In a Decadic Pulse Dialing, also
called Loop Disconnect Dialing, a Direct-Current Pulse Train, representing each
Digit, is produced by interrupting a continous Signal according to a defined
Ratio.
A
Decadic Pulse Telephone is equipped with a Rotary Dial with a Finger Plate over
it. The Rotary Dial is designed to send Electrical Pulses. Figure 2 shows the
Circuit of a Pulse Telephone. The 3 spring contacts: the Impulsing Contact
(ICT), the Bypass Switch 1 (BP1) and the Bypass Switch 2 (BP2) are mounted
inside the Rotary Dial.
Making a Call:
When the Customer lifts the Handset
(Off Hook), the Hook Switch is closed and a DC Loop between the Telephone
Exchange and the Customer is formed, Figure 1. Therefore, a Continous Current DC
flows through the Loop.
- While Dialing a Number, for example, 5, the Dial is drawn round in the Clockwise Direction to the Finger Stop Position and released. The Finger Plate will then return to the Rest Position under the influence of a spring. Meanwhile, the BP2 in the Rotary Dial is closed to make a Short Circuit, thus you hear no disturbance in the Earphone during Dialing.
- The ICT then generates the Dial Pulses by closing and opening itself to interrupt the DC Current loop. The number of interruptions is equivalent to the Dialed Digit. This type of Telephone generates two additional Pulses, which are eliminated by the BP1.
- Besides, there is also a mechanical device called Centrifugal Governor, mounted inside the Rotary Dial, it helps to maintain a uniform speed of rotation.
- After finishing Dialing, the Pulses are then decoded at the Telephone Exchange that make a connection to the Called Customer. The Voice Signals from the Customer will be transmitted to the Earphone through an Isolating Transformer. The Earphone is connected to the Secondary Winding of the Isolating Transformer. This protects the Earphone from being damaged by DC Current. The Transformer also provides an Electrical Isolation between the Telephone Exchange and the Ear. In addition, two anti-parallel Diodes protects the Ear from too much Noise. If the Voltage in the Secondary Winding is over a certain level, one of the Diodes starts to Conduct and makes a Short Circuit for the Earphone.
- While talking to the Microphone, the Sound Signal is transmitted to the Called Customer through the Telephone Exchange.
Receiving a Call:
When the Handset is on the Cradle,
the Telephone is said to be "On the Hook", or ready to receive a
Call,
- Whenever there is an Incoming Call, An AC Ringing Signal from the Telephone Exchange is transmitted to the Telephone. The Ringing Signal is generally 10 mA AC Signal, with a Frequency between 20 and 25 Hz, that activates an Electromagnet which operates a small Hammer to strike the Bells.
