Skype is a telecommunications application that specializes in providing video chat and voice calls between computers, mobile devices, the Xbox One console, smartwatches via the Internet. Skype provides instant messaging services. Users may transmit text, video and images. Skype allows video conference calls. At the end of 2010, there were over 660 million worldwide users, with over 300 million estimated active each month as of August 2015. At one point in February 2012, there were 34 million users concurrently online on Skype. First released in August 2003, Skype was created by the Swede Niklas Zennström and the Dane Janus Friis, in cooperation with Ahti Heinla, Priit Kasesalu, Jaan Tallinn, Estonians who developed the backend, used in the music-sharing application Kazaa. In September 2005, eBay acquired Skype for $2.6 billion. In September 2009, Silver Lake, Andreessen Horowitz, the Canada Pension Plan Investment Board announced the acquisition of 65% of Skype for $1.9 billion from eBay, which attributed to the enterprise a market value of $2.92 billion.
Microsoft bought Skype in May 2011 for $8.5 billion. Skype division headquarters are in Luxembourg, but most of the development team and 44% of all the division's employees are still situated in Tallinn and Tartu, Estonia. Skype allows users to communicate over the Internet by voice, using a microphone, by video using a webcam, by instant messaging. Skype implements a freemium business model. Skype-to-Skype calls are free of charge, while calls to landline telephones and mobile phones are charged via a debit-based user account system called Skype Credit; some network administrators have banned Skype on corporate, government and education networks, citing such reasons as inappropriate usage of resources, excessive bandwidth usage and security concerns. Skype featured a hybrid peer-to-peer and client–server system. Skype has been powered by Microsoft-operated supernodes since May 2012; the 2013 mass surveillance disclosures revealed that Microsoft had granted intelligence agencies unfettered access to supernodes and Skype communication content.
Throughout 2016 and 2017, Microsoft redesigned its Skype clients in a way that transitioned Skype from peer-to-peer service to a centralized Azure service and adjusted the user interfaces of apps to make text-based messaging more prominent than voice calling. Skype for Windows, iOS, Android and Linux received significant, visible overhauls; the name for the software is derived from "Sky peer-to-peer", abbreviated to "Skyper". However, some of the domain names associated with "Skyper" were taken. Dropping the final "r" left the current title "Skype", for which domain names were available. Skype was founded in 2003 by Niklas Zennström, from Sweden, Janus Friis, from Denmark; the Skype software was created by Estonians Ahti Heinla, Priit Kasesalu, Jaan Tallinn. The first public beta version was released on 29 August 2003. In June 2005, Skype entered into an agreement with the Polish web portal Onet.pl for an integrated offering on the Polish market. On 12 September 2005, eBay Inc. agreed to acquire Luxembourg-based Skype Technologies SA for US$2.5 billion in up-front cash and eBay stock, plus potential performance-based consideration.
On 1 September 2009, eBay announced it was selling 65% of Skype to Silver Lake, Andreessen Horowitz, the Canada Pension Plan Investment Board for US$1.9 billion, valuing Skype at US$2.75 billion. On 14 July 2011, Skype partnered with Comcast to bring its video chat service to Comcast subscribers via HDTV sets. On 17 June 2013, Skype released a free video messaging service, which can be operated on Windows, Mac OS, iOS, Android and BlackBerry. On 2 August 2017, Skype teamed up with PayPal to provide a new money send feature, it allows users to transfer funds via the Skype mobile app in the middle of a conversation using PayPal. On 10 May 2011, Microsoft Corporation acquired Skype Communications, S.à r.l for US$8.5 billion. The company was incorporated as a division of Microsoft, which acquired all its technologies with the purchase; the acquisition was completed on 13 October 2011. Shortly after its acquisition, Microsoft began integrating the Skype service with its own products. Along with taking over development of existing Skype desktop and mobile apps, the company developed a dedicated client app for its newly released, touch-focused Windows 8 and Windows RT operating systems.
They were made available from Windows Store when the new OS launched on 26 October 2012. The following year, it became the default messaging app for Windows 8.1, replacing the Windows 8 Messaging app at the time, became pre-installed software on every device that came with or upgraded to 8.1. When the company introduced Office 2013 on 27 February 2013, it was announced that 60 Skype world minutes per month would be included in Office 365 consumer plans. Furthermore, Microsoft discontinued two of its own products in favor of Skype: In a month-long transition period from 8 to 30 April 2013, Microsoft phased out its long-standing Windows Live Messenger instant messaging service in favor of Skype, although Messenger continued in mainland China. On 11 November 2014, Microsoft announced; the latest version of the communication software combines features of Lync and the consumer Skype software. There are two user interfaces – organizations can switch their users from the default Skype for Business interface to the Lync interface.
On 12 August 2013, Skype released the 4.10 update to the app for Apple iPhone and iPad that allows HD quality video for iPhone 5 and fourth-generation iPads. On 20 November 2014, Microsoft Office's team announced that a
A telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the airing of telephone callers, the broadcast-quality telephone hybrid is known as a broadcast telephone hybrid or telephone balance unit; the need for hybrids comes from the nature of analog plain old telephone service home or small business telephone lines, where the two audio directions are combined on a single two-wire pair. Within the telephone network and transmission are always four-wire circuits with the two signals being separated. Hybrids perform the necessary conversion. In older analog networks, conversion to four-wire was required so that repeater amplifiers could be inserted in long-distance links. In today's digital systems, each speech direction must be transported independently; the line cards in a telephone central office switch that are interfaced to analog lines include hybrids that adapt the four-wire network to the two-wire circuits that connect most subscribers.
The search for better telephone hybrids and echo cancelers was an important motive for the development of DSP algorithms and hardware at Bell Labs, NEC, other sites. The fundamental principle is that of impedance matching; the incoming signal is applied to both the telephone line and a "balancing network", designed to have the same impedance as the line. The outgoing signal is derived by subtracting the two, thus canceling the incoming signal from the outgoing signal. Early hybrids were made with transformers configured as hybrid coils that had an extra winding that could be connected out of phase; the name hybrid comes from these special mixed-winding transformers. An effective hybrid would have high trans-hybrid loss, which means that little of the incoming audio would appear on the outgoing port. Too much leakage can cause echoes when there is a delay in the transmission path, as there is with satellite, mobile phone, VoIP links; this is a result of a talker's voice traversing to the far-end hybrid and returning to his own receiver with insufficient attenuation.
ITU-T Recommendation G.131 describes the relationship of echo delay vs. amplitude to listener annoyance. At 100ms, 45 dB return loss is required for less than 1% of test subjects to express dissatisfaction. Good cancellation depends upon the balancing network having a frequency-vs.-impedance characteristic that matches the line. Since telephone line impedances vary depending upon many factors and the relationship is not always smooth, analog hybrids are able to achieve only a few dB of guaranteed isolation. For this reason, modern hybrids use digital signal processing to implement an adaptive least mean squares filter that automatically detects the line's impedance across the voice frequency range and adjusts to it; these may reach greater than 30 measured with white noise as the send signal. DSP hybrids are called "line echo cancellers". Hybrids and cancellers are sometimes combined with echo suppressors; these work on the assumption that only one of the two parties to a conversation is speaking at a given time.
The suppressor switches a loss into the inactive speech path, thus enhancing the echo-cancelling effect of the hybrid at the expense of simultaneous two-way conversation. Despite being inherently four-wire, VoIP systems require hybrids when they interface to two-wire lines. A VoIP-to-Telco gateway used to interface a VoIP PBX to analog lines would contain hybrids to perform the required conversion. End-end VoIP needs no hybrids. In broadcast studio facilities, the name for the functional part has come to refer to the whole, a telephone hybrid is the device that packages all the functions needed to connect telephone lines to studio audio systems, providing electrical and physical interface between the telco lines and studio equipment; these devices include processing in addition to the hybrid function, such as dynamics control and equalization. Some have dynamic EQ that adjusts parameters automatically to maintain spectral consistency from varying source audio; some incorporate acoustic echo cancellation to allow setups with acoustic paths between loudspeakers carrying phone audio and microphones feeding the phone lines.
In studio applications, a hybrid needs good send-to-receive isolation. When too much of the host audio appears at the hybrid's output, there will be a number of defects. Distortion of the host's voice can result from the telephone line's changing the phase of the send audio before it returns, with varying shifts at different frequencies; the original and leakage audio are mixed at the console and combine in and out of phase at the various frequencies. When this occurs, the host sounds either hollow or tinny as the phase cancellation affects some frequencies more than others. Audio feedback can result from the acoustic coupling created when callers must be heard on a loudspeaker; when lines are conferenced and the gain around the loop of the multiple hybrids is greater than unity, feedback "singing" will be audible. If the leakage is high, operators will not be able to control the relative levels of the host audio
The carbon microphone known as carbon button microphone, button microphone, or carbon transmitter, is a type of microphone, a transducer that converts sound to an electrical audio signal. It consists of two metal plates separated by granules of carbon. One plate is thin and faces toward the speaking person, acting as a diaphragm. Sound waves striking the diaphragm cause it to vibrate, exerting a varying pressure on the granules, which in turn changes the electrical resistance between the plates. Higher pressure lowers the resistance. A steady direct current is passed between the plates through the granules; the varying resistance results in a modulation of the current, creating a varying electric current that reproduces the varying pressure of the sound wave. In telephony, this undulating current is directly passed through the telephone wires to the central office. In public address systems it is amplified by an audio amplifier; the frequency response of the carbon microphone, however, is limited to a narrow range, the device produces significant electrical noise.
Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals. They were used in telephone systems until the 1980s, while other applications used different microphone designs much earlier, their low cost, inherently high output and frequency response characteristic were well suited for telephony. For plain old telephone service, carbon-microphone based telephones can still be used without modification. Carbon microphones modified telephone transmitters, were used in early AM radio broadcasting systems, but their limited frequency response, as well as a high noise level, led to their abandonment in those applications by the late 1920s, they continued to be used for low-end public address, military and amateur radio applications for some decades afterward. The first microphone that enabled proper voice telephony was the carbon microphone; this was independently developed around 1878 by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US.
Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, most historians credit him with its invention. Hughes' device used loosely packed carbon granules - the varying pressure exerted on the granules by the diaphragm from the acoustic waves caused the resistance of the carbon to vary proportionally, allowing a accurate electrical reproduction of the sound signal. Hughes coined the word microphone, he demonstrated his apparatus to the Royal Society by magnifying the sound of insects scratching through a sound box. Contrary to Edison, Hughes decided not to take out a patent. In America and Berliner fought a long legal battle over the patent rights. A federal court awarded Edison full rights to the invention, stating "Edison preceded Berliner in the transmission of speech... The use of carbon in a transmitter is, beyond controversy, the invention of Edison" and the Berliner patent was ruled invalid; the carbon microphone is the direct prototype of today's microphones and was critical in the development of telephony and the recording industries.
Carbon granules were used between carbon buttons. Carbon microphones were used in telephones from 1890 until the 1980s. Carbon microphones can be used as amplifiers; this capability was used in early telephone repeaters, making long distance phone calls possible in the era before vacuum tube amplifiers. In these repeaters, a magnetic telephone receiver was mechanically coupled to a carbon microphone; because a carbon microphone works by varying a current passed through it, instead of generating a signal voltage as with most other microphone types, this arrangement could be used to boost weak signals and send them down the line. These amplifiers were abandoned with the development of vacuum tubes, which offered higher gain and better sound quality. After vacuum tubes were in common use, carbon amplifiers continued to be used during the 1930s in portable audio equipment such as hearing aids; the Western Electric 65A carbon amplifier was 1.2" in diameter and 0.4" high and weighed less than 1.4 ounces.
Such carbon amplifiers did not require the heavy bulky batteries and power supplies used by vacuum tube amplifiers. By the 1950s, carbon amplifiers for hearing aids had been replaced by miniature vacuum tubes. However, carbon amplifiers are still being sold. An illustration of the amplification provided by carbon microphones was the oscillation caused by feedback, which resulted in an audible squeal from the old "candlestick telephone" if its earphone was placed near the carbon microphone. Early AM radio transmitters relied on carbon microphones for voice modulation of the radio signal. In the first long-distance audio transmissions by Reginald Fessenden in 1906, a continuous wave from an Alexanderson alternator was fed directly to the transmitting antenna through a water-cooled carbon microphone. Systems using vacuum tube oscillators used the output from a carbon microphone to modulate the grid bias of the oscillator or output tube to achieve modulation. Apart from legacy telephone installations in Third World countries, carbon microphones are still used today in certain niche applications in the developed world.
An example is the Shure 104c, still in demand because of its wide compatibility with existing equipment. The principal advantage of carbon microphones over
Wireless telegraphy means transmission of telegraph signals by radio waves. Before about 1910 when radio became dominant, the term wireless telegraphy was used for various other experimental technologies for transmitting telegraph signals without wires, such as electromagnetic induction, ground conduction telegraph systems. Radiotelegraphy was the first means of radio communication, it continued to be the only type of radio transmission during the first three decades of radio, called the "wireless telegraphy era" up until World War I, when the development of amplitude modulation radiotelephony allowed sound to be transmitted by radio. In radiotelegraphy, information is transmitted by pulses of radio waves of two different lengths called "dots" and "dashes", which spell out text messages in Morse code. In a manual system, the sending operator taps on a switch called a telegraph key which turns the transmitter on and off, producing the pulses of radio waves. At the receiver the pulses are audible in the receiver's speaker as beeps, which are translated back to text by an operator who knows Morse code.
Radiotelegraphy was used for long distance person-to-person commercial and military text communication throughout the first half of the 20th century. It became a strategically important capability during the two world wars, since a nation without long distance radiotelegraph stations could be isolated from the rest of the world by an enemy cutting its submarine telegraph cables. Beginning about 1908, powerful transoceanic radiotelegraphy stations transmitted commercial telegram traffic between countries at rates up to 200 words per minute. Radiotelegraphy was transmitted by several different modulation methods during its history; the primitive spark gap transmitters used until 1920 transmitted damped waves, which had large bandwidth and tended to interfere with other transmissions. This type of emission was banned by 1930; the vacuum tube transmitters which came into use after 1920 transmitted code by pulses of unmodulated sinusoidal carrier wave called continuous waves, still used today. To make CW transmissions audible, the receiver requires a circuit called a beat frequency oscillator.
A third type of modulation, frequency shift keying was used by radioteletypes. Morse code radiotelegraphy was replaced by radioteletype networks in most high volume applications by World War 2. Today it is nearly obsolete, the only remaining users are the radio amateur community and some limited training by the military for emergency use. Wireless telegraphy or radiotelegraphy called CW, ICW transmission, or on-off keying, designated by the International Telecommunication Union as emission type A1A, is a radio communication method in which the sending operator taps on a switch called a telegraph key, which turns the radio transmitter on and off, producing pulses of unmodulated carrier wave of different lengths called "dots" and "dashes", which encode characters of text in Morse code. At the receiving location the code is audible in the radio receiver's earphone or speaker as a sequence of buzzes or beeps, translated back to text by an operator who knows Morse code. Although this type of communication has been replaced since its introduction over 100 years ago by other means of communication it is still used by amateur radio operators as well as some military services.
A CW coastal station, KSM, still exists in California, run as a museum by volunteers, occasional contacts with ships are made. Radio beacons in the aviation service, but as "placeholders" for commercial ship-to-shore systems transmit Morse but at slow speeds; the US Federal Communications Commission issues a lifetime commercial Radiotelegraph Operator License. This requires passing a simple written test on regulations, a more complex written exam on technology, demonstrating Morse reception at 20 words per minute plain language and 16 wpm code groups. Wireless telegraphy is still used today by amateur radio hobbyists where it is referred to as radio telegraphy, continuous wave, or just CW. However, its knowledge is not required to obtain any class of amateur license. Continuous wave radiotelegraphy is regulated by the International Telecommunication Union as emission type A1A. Efforts to find a way to transmit telegraph signals without wires grew out of the success of electric telegraph networks, the first instant telecommunication systems.
Developed beginning in the 1830s, a telegraph line was a person-to-person text message system consisting of multiple telegraph offices linked by an overhead wire supported on telegraph poles. To send a message, an operator at one office would tap on a switch called a telegraph key, creating pulses of electric current which spelled out a message in Morse code; when the key was pressed, it would connect a battery to the telegraph line, sending current down the wire. At the receiving office the current pulses would operate a telegraph sounder, a device which would make a "click" sound when it received each pulse of current; the operator at the receiving station who knew Morse code would translate the clicking sounds to text and write down the message. The ground was used as the return path for current in the telegraph circuit, to avoid having to use a second overhead wire. By the 1860s, telegraph was the standard way to send most urgent commercial and milita
Telephony is the field of technology involving the development and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the development of the telephone. Telephony is referred to as the construction or operation of telephones and telephonic systems and as a system of telecommunications in which telephonic equipment is employed in the transmission of speech or other sound between points, with or without the use of wires; the term is used to refer to computer hardware and computer network systems, that perform functions traditionally performed by telephone equipment. In this context the technology is referred to as Internet telephony, or voice over Internet Protocol; the first telephones were connected directly in pairs. Each user had a separate telephone wired to the locations he or she might wish to reach; this became inconvenient and unmanageable when people wanted to communicate with more than a few people.
The inventions of the telephone exchange provided the solution for establishing telephone connections with any other telephone in service in the local area. Each telephone was connected to the exchange via the local loop. Nearby exchanges in other service areas were connected with trunk lines and long distance service could be established by relaying the calls through multiple exchanges. Switchboards were manually operated by an attendant referred to as the "switchboard operator"; when a customer cranked a handle on the telephone, it turned on an indicator on the board in front of the operator, who would plug the operator headset into that jack and offer service. The caller had to ask for the called party by name by number, the operator connected one end of a circuit into the called party jack to alert them. If the called station answered, the operator disconnected their headset and completed the station-to-station circuit. Trunk calls were made with the assistance of other operators at other exchangers in the network.
In modern times, most telephones are plugged into telephone jacks. The jacks are connected by inside wiring to a drop wire. Cables bring a large number of drop wires from all over a district access network to one wire center or telephone exchange; when a telephone user wants to make a telephone call, equipment at the exchange examines the dialed telephone number and connects that telephone line to another in the same wire center, or to a trunk to a distant exchange. Most of the exchanges in the world are interconnected through a system of larger switching systems, forming the public switched telephone network. After the middle of the 20th century and data became important secondary users of the network created to carry voices, late in the century, parts of the network were upgraded with ISDN and DSL to improve handling of such traffic. Today, telephony uses digital technology in the provisioning of telephone systems. Telephone calls can be provided digitally, but may be restricted to cases in which the last mile is digital, or where the conversion between digital and analog signals takes place inside the telephone.
This advancement has reduced costs in communication, improved the quality of voice services. The first implementation of this, ISDN, permitted all data transport from end-to-end speedily over telephone lines; this service was made much less important due to the ability to provide digital services based on the IP protocol. Since the advent of personal computer technology in the 1980s, computer telephony integration has progressively provided more sophisticated telephony services and controlled by the computer, such as making and receiving voice and data calls with telephone directory services and caller identification; the integration of telephony software and computer systems is a major development in the evolution of office automation. The term is used in describing the computerized services of call centers, such as those that direct your phone call to the right department at a business you're calling. It's sometimes used for the ability to use your personal computer to initiate and manage phone calls.
CTI is not a new concept and has been used in the past in large telephone networks, but only dedicated call centers could justify the costs of the required equipment installation. Primary telephone service providers are offering information services such as automatic number identification, a telephone service architecture that separates CTI services from call switching and will make it easier to add new services. Dialed Number Identification Service on a scale is wide enough for its implementation to bring real value to business or residential telephone usage. A new generation of applications is being developed as a result of standardization and availability of low cost computer telephony links. Starting with the introduction of the transistor, invented in 1947 by Bell Laboratories, to amplification and switching circuits in the 1950s, through development of computer-based electronic switching systems, the public switched telephone network has evolved towards automation and digitization of signaling and audio transmissions.
Digital telephony is the use of digital electronics in the operation and provisioning of telephony systems and services. Since the 1960s a digital core network has replaced the traditional analog transmission and signaling systems, much of the access network has been digitized. Digital
A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. In 1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced intelligible replication of the human voice; this instrument was further developed by many others. The telephone was the first device in history that enabled people to talk directly with each other across large distances. Telephones became indispensable to businesses and households and are today some of the most used small appliances; the essential elements of a telephone are a microphone to speak into and an earphone which reproduces the voice in a distant location. In addition, most telephones contain a ringer to announce an incoming telephone call, a dial or keypad to enter a telephone number when initiating a call to another telephone.
The receiver and transmitter are built into a handset, held up to the ear and mouth during conversation. The dial may be located either on a base unit to which the handset is connected; the transmitter converts the sound waves to electrical signals which are sent through a telephone network to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones are duplex devices; the first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were replaced by manually operated centrally located switchboards; these exchanges were soon connected together forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission between mobile stations on ships and automobiles in the mid-20th century. Hand-held mobile phones were introduced for personal service starting in 1973.
In decades their analog cellular system evolved into digital networks with greater capability and lower cost. Convergence has given most modern cell phones capabilities far beyond simple voice conversation, they may be able to record spoken messages and receive text messages and display photographs or video, play music or games, surf the Internet, do road navigation or immerse the user in virtual reality. Since 1999, the trend for mobile phones is smartphones that integrate all mobile communication and computing needs. A traditional landline telephone system known as plain old telephone service carries both control and audio signals on the same twisted pair of insulated wires, the telephone line; the control and signaling equipment consists of three components, the ringer, the hookswitch, a dial. The ringer, or beeper, light or other device, alerts the user to incoming calls; the hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call.
A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the 1960s dials used exclusively the rotary technology, replaced by dual-tone multi-frequency signaling with pushbutton telephones. A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interference and crosstalk better than a single wire or an untwisted pair; the strong outgoing speech signal from the microphone does not overpower the weaker incoming speaker signal with sidetone because a hybrid coil and other components compensate the imbalance. The junction box arrests lightning and adjusts the line's resistance to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths; the line voltages are negative compared to earth. Negative voltage attracts positive metal ions toward the wires.
The landline telephone contains a switchhook and an alerting device a ringer, that remains connected to the phone line whenever the phone is "on hook", other components which are connected when the phone is "off hook". The off-hook components include a transmitter, a receiver, other circuits for dialing and amplification. A calling party wishing to speak to another party will pick up the telephone's handset, thereby operating a lever which closes the switchhook, which powers the telephone by connecting the transmitter and related audio components to the line; the off-hook circuitry has a low resistance which causes a direct current, which comes down the line from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, sends a dial tone to indicate readiness. On a modern push-button telephone, the caller presses the number keys to send the telephone number of the called party; the keys control a tone generator circuit. A rotary-dial telephone uses pulse
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done. Conversion to an intermediate frequency is useful for several reasons; when several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors have higher gains so fewer stages are required. It's easier to make selective filters at lower fixed frequencies. There may be several such stages of intermediate frequency in a superheterodyne receiver. Intermediate frequencies are used for three general reasons.
At high frequencies, signal processing circuitry performs poorly. Active devices such as transistors cannot deliver much amplification. Ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in satellite dishes, the microwave downlink signal received by the dish is converted to a much lower IF at the dish, to allow a inexpensive coaxial cable to carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive waveguide. A second reason, in receivers that can be tuned to different frequencies, is to convert the various different frequencies of the stations to a common frequency for processing, it is difficult to build multistage amplifiers and detectors that can have all stages track in tuning different frequencies, but it is comparatively easy to build tunable oscillators.
Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, all processing after, done at the same fixed frequency, the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers. A more important advantage is; the bandwidth of a filter is proportional to its center frequency. In receivers like the TRF in which the filtering is done at the incoming RF frequency, as the receiver is tuned to higher frequencies its bandwidth increases; the main reason for using an intermediate frequency is to improve frequency selectivity. In communication circuits, a common task is to separate out or extract signals or components of a signal that are close together in frequency; this is called filtering. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance subcarrier from a TV signal.
With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency. FM and television broadcasting with their narrow channel widths, as well as more modern telecommunications services such as cell phones and cable television, would be impossible without using frequency conversion; the most used intermediate frequencies for broadcast receivers are around 455 kHz for AM receivers and 10.7 MHz for FM receivers. In special purpose receivers other frequencies can be used. A dual-conversion receiver may have two intermediate frequencies, a higher one to improve image rejection and a second, lower one, for desired selectivity. A first intermediate frequency may be higher than the input signal, so that all undesired responses can be filtered out by a fixed-tuned RF stage. In a digital receiver, the analog to digital converter operates at low sampling rates, so input RF must be mixed down to IF to be processed.
Intermediate frequency tends to be lower frequency range compared to the transmitted RF frequency. However, the choices for the IF are most dependent on the available components such as mixer, filters and others that can operate at lower frequency. There are other factors involved in deciding the IF frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters. Modern satellite television receivers use several intermediate frequencies; the 500 television channels of a typical system are transmitted from the satellite to subscribers in the Ku microwave band, in two subbands of 10.7 - 11.7 and 11.7 - 12.75 GHz. The downlink signal is received by a satellite dish. In the box at the focus of the dish, called a low-noise block downconverter, each block of frequencies is converted to the IF range of 950 - 2150 MHz by two fixed frequency local oscillators at 9.75 and 10.6 GHz. One of the two blocks is selected by a control signal from the set top box inside, which switches on one of the local oscillators.
This IF is carried into the building to the television receiver on a coaxial cable. At the cable company's set top box, the signal is converted to a lower IF of 480 MHz for filtering, by a variable frequency oscillator; this is sent through a 30 MHz bandpass filter, which selects the signal from one of the transponders on the satellite, which carries several channels. Further