2825R TWO DIVER RACK MOUNT COMMUNICATOR USER MANUAL
2825R-UM
REV 3.0
30
7.2 What Is Full Duplex (4-Wire)?
It is a 4-Wire dual communication path system that uses special audio amplifiers on the
microphone and earphone connection to eliminate the issues associated with Round Robin
systems. It allows everyone on the communicator to talk to each other just as if they were on
a telephone.
7.2.1
Why Are Special Amplifiers Needed?
When developing a 4-Wire communication system, the biggest problem facing the
designer is oscillation caused by feedback. This can occur in two ways. The most
common is acoustic feedback, also called the Larsen effect, which occurs when the
microphone picks up the sound from the speaker and feeds it back into the
amplifier. This signal is amplified and sent out the speaker at a higher level. Given
the right conditions, this process repeats until the amplifier reaches maximum
signal level. The result is usually a high pitch, howling sound commonly heard in
public address systems when the volume is turned up too high.
The solution for acoustic feedback is to turn down the amplifier volume and to
isolate the speaker from the microphone. In a dive helmet, the diver's head makes
a good acoustic isolator. On the surface, a tender using a headset may get some
acoustic feedback via the PANEL SPEAKER. The solution is to put some distance
between the tender and the PANEL SPEAKER by using the Amron Model 2822-28
Walk-and-Talk Module accessory. Alternatively, the PANEL SPEAKER can be
turned off using the SPEAKER SWITCH located on the front panel.
There exists a second, more difficult to avoid feedback path that can also cause
oscillation. In 4-Wire mode, there are two sets of wire pairs. One pair carries the
signal from the diver microphone to the communicator microphone input. This is an
extremely low level signal, typically in the range of about 1mVRMS. The second
carries the output signal from the communicator power amplifier to the diver's
earphone. This signal can be as high as 4VRMS. The earphone signal is typically
about 1000 times greater than the microphone signal and can be as much as 4000
times greater. If the signal on the earphone wire pair were to couple to the
microphone wire pair, the result would be a feedback path that can lead to
oscillation. To prevent such coupling, the dive cables are constructed using two
individual twisted wire pairs and in high quality cables, such as Amron's CC1
communication cable, each pair can be shielded to provide additional protection.
Shielding does decrease the amount of coupling between the wire pairs but without
special amplifiers, the full capability of using twisted wire pairs is lost.
Amron Diver Communicators are designed with a special balanced circuit,
differential input and output amplifiers. In a balanced circuit, each wire in the
twisted pair carries an equal and opposite signal. Each wire generates an
electromagnetic field that is in opposition with the field of other wire. The net result
is that strength of the radiated electromagnetic field is significantly reduced if not
totally eliminated. In addition, any external fields, either from the other wire pair or
an external noise source, will be coupled to both wires equally creating a common-
mode signal. The differential microphone amplifier in Amron Diver Communicators
cancels the common-mode signal while amplifying the signal from the microphone.