In a Decadic Pulse Dialing

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Trong một Pulse Decadic Quay số, còn được gọi là Vòng Ngắt kết nối quay số, một xung trực tiếp hiện tại Đào tạo, đại diện cho mỗi chữ số, được sản xuất bởi gián đoạn một tín hiệu liên tục theo với một tỷ số được xác định. Hình 1 cho thấy một Decadic xung điện thoại. Hình 1: Điện thoại Decadic Pulse Một Decadic Pulse Điện thoại được trang bị một Dial quay với một mảng ngón tay trên nó. Các Dial quay được thiết kế để gửi các xung điện. Hình 2 cho thấy các vi mạch của một Xung điện thoại. Trong...

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Nội dung Text: In a Decadic Pulse Dialing

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.