Telephone Hybrid - Broadcast Telephone Hybrids

Broadcast Telephone Hybrids

In broadcast studio facilities, the name for the functional part has come to refer to the whole, and a Telephone Hybrid is the device that packages all the functions needed to connect telephone lines to studio audio systems, providing electrical and physical interface between the Telco lines and studio equipment These devices often include processing in addition to the hybrid function, such as dynamics control, filtering, and equalization. Some have dynamic EQ that adjusts parameters automatically to maintain spectral consistency from widely varying source audio. Some incorporate acoustic echo cancellation to allow setups with acoustic paths between loudspeakers carrying phone audio and microphones feeding the phone lines.

In studio application, a hybrid needs particularly good send-to-receive isolation. When too much of the host audio appears at the hybrid’s output, there will be a number of defects. 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. 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 a 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 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. ISDN and VoIP Telco connections 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 rack-mount units 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, two, or more hybrids might be packaged within a unit. There are variations to accommodate either POTS, ISDN, or VoIP 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, with a button to activate either the handset or the adaptor. These are simple passive devices that pass the audio from the telephone without processing.

At the other end of the spectrum are systems that can handle multiple lines and connect to a computer so that a producer can follow who is on which line, communicate with the host, and manage the order of taking calls.

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

In British English, broadcast telephone hybrids are 'telephone balance units' (TBUs)