each direction must be processed independently. The analog line card in a telephone central office switch includes a hybrid that adapts the four-wire network to the two-wire circuit that connects most subscribers.

The fundamental principle is that of impedance matching. The send signal is applied to both the telephone line and a ‘balancing network’ that is designed to have the same impedance as the line. The receive signal is derived by subtracting the two, thus canceling the send audio. Early hybrids were made with transformers configured as hybrid coils that had an extra winding which could be connected out of phase. The name ‘hybrid’ comes from these special mixed-winding transformers.

An effective hybrid would have good trans-hybrid loss, which means that relatively little of the send audio would appear on the receive 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 connections. This is a result of a speaker’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, 45dB return loss is required for less than 1% of test subjects to express dissatisfaction.

The cancellation depends upon the balancing network having a frequency-vs.-impedance characteristic that matches the line accurately. Since telephone line impedances vary depending upon length and other factors, and the curve 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 filter that automatically detects the line’s impedance across the voice frequency range and precisely adjusts to it. These may reach 30-50dB trans-hybrid loss.

Broadcast Telephone Hybrids

In broadcast studios and other audio production facilities, a telephone hybrid (in British, ‘Telephone Balance Units’, or ‘TBUs’) is the piece of equipment that is used to interface telephone lines to studio mixing consoles. In addition to the core hybrid function, these often have features such as automatic gain control, filtering, noise-gating, and acoustic feedback reduction processing.

In studio application, a hybrid needs particularly good send-to-receive isolation. When too much of the host audio appears at the telephone output, there will be a number of unwanted effects:

• Distortion of the host's voice. The telephone line will change the phase of the send audio before it returns, with varying shifts at different frequencies. The host audio will suffer degradation as the original and leakage audio are mixed at the console and combine in and out of phase at the various frequencies. When this occurs, the announcer sounds either hollow or tinny as the phase cancellation affects some frequencies more than others.
• Audio feedback can result from the acoustic coupling created when callers must be heard on an open loudspeaker.
• When lines are conferenced and the gain around the loop of the multiple hybrids is greater than unity, feedback singing will be audible.
• If the leakage is very high, operators will not be able to control the relative levels of the local 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 the ancillary functions.

ISDN and VoIP theoretically have no need for hybrids. However, calls that have ISDN or VoIP on one end usually terminate to an analog line at the other, and so there is a significant source of echo from both the Telco hybrid on the line card and the phone itself. Acoustic coupling, when the microphone picks up the output of the earpiece, is another potential source of echo. Electrical pickup between analog circuits (crosstalk) is yet another. Even low echo levels can be audible when there is a long delay, as is usually the case with VoIP.

Telephone hybrids intended for studio application are usually 1U rack-mount units ^[1][2][3] that have RJ-style connectors for the telephone line and either balanced analog or AES3 audio inputs/outputs on XLR connectors for the studio equipment connection. One or more hybrids might be packaged within a single unit. There are variations to accommodate either POTS or ISDN telephone lines. In addition to the audio functions, hybrids can include extra capabilities such as auto-answer/disconnect, DTMF detection/generation, and Caller ID detection.

Note that telephone hybrids must be fed from the mixing console with mix-minus to avoid feedback.

Some inexpensive adaptors connect between a telephone and its handset, and have a button to activate either the handset or the adaptor. Costing around US$100, these are simple passive devices that pass the audio from the telephone without processing.

At the other end of the spectrum are systems that cost thousands of dollars, but can handle multiple lines and usually connect to a computer so that a producer can keep up with who is on which line and manage which goes on the air next.

Telephone hybrids are also used to interface production intercom systems to the telephone network.

Related Equipment

Telephone hybrids are used to put calls on the air from common telephones. POTS codecs, ISDN codecs, and IP codecs are used for remote broadcasts when it is possible to have specialized equipment at both ends of the connection. POTS codecs work with analog lines, ISDN codecs with ISDN lines, and IP codecs with any link that can provide IP connectivity. These employ audio data compression and decompression at each end. Audio bandwidths up to 20kHz can be achieved this way. Compression is often via MPEG MP3 or AAC.

Autopatches are telephone hybrids used by Amateur Radio operators to interface their radio equipment with telephone lines.

Notes

See also

• Echo cancellation
• Adaptive feedback cancellation
• Least mean squares filter
• Hybrid coil
• Remote pickup unit

External links

• Modeling hybrids as 2 x 2 matrices
• US patent 3970805: transistorised hybrid
• ITU-T Recommendations: Series G