In a Decadic Pulse Dialing docx

Figure 1: Decadic Pulse Telephone A Decadic Pulse Telephone is equipped with a Rotary Dial with a Finger Plate over it.. Figure 2: The Telephone is Ready to make a Call when the Hook Swi

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 Figure 1 shows a Decadic Pulse Telephone

Figure 1: Decadic Pulse Telephone

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

Figure 2: The Telephone is Ready to make a Call when the Hook Switch is Closed The Handset is Off the Hook

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, Figure 3 The Hook Switch is opened and the path to the right part of the circuit is disconnected

• 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

Figure 3: The Telephone is Ready to receive a Call when the Hook Switch is Opened The Handset is On the Hook

The DTMF Telephone, as its name implied, is based on a concept known as Dual Tone Multi-Frequency (DTMF), Figure 1 It generates a combination of two Tones for each Dialed Digit, and sends the Digits to the Telephone Exchange by Hearable Tones instead

of Electrical Pulses as in Decadic Pulse Telephone

Figure 4: DTMF Telephone

The DTMF Telephone is equipped with a Pushbutton Dial, in which 10 Dialing Digits (0 through 9), the Star “ ” and the Pound “#” symbols are assigned to specific Pushbuttons The Pushbuttons are arranged in an two-dimensional array with four Rows and three Columns, as shown in Figure 2 Each Row and Each Columns is assigned a Tone of a specific Frequency, the Columns having Tones of higher Frequencies and the Rows having Tones of lower Frequencies When a Button is pushed, a Dual-Tone Signal is generated This Signal is a combination of Two Tones of different Frequencies, one from the Lower Frequency Group and the other from the Upper Frequency Group, and it is the reason for calling it “Dual Tone Multi-Frequency” In this way, 7 (4 + 3) Tones of different Frequencies are used to generate 12 (4 x 3) combinations For example, pushing the Button “5”, the Tones of 770 Hz and 1336 Hz are transmitted together to the

Telephone Exchange This Signal is decoded by the Telephone Exchange in order to determine which Digit was Dialed

Figure 5: The Pushbutton Dial and its corresponding Frequencies Pairs

Making a Call:

When the Customer lifts the Handset (Off Hook), the Hook Switch is closed and a Circuit connection between the Telephone Exchange and the Customer Telephone is formed

Generation of Signaling Tones

• As has been said, the operation of any Pushbutton generates a Signal composed of Two Tones, which last as long as the Button is pushed Figure 3 shows the Circuit

of a DTMF Telephone

Figure 6: The Telephone is Ready to make a Call when the Hook Switch is

Closed The Handset is Off the Hook

• There are two Oscillation Circuits for generating Tones at different Frequencies Each Circuit consists of a Three-winding Coil (A, A', A" and B, B', B") and a Capacitor (CA and CB) Windings A and B have a number of Spring Contacts, divided in to Group KA and Group KB There are seven Cranks under the

Pushbutton Dial, they are shown as the dotted lines in Figure 3, four of them corresponds to Rows and three to Columns The operation of pushing a Button results in the actuation of a Horizontal Crank and a Vertical Crank When a Crank

is actuated, it will close the corresponding Spring Contact The closure of one of the KA and one of the KB Contacts connects each Capacitor to one of the taps on the associated Winding A and B In this way, the Oscillation Circuits

corresponding to the Dialed Number are setup

• Then, the actuation of the Horizontal Cranks will also put a Common Switch K beside the Pushbutton Array in motion The Common Switch K will operate a set

of Contacts in sequential order as listed in Figure 3 The order and function of each Contact is stated in the following:

1 Attenuates the Dial Tones in the Earphone so the Customer hears the Dial Signal at a comfortable level

2 Powers up the Transistor

3 Disables the Microphone so that no other Noise is received from it to interfere the Dial Signal

4 Initiates the Dual-Tone Signal from the Oscillation Circuits This Signal is sustained by Feedback Amplification through the Transistor and the Transformer action between the Secondary (A', B') and Tertiary Windings (A", B") of each Coil

• The whole Signal Generation Circuit is mounted on the back of the Pushbutton Panel, making the Pushbutton Dial a self-contained unit that can be substituted for the Rotary Dial in a Decadic Pulse Telephone The other parts of a DTMF

Telephone are similar to those of a Decadic Pulse Telephone, thus the process of Making and Receiving a Call is similar to that of a Pulse Telephone At the

Telephone Exchange, the Tones are decoded and a Connection is made to the Called Customer

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 That is, the Hook Switch in Figure 3 is opened and the path to the right part of the Circuit is disconnected

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 a pair of Electromagnet which operates a small Hammer to strike the Bells