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fixed rate. This switch is in series with the hook switch. Opening the switch interrupts the current in the local
loop. A series of current pulses is thus sent out on the local loop as shown in Figure 3. The number of pulses
sent corresponds to the digit dialed. Dialing a 0 sends ten pulses.
The dial pulses are sent at a rate of 10 pulses per second (100mS between pulses). Each pulse consists of a
mark interval (loop current). In America the mark interval is 40mS and the space interval is 60mS giving a
mark/space ratio of 40/60. In Europe the mark space ratio is usually 33/67.
Tone Dialer
Tone dialing is accomplished with a keyboard of 12 keys
arranged in 4 rows and 3 columns. As seen in Figure 4, low
frequencies of 697, 770, 852 and 941 are associated with rows
R1 through R4 and high frequencies of 1,209, 1,336 and
1,477Hz are associated with columns C1 through C3. To send
each digit, two frequencies are sent to the Central Office
simultaneously.
For this reason, this method of dialing is
referred to as dual tone multifrequency (DTMF). The different
frequencies are generated by connecting a capacitor to
different taps of a transformer to establish a resonant circuit of
the correct frequency.
mechanically connected to switch SR1. Similarly each of the
other rows and columns are connected to their corresponding
switch.
Thus, pressing any key closes two switches and
generates two frequencies. Pressing a 6 for example, closes
switches SR2 and SC3 and generates 770 and 1,477Hz.
TRANSMITTER
The Transmitter consists of a metal diaphragm and a metal case insulated from each other
as shown in Figure 5. The case is filled with carbon granules. Wires are connected to the
case and the diaphragm and a current is put through the carbon granules. When you
speak into the transmitter, the sound waves of your voice strike the diaphragm and cause
it to vibrate.
This causes the carbon granules to compress and expand.
compressed, the resistance of the carbon granules is less than when expanded. The
change of resistance causes a corresponding change in the current. The current thus
varies in step with the sound waves of your voice. In a newer electronic type telephone
the carbon granule transmitter may be replaced by an electret or other type microphone.
An electret microphone is made up of a capacitor with a dielectric material which holds a
permanent electric charge. Sound waves striking a plate of this capacitor cause the plate
to vibrate and thus generate a small voltage across the capacitor. This voltage is amplified
by a field effect transistor (FET) mounted inside of the microphone. The signal from the
microphone is then amplified before being sent to the Central Office via the local loop.
RECEIVER
There are several different types of receivers. In principle, they work the same
as the speakers in your radio and TV. The speaker consists of a small coil
attached to a diaphragm. The coil is mounted over a permanent magnet as
shown in Figure 6. Coil current in one direction causes the coil and diaphragm
to be repelled from the permanent magnet. Coil current in the other direction
causes the coil and diaphragm to be attracted to the permanent magnet. If a
current of audio frequency is sent through the coil, the diaphragm vibrates and
generates sound waves in step with the current. Thus, if the current from the
transmitter is sent through the coil, the sound produced will duplicate the
sound striking the transmitter.
Each of the 3 keys of row 1 is
1,209
R1 697Hz
R2 770Hz
R3 852Hz
R4 941Hz
-3-
C1
C2
C3
1,336Hz
1,477Hz
1
2
3
4
5
6
7
8
9
0
#
*
Figure 4
Diaphragm
When
Figure 5
Coil
Permanent
Magnet
Speaker
Frame
Figure 6
SR1
SR2
SR3
SR4
SC3
SC2
SC1
Insulator
Case
Carbon
Granules
Current
Diaphragm
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