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83-423: This carrier wave usually has a much higher frequency than the message signal does. This is because it is impractical to transmit signals with low frequencies. Generally, to receive a radio wave one needs a radio antenna with length that is one-fourth of wavelength. For low frequency radio waves, wavelength is on the scale of kilometers and building such a large antenna is not practical. In radio communication ,

166-420: A signal transition . Information can be transmitted either during the given time interval, or encoded as the presence or absence of a change in the received signal. Significant conditions are recognized by an appropriate device called a receiver, demodulator, or decoder. The decoder translates the actual signal received into its intended logical value such as a binary digit (0 or 1), an alphabetic character,

249-537: A 64QAM modem, M = 64. In a line code, these may be M different voltage levels. By taking information per pulse N in bit/pulse to be the base-2- logarithm of the number of distinct messages M that could be sent, Hartley constructed a measure of the gross bit rate R as: where f s is the baud rate in symbols/second or pulses/second. (See Hartley's law ). Modulation is used in passband filtered channels such as telephone lines, radio channels and other frequency division multiplex (FDM) channels. In

332-539: A baseband channel such as a telegraph line, a serial cable or a Local Area Network twisted pair cable, data is transferred using line codes; i.e., pulses rather than sinewave tones. In this case, the baud rate is synonymous to the pulse rate in pulses/second. The maximum baud rate or pulse rate for a base band channel is called the Nyquist rate , and is double the bandwidth (double the cut-off frequency). The simplest digital communication links (such as individual wires on

415-441: A constellation diagram , showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol. PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal I + jQ (where j is the imaginary unit ). The resulting so called equivalent lowpass signal or equivalent baseband signal

498-421: A line code , symbol rate , modulation rate or baud rate is the number of symbol changes, waveform changes, or signaling events across the transmission medium per unit of time . The symbol rate is measured in baud (Bd) or symbols per second . In the case of a line code, the symbol rate is the pulse rate in pulses per second. Each symbol can represent or convey one or several bits of data. The symbol rate

581-441: A specific range of frequencies . The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz (20 kHz), though the high frequency limit usually reduces with age. Other species have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz. In many media, such as air, the speed of sound is approximately independent of frequency, so

664-415: A synchronous data transmission system, the tone can only be changed from one frequency to another at regular and well-defined intervals. The presence of one particular frequency during one of these intervals constitutes a symbol. (The concept of symbols does not apply to asynchronous data transmission systems.) In a modulated system, the term modulation rate may be used synonymously with symbol rate. If

747-513: A challenging topic in telecommunication systems and computer engineering. Such systems have many civil and military applications. Moreover, blind recognition of modulation type is an important problem in commercial systems, especially in software-defined radio . Usually in such systems, there are some extra information for system configuration, but considering blind approaches in intelligent receivers, we can reduce information overload and increase transmission performance. Obviously, with no knowledge of

830-534: A cosine waveform) and a quadrature phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite number of amplitudes and then summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK. In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits . Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises

913-547: A data rate of double the symbol rate. In a more complex scheme such as 16-QAM , four bits of data are transmitted in each symbol, resulting in a bit rate of four times the symbol rate. Although it is common to choose the number of symbols to be a power of 2 and send an integer number of bits per baud, this is not required. Line codes such as bipolar encoding and MLT-3 use three carrier states to encode one bit per baud while maintaining DC balance . The 4B3T line code uses three 3-ary modulated bits to transmit four data bits,

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996-452: A digital modulation method provided by a modem , each symbol is typically a sine wave tone with a certain frequency, amplitude and phase. Symbol rate, baud rate, is the number of transmitted tones per second. One symbol can carry one or several bits of information. In voiceband modems for the telephone network, it is common for one symbol to carry up to 7 bits. Conveying more than one bit per symbol or bit per pulse has advantages. It reduces

1079-512: A discrete alphabet to be transmitted. This alphabet can consist of a set of real or complex numbers , or sequences, like oscillations of different frequencies, so-called frequency-shift keying (FSK) modulation. A more complicated digital modulation method that employs multiple carriers, orthogonal frequency-division multiplexing (OFDM), is used in WiFi networks, digital radio stations and digital cable television transmission. In analog modulation,

1162-412: A finite number of M alternative symbols (the modulation alphabet ). A simple example: A telephone line is designed for transferring audible sounds, for example, tones, and not digital bits (zeros and ones). Computers may, however, communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to

1245-497: A five-bit code for telegraphs which was standardized internationally and is commonly called Baudot code . More than two voltage levels are used in advanced techniques such as FDDI and 100/1,000 Mbit /s Ethernet LANs, and others, to achieve high data rates. 1,000 Mbit/s Ethernet LAN cables use four wire pairs in full duplex (250 Mbit/s per pair in both directions simultaneously), and many bits per symbol to encode their data payloads. In digital television transmission

1328-411: A fractional error of Δ f f = 1 2 f T m {\textstyle {\frac {\Delta f}{f}}={\frac {1}{2fT_{\text{m}}}}} where T m {\displaystyle T_{\text{m}}} is the timing interval and f {\displaystyle f} is the measured frequency. This error decreases with frequency, so it

1411-478: A heart beats at a frequency of 120 times per minute (2 hertz), the period—the time interval between beats—is half a second (60 seconds divided by 120). For cyclical phenomena such as oscillations , waves , or for examples of simple harmonic motion , the term frequency is defined as the number of cycles or repetitions per unit of time. The conventional symbol for frequency is f or ν (the Greek letter nu )

1494-416: A known frequency near the unknown frequency is mixed with the unknown frequency in a nonlinear mixing device such as a diode . This creates a heterodyne or "beat" signal at the difference between the two frequencies. If the two signals are close together in frequency the heterodyne is low enough to be measured by a frequency counter. This process only measures the difference between the unknown frequency and

1577-469: A motherboard or the RS-232 serial port/COM port) typically have a symbol rate equal to the gross bit rate. Common communication links such as 10 Mbit/s Ethernet ( 10BASE-T ), USB , and FireWire typically have a data bit rate slightly lower than the baud rate, due to the overhead of extra non-data symbols used for self-synchronizing code and error detection . J. M. Emile Baudot (1845–1903) worked out

1660-412: A musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or 1000 baud . Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit rate

1743-429: A narrowband analog signal over an analog baseband channel as a two-level signal by modulating a pulse wave . Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e., as a quantized discrete-time signal ) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example, some line code . These are not modulation schemes in

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1826-594: A proper class. Another recent approach is based on feature extraction. Digital baseband modulation changes the characteristics of a baseband signal, i.e., one without a carrier at a higher frequency. This can be used as equivalent signal to be later frequency-converted to a carrier frequency, or for direct communication in baseband. The latter methods both involve relatively simple line codes , as often used in local buses, and complicated baseband signalling schemes such as used in DSL . Pulse modulation schemes aim at transferring

1909-415: A rate of 1.3 3 bits per baud. Modulating a carrier increases the frequency range, or bandwidth , it occupies. Transmission channels are generally limited in the bandwidth they can carry. The bandwidth depends on the symbol (modulation) rate (not directly on the bit rate ). As the bit rate is the product of the symbol rate and the number of bits encoded in each symbol, it is clearly advantageous to increase

1992-402: A repeating event is accomplished by counting the number of times that event occurs within a specific time period, then dividing the count by the period. For example, if 71 events occur within 15 seconds the frequency is: f = 71 15 s ≈ 4.73 Hz . {\displaystyle f={\frac {71}{15\,{\text{s}}}}\approx 4.73\,{\text{Hz}}.} If

2075-565: A single cable to customers. Since each carrier occupies a different frequency, the channels do not interfere with each other. At the destination end, the carrier signal is demodulated to extract the information bearing modulation signal. A modulator is a device or circuit that performs modulation. A demodulator (sometimes detector ) is a circuit that performs demodulation , the inverse of modulation. A modem (from mod ulator– dem odulator), used in bidirectional communication, can perform both operations. The lower frequency band occupied by

2158-425: Is synchronous modulation . The most common digital modulation techniques are: MSK and GMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one-symbol-time duration (total response signaling). OFDM

2241-551: Is 1/1,000 second = 1 millisecond. The term baud rate has sometimes incorrectly been used to mean bit rate, since these rates are the same in old modems as well as in the simplest digital communication links using only one bit per symbol, such that binary "0" is represented by one symbol, and binary "1" by another symbol. In more advanced modems and data transmission techniques, a symbol may have more than two states, so it may represent more than one binary digit (a binary digit always represents one of exactly two states). For this reason,

2324-444: Is a complex-valued representation of the real-valued modulated physical signal (the so-called passband signal or RF signal ). These are the general steps used by the modulator to transmit data: At the receiver side, the demodulator typically performs: As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because

2407-435: Is also used. The period T is the time taken to complete one cycle of an oscillation or rotation. The frequency and the period are related by the equation f = 1 T . {\displaystyle f={\frac {1}{T}}.} The term temporal frequency is used to emphasise that the frequency is characterised by the number of occurrences of a repeating event per unit time. The SI unit of frequency

2490-425: Is based on the idea of frequency-division multiplexing (FDM), but the multiplexed streams are all parts of a single original stream. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The modulated sub-carriers are summed to form an OFDM signal. This dividing and recombining help with handling channel impairments. OFDM

2573-469: Is called chip rate , which also is the pulse rate of the equivalent base band signal. However, in spread spectrum systems, the term symbol may also be used at a higher layer and refer to one information bit, or a block of information bits that are modulated using for example conventional QAM modulation, before the CDMA spreading code is applied. Using the latter definition, the symbol rate is equal to or lower than

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2656-495: Is called a radio wave . Likewise, an electromagnetic wave with a frequency higher than 8 × 10  Hz will also be invisible to the human eye; such waves are called ultraviolet (UV) radiation. Even higher-frequency waves are called X-rays , and higher still are gamma rays . All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called electromagnetic radiation . They all travel through vacuum at

2739-407: Is considered as a modulation technique rather than a multiplex technique since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the orthogonal frequency-division multiple access (OFDMA) and multi-carrier code-division multiple access (MC-CDMA) schemes, allowing several users to share

2822-405: Is expressed with the unit reciprocal second (s ) or, in the case of radioactivity, with the unit becquerel . It is defined as a rate , f = N /Δ t , involving the number of entities counted or the number of events happened ( N ) during a given time duration (Δ t ); it is a physical quantity of type temporal rate . Symbol rate In a digitally modulated signal or

2905-411: Is generally a problem at low frequencies where the number of counts N is small. An old method of measuring the frequency of rotating or vibrating objects is to use a stroboscope . This is an intense repetitively flashing light ( strobe light ) whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down. When

2988-407: Is one of the signal 's parameters chosen to represent information . A significant condition could be an electric current (voltage, or power level), an optical power level, a phase value, or a particular frequency or wavelength . The duration of a significant condition is the time interval between successive significant instants. A change from one significant condition to another is called

3071-402: Is red light, 800 THz ( 8 × 10  Hz ) is violet light, and between these (in the range 400–800 THz) are all the other colors of the visible spectrum . An electromagnetic wave with a frequency less than 4 × 10  Hz will be invisible to the human eye; such waves are called infrared (IR) radiation. At even lower frequency, the wave is called a microwave , and at still lower frequencies it

3154-476: Is related to angular frequency (symbol ω , with SI unit radian per second) by a factor of 2 π . The period (symbol T ) is the interval of time between events, so the period is the reciprocal of the frequency: T = 1/ f . Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals ( sound ), radio waves , and light . For example, if

3237-419: Is related to the gross bit rate , expressed in bits per second . A symbol may be described as either a pulse in digital baseband transmission or a tone in passband transmission using modems. A symbol is a waveform, a state or a significant condition of the communication channel that persists , for a fixed period of time. A sending device places symbols on the channel at a fixed and known symbol rate, and

3320-493: Is termed rotational frequency , is revolution per minute , abbreviated r/min or rpm. 60 rpm is equivalent to one hertz. As a matter of convenience, longer and slower waves, such as ocean surface waves , are more typically described by wave period rather than frequency. Short and fast waves, like audio and radio, are usually described by their frequency. Some commonly used conversions are listed below: For periodic waves in nondispersive media (that is, media in which

3403-445: Is that it is impractical to know the reference phase of the transmitter.) By increasing the number of states that the carrier signal can take, the number of bits encoded in each symbol can be greater than one. The bit rate can then be greater than the symbol rate. For example, a differential phase-shift keying system might allow four possible jumps in phase between symbols. Then two bits could be encoded at each symbol interval, achieving

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3486-617: Is the hertz (Hz), named after the German physicist Heinrich Hertz by the International Electrotechnical Commission in 1930. It was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, officially replacing the previous name, cycle per second (cps). The SI unit for the period, as for all measurements of time, is the second . A traditional unit of frequency used with rotating mechanical devices, where it

3569-400: Is the speed of light in vacuum, and this expression becomes f = c λ . {\displaystyle f={\frac {c}{\lambda }}.} When monochromatic waves travel from one medium to another, their frequency remains the same—only their wavelength and speed change. Measurement of frequency can be done in the following ways: Calculating the frequency of

3652-558: Is three bits per second. In the Navy, more than one flag pattern and arm can be used at once, so the combinations of these produce many symbols, each conveying several bits, a higher data rate. If N bits are conveyed per symbol, and the gross bit rate is R , inclusive of channel coding overhead, the symbol rate can be calculated as: In that case M = 2 different symbols are used. In a modem, these may be sinewave tones with unique combinations of amplitude, phase and/or frequency. For example, in

3735-552: Is twice the symbol rate, i.e. 2000 bits per second. According to one definition of digital signal , the modulated signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion . Most textbooks would consider digital modulation schemes as a form of digital transmission , synonymous to data transmission; very few would consider it as analog transmission . The most fundamental digital modulation techniques are based on keying : In QAM, an in-phase signal (or I, with one example being

3818-426: Is usually expressed as a fraction; i.e., 1/2, 3/4, etc. In the case of 3/4 FEC, for every 3 bits of data, you are sending out 4 bits, one of which is for error correction. Example: then In digital terrestrial television ( DVB-T , DVB-H and similar techniques) OFDM modulation is used; i.e., multi-carrier modulation. The above symbol rate should then be divided by the number of OFDM sub-carriers in view to achieve

3901-464: The frequency of the carrier wave is varied by the modulation signal. These were the earliest types of modulation, and are used to transmit an audio signal representing sound in AM and FM radio broadcasting . More recent systems use digital modulation , which impresses a digital signal consisting of a sequence of binary digits (bits), a bitstream , on the carrier, by means of mapping bits to elements from

3984-411: The symbol that is represented by the particular phase, frequency or amplitude. If the alphabet consists of M = 2 N {\displaystyle M=2^{N}} alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate ) is f S {\displaystyle f_{S}} symbols/second (or baud ),

4067-528: The OFDM symbol rate. See the OFDM system comparison table for further numerical details. Some communication links (such as GPS transmissions, CDMA cell phones, and other spread spectrum links) have a symbol rate much higher than the data rate (they transmit many symbols called chips per data bit). Representing one bit by a chip sequence of many symbols overcomes co-channel interference from other transmitters sharing

4150-517: The QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers. Automatic digital modulation recognition in intelligent communication systems is one of the most important issues in software-defined radio and cognitive radio . According to incremental expanse of intelligent receivers, automatic modulation recognition becomes

4233-486: The alternating current in household electrical outlets is 60 Hz (between the tones B ♭ and B; that is, a minor third above the European frequency). The frequency of the ' hum ' in an audio recording can show in which of these general regions the recording was made. Aperiodic frequency is the rate of incidence or occurrence of non- cyclic phenomena, including random processes such as radioactive decay . It

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4316-503: The baud rate value will often be lower than the gross bit rate. Example of use and misuse of "baud rate" : It is correct to write "the baud rate of my COM port is 9,600" if we mean that the bit rate is 9,600 bit/s, since there is one bit per symbol in this case. It is not correct to write "the baud rate of Ethernet is 100 megabaud " or "the baud rate of my modem is 56,000" if we mean bit rate. See below for more details on these techniques. The difference between baud (or signaling rate) and

4399-422: The bit error rate. An optimal symbol set design takes into account channel bandwidth, desired information rate, noise characteristics of the channel and the receiver, and receiver and decoder complexity. Many data transmission systems operate by the modulation of a carrier signal . For example, in frequency-shift keying (FSK) , the frequency of a tone is varied among a small, fixed set of possible values. In

4482-413: The bit rate over a fixed bandwidth (and therefore a fixed maximum symbol rate), leading to increasing bits per symbol. For example, ITU-T V.29 specifies 4 bits per symbol, at a symbol rate of 2,400 baud, giving an effective bit rate of 9,600 bits per second. The history of spread spectrum goes in the opposite direction, leading to fewer and fewer data bits per symbol in order to spread the bandwidth. In

4565-441: The bit rate. The disadvantage of conveying many bits per symbol is that the receiver has to distinguish many signal levels or symbols from each other, which may be difficult and cause bit errors in case of a poor phone line that suffers from low signal-to-noise ratio. In that case, a modem or network adapter may automatically choose a slower and more robust modulation scheme or line code, using fewer bits per symbol, in view to reduce

4648-406: The carrier signal has only two states, then only one bit of data (i.e., a 0 or 1) can be transmitted in each symbol. The bit rate is in this case equal to the symbol rate. For example, a binary FSK system would allow the carrier to have one of two frequencies, one representing a 0 and the other a 1. A more practical scheme is differential binary phase-shift keying , in which the carrier remains at

4731-443: The case of GPS, we have a data rate of 50 bit/s and a symbol rate of 1.023 Mchips/s. If each chip is considered a symbol, each symbol contains far less than one bit (50 bit/s / 1,023 ksymbols/s ≈ 0.000,05 bits/symbol). The complete collection of M possible symbols over a particular channel is called a M-ary modulation scheme. Most modulation schemes transmit some integer number of bits per symbol b , requiring

4814-409: The complete collection to contain M = 2 different symbols. Most popular modulation schemes can be described by showing each point on a constellation diagram , although a few modulation schemes (such as MFSK , DTMF , pulse-position modulation , spread spectrum modulation) require a different description. In telecommunication , concerning the modulation of a carrier , a significant condition

4897-487: The conventional sense since they are not channel coding schemes, but should be considered as source coding schemes, and in some cases analog-to-digital conversion techniques. Frequency Frequency (symbol f ), most often measured in hertz (symbol: Hz), is the number of occurrences of a repeating event per unit of time . It is also occasionally referred to as temporal frequency for clarity and to distinguish it from spatial frequency . Ordinary frequency

4980-521: The data rate (or bit rate) is like a man using a single semaphore flag who can move his arm to a new position once each second, so his signaling rate (baud) is one symbol per second. The flag can be held in one of eight distinct positions: Straight up, 45° left, 90° left, 135° left, straight down (which is the rest state, where he is sending no signal), 135° right, 90° right, and 45° right. Each signal (symbol) carries three bits of information. It takes three binary digits to encode eight states. The data rate

5063-407: The data rate is N f S {\displaystyle Nf_{S}} bit/second. For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate. In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on

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5146-421: The frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integer multiple of

5229-416: The latter if the former is fixed. However, for each additional bit encoded in a symbol, the constellation of symbols (the number of states of the carrier) doubles in size. This makes the states less distinct from one another which in turn makes it more difficult for the receiver to detect the symbol correctly in the presence of disturbances on the channel. The history of modems is the attempt at increasing

5312-416: The modulated carrier is transmitted through space as a radio wave to a radio receiver . Another purpose of modulation is to transmit multiple channels of information through a single communication medium , using frequency-division multiplexing (FDM). For example, in cable television (which uses FDM), many carrier signals, each modulated with a different television channel , are transported through

5395-426: The modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include: In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from

5478-404: The modulation signal is called the baseband , while the higher frequency band occupied by the modulated carrier is called the passband . In analog modulation , an analog modulation signal is "impressed" on the carrier. Examples are amplitude modulation (AM) in which the amplitude (strength) of the carrier wave is varied by the modulation signal, and frequency modulation (FM) in which

5561-545: The number of counts is not very large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error into the count of between zero and one count, so on average half a count. This is called gating error and causes an average error in the calculated frequency of Δ f = 1 2 T m {\textstyle \Delta f={\frac {1}{2T_{\text{m}}}}} , or

5644-470: The receiving device has the job of detecting the sequence of symbols in order to reconstruct the transmitted data. There may be a direct correspondence between a symbol and a small unit of data . For example, each symbol may encode one or several binary digits (bits). The data may also be represented by the transitions between symbols, or even by a sequence of many symbols. The symbol duration time , also known as unit interval , can be directly measured as

5727-410: The reference frequency. To convert higher frequencies, several stages of heterodyning can be used. Current research is extending this method to infrared and light frequencies ( optical heterodyne detection ). Visible light is an electromagnetic wave , consisting of oscillating electric and magnetic fields traveling through space. The frequency of the wave determines its color: 400 THz ( 4 × 10 Hz)

5810-560: The rotation rate of a shaft, mechanical vibrations, or sound waves , can be converted to a repetitive electronic signal by transducers and the signal applied to a frequency counter. As of 2018, frequency counters can cover the range up to about 100 GHz. This represents the limit of direct counting methods; frequencies above this must be measured by indirect methods. Above the range of frequency counters, frequencies of electromagnetic signals are often measured indirectly utilizing heterodyning ( frequency conversion ). A reference signal of

5893-445: The same frequency channel, including radio jamming , and is common in military radio and cell phones . Despite the fact that using more bandwidth to carry the same bit rate gives low channel spectral efficiency in (bit/s)/Hz, it allows many simultaneous users, which results in high system spectral efficiency in (bit/s)/Hz per unit of area. In these systems, the symbol rate of the physically transmitted high-frequency signal rate

5976-430: The same frequency, but can be in one of two phases. During each symbol, the phase either remains the same, encoding a 0, or jumps by 180°, encoding a 1. Again, only one bit of data (i.e., a 0 or 1) is transmitted by each symbol. This is an example of data being encoded in the transitions between symbols (the change in phase), rather than the symbols themselves (the actual phase). (The reason for this in phase-shift keying

6059-541: The same physical medium by giving different sub-carriers or spreading codes to different users. Of the two kinds of RF power amplifier , switching amplifiers ( Class D amplifiers ) cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA , but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often

6142-414: The same speed (the speed of light), giving them wavelengths inversely proportional to their frequencies. c = f λ , {\displaystyle \displaystyle c=f\lambda ,} where c is the speed of light ( c in vacuum or less in other media), f is the frequency and λ is the wavelength. In dispersive media , such as glass, the speed depends somewhat on frequency, so

6225-461: The strobing frequency will also appear stationary. Higher frequencies are usually measured with a frequency counter . This is an electronic instrument which measures the frequency of an applied repetitive electronic signal and displays the result in hertz on a digital display . It uses digital logic to count the number of cycles during a time interval established by a precision quartz time base. Cyclic processes that are not electrical, such as

6308-427: The symbol rate calculation is: The 204 is the number of bytes in a packet including the 16 trailing Reed–Solomon error correction bytes. The 188 is the number of data bytes (187 bytes) plus the leading packet sync byte (0x47). The bits per symbol is the (modulation's power of 2) × (Forward Error Correction). So for example, in 64-QAM modulation 64 = 2 so the bits per symbol is 6. The Forward Error Correction (FEC)

6391-455: The time between transitions by looking into an eye diagram of an oscilloscope . The symbol duration time T s can be calculated as: where f s is the symbol rate. For example, a baud rate of 1 kBd = 1,000 Bd is synonymous to a symbol rate of 1,000 symbols per second. In case of a modem, this corresponds to 1,000 tones per second, and in case of a line code, this corresponds to 1,000 pulses per second. The symbol duration time

6474-406: The time required to send a given quantity of data over a limited bandwidth . A high spectral efficiency in (bit/s)/Hz can be achieved; i.e., a high bit rate in bit/s although the bandwidth in hertz may be low. The maximum baud rate for a passband for common modulation methods such as QAM , PSK and OFDM is approximately equal to the passband bandwidth. Voiceband modem examples: In case of

6557-495: The transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information, etc., blind identification of the modulation is made fairly difficult. This becomes even more challenging in real-world scenarios with multipath fading, frequency-selective and time-varying channels. There are two main approaches to automatic modulation recognition. The first approach uses likelihood-based methods to assign an input signal to

6640-549: The transmitter-receiver pair has prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations. Asynchronous methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier signal . In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred. The opposite

6723-473: The wave speed is independent of frequency), frequency has an inverse relationship to the wavelength , λ ( lambda ). Even in dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ . {\displaystyle f={\frac {v}{\lambda }}.} In the special case of electromagnetic waves in vacuum , then v = c , where c

6806-419: The wavelength is not quite inversely proportional to frequency. Sound propagates as mechanical vibration waves of pressure and displacement, in air or other substances. In general, frequency components of a sound determine its "color", its timbre . When speaking about the frequency (in singular) of a sound, it means the property that most determines its pitch . The frequencies an ear can hear are limited to

6889-455: The wavelength of the sound waves (distance between repetitions) is approximately inversely proportional to frequency. In Europe , Africa , Australia , southern South America , most of Asia , and Russia , the frequency of the alternating current in household electrical outlets is 50 Hz (close to the tone G), whereas in North America and northern South America, the frequency of

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