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Tone Dialer
Tone dialing is accomplished with a keyboard of 12
keys arranged in 4 rows and 3 columns. As seen in
Figure 3-4, low frequencies of 697, 770, 852 and
941 are associated with rows R1 through R4 and
high frequencies of 1209, 1336 and 1477Hz 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. Each of the 3 keys of row 1 are
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
Figure 3-4
1477Hz.
Transmitter
The Transmitter consists of a metal diaphragm and a metal case insulated from each other as shown in Figure 3-5.
The case is filled with carbon granules. When you speak into the transmitter, the sound waves of your voice strike
the diaphragm and causes it to vibrate. This causes the carbon granules to compress and expand. When
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.
Figure 3-5
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