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Hard-Wire
Underwater Communications
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by Michael R. Pelissier
 Hard-wire
communications typically means that the divers will be using
a system similar to an intercom. The divers will be wearing a
helmet or full face mask (ffm) of some kind equipped with an
earphones and a microphone. These can be wired in either the
two wire or four wire mode which will be outlined below. For
standard dives, the intercoms are referred to as "Air Radio's".
Air, meaning, they are not using mixed gas, and radio, because
most divers don't know the unit is not a radio at all. It does
not send out radio signals, it simply uses the communication
wire to carry the signal. If the diving profile calls for long
durations or deep depths, mixed gases may be used. When you use
Helium as one of the gases, which is common on deep dives, it
makes your speech sound like Donald Duck and is hard to understand.
Thus, the Helium unscrambler intercom was designed. It is commonly
called a "Helium Unscrambler". It is basically the
same intercom as an Air Intercom but has a specially designed
circuit to make the speech sound intelligible to the topside
listener. |
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Two Wire Mode: In the beginning of Commercial diving,
most all divers were using Hard-wire boxes wired in the two wire
mode. Dive boxes, as the intercoms were commonly called, were
available for both one or two divers. The boxes were crude. They
usually had simple electronics, some type of water resistant
housing, 6 volt spring type batteries, a panel speaker, binding
posts and a push-to-talk control. The helmet or ffm had to be
wired the same to match the box.
The ffm typically had two speakers
and a microphone. The two speakers and the microphone was wired
together. If the communication wire only had two wires, one would
go on one side of the microphone and earphones and the other
wire went to the other side of the microphone earphones. If the
communication cable had four wires, the diver usually twisted
two of the four wires together and twisted the remaining two
wires together. The the two pairs were connected the same as
having two wires (e.g. 2 wires twisted together on one side of
the microphone earphones, two wires twisted together on the other
side of the microphone earphones). The ffm usually had some sort
of water proof connector installed to keep the water tight integrity.
The topside box usually had banana
plug style binding posts. If it was a two diver box, it would
have two binding posts. One for diver "A" and one for
diver "B". Again, if you only had 2 wires in the communication
cable, one wire would go on one side of the binding post and
the other on the other side of the binding post. If you had four
wires, two would be twisted together and put on one side of the
binding post and the other would be twisted and connected on
the other side of the binding post (see Figure 1). |
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When the air box was powered ON,
the diver would be the primary talkers. You would hear him all
the time via the front panel speaker located on the air box.
If you were using a two diver box and both divers were down,
you would hear both divers all the time. However, the divers
could not hear each other. The earphones and microphones being
tired together on the divers end, made them both earphones and
microphones. This meant that as the diver off gassed and bubbles
passed by the earphones, the earphones picked up the noise. This
type of communications was noisy and sometimes hard to understand
what the divers were saying through all the nose two divers could
make. |
Figure 1, set up for two wire communications |
 When
the topside tender wanted to talk to the divers, he pushed a
push-to-talk switch (PTT) and talked into the air boxes front
panel speaker. This reversed the communications and now made
all the communications go down to the divers. The divers earphones
and microphones became one. As long as the tender held down the
PTT switch, the divers heard the message. When the tender released
the PTT button, the divers became the primary talkers again.
Most 2 wire boxes had a Cross talk switch which allowed the tender
to let the divers talk to each other. However, this kept the
tender busy since he had to toggle a switch from diver 1 to diver
2 and visa-versa Most of the time the tender would not let the
divers talk to each other unless it was an emergency. The two
wire mode is still being used by some Commercial divers to date. |
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Four Wire Mode: |
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The 4-wire mode will give you
optimum performance and intelligibility. When using this method,
everyone hears each other simultaneously. Therefore, the earphones,
speakers and microphones must be isolated to prevent feedback.
All earphones are connected in parallel. A 4 wire cable is required
for all stations. Two of the 4 wires are used for the microphone
and the other two for the  headset
and earphones. The air box is designed with a banana style binding
post for the earphone and one for the microphone. Each diver
would have his own set of earphone microphone binding posts.
It is common for the microphone binding post to be red and the
earphone binding post to be black in color. The helmet and/or
ffm must be wired the same as the topside box. An advantage in
owning a 4 wire box is that it usually can be wired in either
the 2 or 4 wire mode. A two wire box can only operate in the
2 wire mode.
To wire the helmet or ffm, you
must use a communication cable with 4 wires. Two wires will be
used to connected to the earphones. The other two wires are used
for the microphone. The topside box is wired the same. Two wires
on the red microphone binding post and two wires on the black
earphone post. The topside tender has to use a headset with boom
microphone when talking in the 4 wire mode. Due to the fact you
cannot use the front panel speaker as a microphone and earphone
like in the two wire mode, the headset and boom microphone is
mandatory (see Figure 2). |
Figure 2, set up for four wire communications |
The first considerable thing you
will notice is that the communication form the divers is cleaner
and quitter. This is due primarily because the earphones are
now earphones only and not doubling as microphones. When the
divers off gas, the earphones do not pick up any noise. Also,
with all divers on line together, everyone can talk at the same
time. This is safer! In the two wire mode, when the tender is
talking to a diver, he cannot talk back until the tender releases
the PTT button.
This can be dangerous if the diver has an emergency that requires
immediate attention. |
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 The
topside tender could ramble on for seconds or minutes. Until
that tender releases the PTT button, the diver can only listen.
In the four wire  mode,
everyone is on line similar to a telephone conference call. Why
anyone would use anything other then a good four wire system
makes no sense to me!
We offer both styles of communications.
Our MK2-DCI is a two diver air intercom that will operate in
the 2 or 4 wire mode. Our ComBox-1 is a one diver air intercom
designed to operate in the 2 wire mode only. Our BuddyLine is
a 1 or 2 diver four wire system designed for fast response time. |
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MK2-DCI, Two Diver Air Intercom |
GO |
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ComBox, One Diver Air Intercom |
GO |
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MK-7 Buddy Line Two Diver Portable Air Intercom |
GO |
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Accessories |
GO |
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Basic concepts of underwater communications |
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Technical Talk |
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