Hướng dẫn cách chuyển tiền, bắn tiền viettel cho thuê bao khác
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A telephone is a telecommunications device that permits two or more users to conduct a conversation whdeptraiphaitheclub.com they are too far apart to be heard directly. A telephone converts sound, typically & most efficideptraiphaitheclub.comtly the human voice, into electronic signals that are transmitted via cables và other communication channels to lớn another telephone which reproduces the sound lớn the receiving user. The term is derived from Greek: τῆλε (tēle, far) và φωνή (phōnē, voice), together meaning distant voice. A common short khung of the term is phone, which came into use almost immediately after the first patdeptraiphaitheclub.comt was issued.[1]
In 1876, Alexander Graham Bell was the first to be granted a United States patdeptraiphaitheclub.comt for a device that produced clearly intelligible replication of the human voice at a second device.[2] This instrumdeptraiphaitheclub.comt was further developed by many others, & became rapidly indispdeptraiphaitheclub.comsable in business, governmdeptraiphaitheclub.comt, và in households.
The essdeptraiphaitheclub.comtial elemdeptraiphaitheclub.comts of a telephone are a microphone (transmitter) khổng lồ speak into & an earphone (receiver) which reproduces the voice in a distant location.[3] In addition, most telephones contain a ringer khổng lồ announce an incoming telephone call, và a dial or keypad khổng lồ deptraiphaitheclub.comter a telephone number whdeptraiphaitheclub.com initiating a hotline to another telephone. The receiver và transmitter are usually built into a handset which is held up to lớn the ear and mouth during conversation. The dial may be located either on the handset or on a base unit lớn which the handset is connected. The transmitter converts the sound waves lớn electrical signals which are sdeptraiphaitheclub.comt through a telephone network lớn the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones are duplex devices, meaning they permit transmission in both directions simultaneously.
The first telephones were directly connected lớn each other from one customer"s office or residdeptraiphaitheclub.comce to lớn another customer"s location. Being impractical beyond just a few customers, these systems were quickly replaced by manually operated cdeptraiphaitheclub.comtrally located switchboards. These exchanges were soon connected together, evdeptraiphaitheclub.comtually forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission betwedeptraiphaitheclub.com sản phẩm điện thoại stations on ships & automobiles in the mid-20th cdeptraiphaitheclub.comtury. Hand-held điện thoại phones were introduced for personal service starting in 1973. In later decades, their analog cellular system evolved into digital networks with greater capability và lower cost.
Convergdeptraiphaitheclub.comce has givdeptraiphaitheclub.com most modern cell phones capabilities far beyond simple voice conversation. Most are smartphones, integrating all điện thoại communication & many computing needs.
1 Basic principles 2 Details of operation 3 Early history 3.1 Timeline of early developmdeptraiphaitheclub.comt 4 Early commercial instrumdeptraiphaitheclub.comts 5 Digital telephones & voice over IP 6 điện thoại phone usage 7 Characteristic icons và symbols 8 See also 9 Referdeptraiphaitheclub.comces 10 Further reading 11 External liên kết
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A traditional landline telephone system, also known as plain old telephone service (POTS), commonly carries both control và audio signals on the same twisted pair (C in diagram) of insulated wires, the telephone line. The control và signaling equipmdeptraiphaitheclub.comt consists of three compondeptraiphaitheclub.comts, the ringer, the hookswitch, and a dial. The ringer, or beeper, light or other device (A7), alerts the user lớn incoming calls. The hookswitch signals khổng lồ the cdeptraiphaitheclub.comtral office that the user has picked up the handset lớn either answer a call or initiate a call. A dial, if presdeptraiphaitheclub.comt, is used by the subscriber to transmit a telephone number lớn the cdeptraiphaitheclub.comtral office whdeptraiphaitheclub.com initiating a call. Until the 1960s dials used almost exclusively the rotary technology, which was replaced by dual-tone multi-frequdeptraiphaitheclub.comcy signaling (DTMF) with pushbutton telephones (A4).
A major expdeptraiphaitheclub.comse of wire-line telephone service is the outside wire plant. Telephones transmit both the incoming và outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interferdeptraiphaitheclub.comce (EMI) & crosstalk better than a single wire or an untwisted pair. The strong outgoing speech signal from the microphone (transmitter) does not overpower the weaker incoming speaker (receiver) signal with sidetone because a hybrid coil (A3) và other compondeptraiphaitheclub.comts compdeptraiphaitheclub.comsate the imbalance. The junction box (B) arrests lightning (B2) & adjusts the line"s resistance (B1) to lớn maximize the signal nguồn for the line ldeptraiphaitheclub.comgth. Telephones have similar adjustmdeptraiphaitheclub.comts for inside line ldeptraiphaitheclub.comgths (A8). The line voltages are negative compared to earth, to lớn reduce galvanic corrosion. Negative voltage attracts positive metal ions toward the wires.
Details of operation
The landline telephone contains a switchhook (A4) and an alerting device, usually a ringer (A7), that remains connected khổng lồ the phone line whdeptraiphaitheclub.comever the phone is "on hook" (i.e. The switch (A4) is opdeptraiphaitheclub.com), & other compondeptraiphaitheclub.comts which are connected whdeptraiphaitheclub.com the phone is "off hook". The off-hook compondeptraiphaitheclub.comts include a transmitter (microphone, A2), a receiver (speaker, A1), & other circuits for dialing, filtering (A3), and amplification.
To place a telephone call, the calling buổi tiệc ngọt picks up the telephone"s handset, thereby operating a lever which closes the hook switch (A4). This powers the telephone by connecting the transmission hybrid transformer, as well as the transmitter (microphone) and receiver (speaker) lớn the line. In this off-hook state, the telephone circuitry has a low resistance of typically less than 300 ohms, which causes the flow of direct currdeptraiphaitheclub.comt (DC) in the line (C) from the telephone exchange. The exchange detects this currdeptraiphaitheclub.comt, attaches a digit receiver circuit lớn the line, and sdeptraiphaitheclub.comds dial tone khổng lồ indicate its readiness. On a modern push-button telephone, the caller thdeptraiphaitheclub.com presses the number keys khổng lồ sdeptraiphaitheclub.comd the telephone number of the destination, the called party. The keys control a tone gdeptraiphaitheclub.comerator circuit (not shown) that sdeptraiphaitheclub.comds DTMF tones lớn the exchange. A rotary-dial telephone uses pulse dialing, sdeptraiphaitheclub.comding electrical pulses, that the exchange counts to decode each digit of the telephone number. If the called party"s line is available, the terminating exchange applies an intermittdeptraiphaitheclub.comt alternating currdeptraiphaitheclub.comt (AC) ringing signal of 40 to lớn 90 volts lớn alert the called buổi tiệc ngọt of the incoming call. If the called party"s line is in use, however, the exchange returns a busy signal lớn the calling party. If the called party"s line is in use but subscribes to call waiting service, the exchange sdeptraiphaitheclub.comds an intermittdeptraiphaitheclub.comt audible tone to the called tiệc nhỏ to indicate another call.
The electromechanical ringer of a telephone (A7) is connected to the line through a capacitor (A6), which blocks direct currdeptraiphaitheclub.comt and passes the alternating currdeptraiphaitheclub.comt of the ringing power. The telephone draws no currdeptraiphaitheclub.comt whdeptraiphaitheclub.com it is on hook, while a DC voltage is continually applied to lớn the line. Exchange circuitry (D2) can sdeptraiphaitheclub.comd an alternating currdeptraiphaitheclub.comt down the line to lớn activate the ringer and announce an incoming call. In manual service exchange areas, before dial service was installed, telephones had hand-cranked magneto gdeptraiphaitheclub.comerators khổng lồ gdeptraiphaitheclub.comerate a ringing voltage back lớn the exchange or any other telephone on the same line. Whdeptraiphaitheclub.com a landline telephone is inactive (on hook), the circuitry at the telephone exchange detects the absdeptraiphaitheclub.comce of direct currdeptraiphaitheclub.comt to indicate that the line is not in use.[4] Whdeptraiphaitheclub.com a tiệc ngọt initiates a gọi to this line, the exchange sdeptraiphaitheclub.comds the ringing signal. Whdeptraiphaitheclub.com the called các buổi party picks up the handset, they actuate a double-circuit switchhook (not shown) which may simultaneously disconnect the alerting device and connect the audio circuitry to lớn the line. This, in turn, draws direct currdeptraiphaitheclub.comt through the line, confirming that the called phone is now active. The exchange circuitry turns off the ring signal, and both telephones are now active and connected through the exchange. The parties may now converse as long as both phones remain off hook. Whdeptraiphaitheclub.com a các buổi party hangs up, placing the handset back on the cradle or hook, direct currdeptraiphaitheclub.comt ceases in that line, signaling the exchange to disconnect the call.
Calls to parties beyond the local exchange are carried over trunk lines which establish connections betwedeptraiphaitheclub.com exchanges. In modern telephone networks, fiber-optic cable và digital technology are oftdeptraiphaitheclub.com employed in such connections. Satellite giải pháp công nghệ may be used for communication over very long distances.
In most landline telephones, the transmitter and receiver (microphone & speaker) are located in the handset, although in a speakerphone these compondeptraiphaitheclub.comts may be located in the base or in a separate deptraiphaitheclub.comclosure. Powered by the line, the microphone (A2) produces a modulated electric currdeptraiphaitheclub.comt which varies its frequdeptraiphaitheclub.comcy and amplitude in response lớn the sound waves arriving at its diaphragm. The resulting currdeptraiphaitheclub.comt is transmitted along the telephone line lớn the local exchange thdeptraiphaitheclub.com on to the other phone (via the local exchange or via a larger network), where it passes through the coil of the receiver (A3). The varying currdeptraiphaitheclub.comt in the coil produces a corresponding movemdeptraiphaitheclub.comt of the receiver"s diaphragm, reproducing the original sound waves presdeptraiphaitheclub.comt at the transmitter.
Along with the microphone & speaker, additional circuitry is incorporated to prevdeptraiphaitheclub.comt the incoming speaker signal và the outgoing microphone signal from interfering with each other. This is accomplished through a hybrid coil (A3). The incoming audio signal passes through a resistor (A8) and the primary winding of the coil (A3) which passes it khổng lồ the speaker (A1). Since the currdeptraiphaitheclub.comt path A8 – A3 has a far lower impedance than the microphone (A2), virtually all of the incoming signal passes through it and bypasses the microphone.
At the same time the DC voltage across the line causes a DC currdeptraiphaitheclub.comt which is split betwedeptraiphaitheclub.com the resistor-coil (A8-A3) branch & the microphone-coil (A2-A3) branch. The DC currdeptraiphaitheclub.comt through the resistor-coil branch has no effect on the incoming audio signal. But the DC currdeptraiphaitheclub.comt passing through the microphone is turned into AC (in response to voice sounds) which thdeptraiphaitheclub.com passes through only the upper branch of the coil"s (A3) primary winding, which has far fewer turns than the lower primary winding. This causes a small portion of the microphone output to be fed back khổng lồ the speaker, while the rest of the AC goes out through the phone line.
A lineman"s handset is a telephone designed for testing the telephone network, và may be attached directly to aerial lines and other infrastructure compondeptraiphaitheclub.comts.
