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Telephone hybrids
A telephone hybrid is a system that is able to make an audio connection between a telephone
line and low frequency equipment. In practice this connection is by no means simple since the
characteristics of the telephone line require the equipment to employ a series of techniques to
ensure good audio quality and immunity to interference.
The bandwidth of a normal telephone line is compressed from about 100 Hz to 3-4 KHz in
order to guarantee a good reproduction of the human voice. A telephone hybrid must have a
wide frequency response in order to adapt to the quality of any line and to avoid adding
further limitations to the system. Unfortunately telephone lines are often sources of intense
disturbances, noise, crackling, fading etc. Furthermore they are often sources or conductors of
static charge and overvoltage, able to destroy circuits which have been imprudently connected
to them. Add the fact that the resistive and capacitative behaviours of line are not uniform,
and thus barely repeatable, and clearly the transmitted audio quality will vary with time.
It is therefore necessary to take particular care in the design of line-audio interfaces in order
to avoid significant degradation of the bandwidth or re-injection of induced interference. The
interface must also ensure complete electrical isolation between the line section and that
connecting to the audio equipment.
Another problem arises during a telephone connection between a remote correspondent
and an interviewer or DJ; the DJ’s voice will be transmitted down the telephone line and,
since the same line is used for transmission and reception, received again with a delay
with respect to the original. It will be re-amplified and looped back resulting in whistles
and Larsen effects. Usually, antilocal circuits are designed to solve this matter by
identifying those audio components which tend to enter into the loop and eliminating
only the part which re-enters. This is indispensable for Audioconferences.
The new Digital Hybrid features with an electronic echo canceller, that allows to reduce
drastically or even eliminate the feedback. In this way the diaphony is improved thanks
to the use of a DSP.
Audioconferencing allows simultaneous communication between remote correspondents that
are interconnected through telephone lines and the hybrid, which interconnects them without
introducing distortions or noise. It allows also to the DJ to participate in the conference. The
DJ, as “manager” of the conference, is able to include or exclude each correspondent from the
conference and can also speak privately to each of them for instance to prepare the interview.
Features of AEV I TB-302 Evolution
AEV ITB-302 is a telephone hybrid that satisfies all today’s demands for studio and broadcast
telephony allowing total management of telephone lines. It is designed in a 19", 1 unit,
rackmounting container, housing 1 or 2 telephone hybrids. Units may be interconnected to
allow expansion to “n” lines whilst maintaining simplicity of use and the possibility of remote
control.
The telephone lines, appropriately filtered and isolated, allow Audioconferencing between them
and the DJ. An internal microprocessor provides a range of functions that, until now, have
been difficult to provide in the same unit. All potentiometers were eliminated in order to solve
wear problems. It is also available a socket on the rear panel to connect a headset.
Installation
AEV ITB-302 telephone hybrid has been designed and manufactured to conform to current
safety standards. The unit should only be installed by competent personnel and in compliance
with operator safety requirements and environmental specification. Great care should be taken
in making earth and ground connections to the unit and associated equipment. As for any
electronic equipment, adequate ventilation for cooling must be ensured in order to guarantee
operating temperatures within the limits defined in the Technical Specifications section. This is
particularly important in case of rack mounting or installation in confined or poorly ventilated
locations. It is advisable to leave a space between each unit and adjacent units (above and
below) even if mounted in a ventilated rack. If well connected to the rack’s ground and earth
connections, the unit will be immune to interference including RF, as long as the high
frequency units have been correctly connected. AEV ITB-302 is anyhow fitted with RF filters on
its input in order to eliminate annoying interference. Gas discharge tubes are fitted to the
telephone lines to guarantee immunity from static charges or over-voltages.
