Download Unit 4 Telephones (fixed

Theme 1 Telephones and Mobile Phones
Unit 4
4.1
Unit 4 Telephones (fixed-line) (Rev 1.0)
Telephones (fixed-line)
Simple Telephone Set
The block diagram of a simple telephone set is shown as below. It consists of a mouthpiece
(microphone), an earpiece (speaker), a switch hook, a dialer, and a ringer. The characteristics
will be described in later sections. Normally, it doesn’t require any extra electricity (like
battery) to make/receive calls.
Based on the dialing technique, there are two types of telephone sets: pulse dialing and tone
dialing. Normally, all tone dialing phones also support pulse dialing features. The user can
make the selection at the side or at the back of the phone (to “T” or “P” positions).
Picture of a Simple Telephone Set
(Pulse Dialing)
Picture of a Simple Telephone Set
(Tone Dialing)
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Theme 1 Telephones and Mobile Phones
4.2
Unit 4 Telephones (fixed-line) (Rev 1.0)
Mouthpiece
The mouthpiece is also referred to as a
microphone. One of the key features for
telephone invention was the use of a
variable-resistance mouthpiece, which
can change the electric current that is
proportional to the sound pressure. The
basic block diagram is shown in the
figure.
Block Diagram of a Mouthpiece
The diaphragm is sensitive to sound waves; the carbon granules are compressed and released
as the sound waves place pressure on the diaphragm. The resistance of the mouthpiece is
reduced when the diaphragm moves inward and is increased when the diaphragm moves
outward. A simple conceptual graphic explanation is shown in the following diagrams.
When the carbon granules are
under pressure, the contact
surface between the carbon
granules is increased and results
in resistance reduction.
Actually, the carbon granules
are of irregular size and shape
with many tiny edges on each
grain. With only a relatively few
edges in contact, the current has
only a few paths through the carbon chamber. However, when the carbon granules are under
pressure, more edges will be in contact resulting in more current paths. The overall resistance
is thus reduced.
The carbon mouthpiece does not respond well to a small acoustic signal and thus is good at
discriminating against background noise. The low cost and long life nature of a carbon type
transmitter are the reasons for its popular use today.
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Theme 1 Telephones and Mobile Phones
4.3
Unit 4 Telephones (fixed-line) (Rev 1.0)
Earpiece
In the 19th century, it was discovered that
if an electric current passed through a
solenoid, it would create a magnetic field
like a magnet. This forms the basis for
earpiece (speaker) construction. It converts
the varying electric current representing the
transmitted speech signal to variation in air
pressure and restoring the original speech.
A typical earpiece consists of many turns
of fine wire that is wound on permanently
magnetized iron driving the diaphragm
which is made of a soft iron material. Under normal conditions, i.e., no signal, the magnetic
field of the magnet attracts the diaphragm and causes it to bend inward, as shown in (a). For
the positive signal, the current in the coil produces a magnetic field which is opposed to the
original field; the overall reduction of the magnetic field will thus make the diaphragm to be
less bent, as shown in (b). For
the negative signal, the
current in the coil produces a
magnetic field which is in the
same direction as the original
field; the overall magnetic
field is increased and the
diaphragm is bent more
towards the magnet, as shown
in (c).
The varying electrical current,
representing speech, flows
through the coils and produces
a varying magnetic field
which causes the diaphragm to vibrate back and forth and reproduce the original speech.
4.4
Switch Hook
A switch hook is used to inform the telephone company the status of the telephone.
• ON hook (handset is hung on hook) → means that the phone is in idle state, the user
has no intention to use the phone.
• OFF hook (handset is lifted up) → gives a signal to the telephone company that the
handset is lifted up, i.e., the user is going to answer a call or make a call.
• When an attempt to make a call is made, a dial tone from the earpiece could be heard
after OFF hook if the connection was made and the service was available.
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Theme 1 Telephones and Mobile Phones
4.5
Unit 4 Telephones (fixed-line) (Rev 1.0)
Dialer
The purpose of dialing is to specify the telephone number of the called party to the telephone
company. There are two types of dialing techniques  pulse dialing and tone dialing.
Pulse Dialing
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Dial pulses are produced by
the rotary dialer in general.
A
sequence
of
pulses
represents each digit.
Each
pulse
creates
interruption to the current
flow in the telephone line.
The dial pulse occurs at a rate of from 9 to 11 pulses per second.
Dialing time is slow, and the time to complete the dialing depends on the dialed
number.
Due to the limited signaling capability and less reliability, this technique is seldom
used now.
Tone Dialing
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Most telephone sets today use
dual-tone
multi-frequency
(DTMF) for dialing or sending
signals;
each
number
is
represented by two tones (i.e.,
dual-tone).
For example, pressing key “7”
generates a 852Hz tone and a
1209Hz tone.
These telephone sets are equipped
with a push-button touchtone
keypad with 12 keys.
The extended keypad is normally
not constructed.
The keypad pattern is normally
fixed, i.e., there won’t be a
keypad with “7”, “8”, and “9” at
the top position of the keypad
from a commercial phone.
The major advantages of using DTMF are (1) fast dialing time of 0.1sec for any digit is
possible, and (2) it can be used in some simple communication services after the call is
connected (e.g., banking services, enquiry systems, etc.).
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Theme 1 Telephones and Mobile Phones
4.6
Unit 4 Telephones (fixed-line) (Rev 1.0)
Ringer
The ringer is used to indicate that a call is coming and it should stop ringing after the phone is
OFF hook. Different countries use different ringing tones. The operation is controlled by the
telephone company and not by the telephone set in general.
4.7
Summary
In this unit, the basic functional units of a telephone’s (fixed-line) user interface are addressed.
These include the mouthpiece, the earpiece, the switch hook, the dialer, and the ringer. Their
operating principles are briefly explained. Apart from dialing, the basic functional units and
the operating principles of a telephone nearly remain the same as during the last century. The
major inventions in telephone are (1) changing the sound into electrical parameter
(current/voltage) for transmission, and then (2) changing the electrical parameter
(current/voltage) back to reproduce the sound at the receiver side.
Key Terms
Dialer: A dialer is used to specify the telephone number of the called party to the telephone
company.
Earpiece: A device which converts the varying electric current representing the transmitted
speech signal to variation in air pressure and restoring the original speech.
Mouthpiece: A variable-resistance device which generates a reasonable large electric current
(or voltage) that is proportional to the sound pressure. It is also referred to as
microphone.
Pulse Dialing: A sequence of pulses represents each digit, and each pulse creates interruption
to the current flow in the telephone line.
Ringer: A device to indicate that a call is coming.
Switch Hook: A switch hook is used to inform the telephone company the status of the
telephone.
Tone Dialing: Use dual-tone multi-frequency (DTMF) for dialing or sending signals; each
number is represented by two tones (i.e., dual-tone).
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