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Encyclopedia > Modulation
Modulation techniques
Analog modulation
AM · SSB · FM · PM · QAM · SM
Digital modulation
OOK · FSK · ASK · PSK · QAM
MSK · CPM · PPM · TCM · OFDM
v  d  e
FHSS · DSSS

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for "MOdulate-DEModulate"). A demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. ... For other uses, see Modem (disambiguation). ...

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. You can say that modems play music for each other. If there are four alternative symbols (corresponding to 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 baud. Since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bit per second.

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The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a filter limits the frequency range to between 300 and 3400 Hz) or a limited radio frequency band. For other uses, see Digital (disambiguation). ... Channel, in communications (sometimes called communications channel), refers to the medium used to convey information from a sender (or transmitter) to a receiver. ... The public switched telephone network (PSTN) is the network of the worlds public circuit-switched telephone networks, in much the same way that the Internet is the network of the worlds public IP-based packet-switched networks. ...

The aim of analog modulation is to transfer an analog lowpass signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel. An analog or analogue signal is any time continuous signal where some time varying feature of the signal is a representation of some other time varying quantity. ... A low-pass filter passes low frequencies fairly well, but attenuates high frequencies. ... The frequency axis of this symbolic diagram would be logarithmically scaled. ... Channel, in communications (sometimes called communications channel), refers to the medium used to convey information from a sender (or transmitter) to a receiver. ...

Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate bandpass channels. Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal. ...

The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a lowpass channel, typically a non-filtered copper wire such as a serial bus or a wired local area network. In telecommunication, a line code is a code chosen for use within a communications system for transmission purposes. ... A low-pass filter passes low frequencies fairly well, but attenuates high frequencies. ... A serial bus is a computer bus that sends data bit by bit down one or a few wires. ... LAN redirects here. ...

The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband lowpass channel or, in some of the schemes, as a bit stream over another digital transmission system. A low-pass filter passes low frequencies fairly well, but attenuates high frequencies. ...

Analog modulation methods

In analog modulation, the modulation is applied continuously in response to the analog information signal. An analog or analogue signal is any time continuous signal where some time varying feature of the signal is a representation of some other time varying quantity. ...

Common analog modulation techniques are: Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. ... Double-sideband suppressed-carrier transmission (DSB-SC): transmission in which (a) frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and (b) the carrier level is reduced to the lowest practical level, ideally completely suppressed. ... Double-sideband reduced carrier transmission (DSB-RC): transmission in which (a) the frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier and (b) the carrier level is reduced for transmission at a fixed level below that which is provided to the modulator. ... Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ... Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ... â€œQAMâ€ redirects here. ... Phase modulation (PM) is the encoding of information into a carrier wave by variation of its phase in accordance with an input signal. ... In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency. ... Phase modulation (PM) is a form of modulation which represents information as variations in the instantaneous phase of a carrier wave. ...

Digital modulation methods

In digital modulation, an analog carrier signal is modulated by a digital bit stream. 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 a finite number of M alternative symbols (the modulation alphabet). For other uses, see Digital (disambiguation). ... Demodulation is the act of removing the modulation from an analog signal. ...

Fundamental digital modulation methods

These are the most fundamental digital modulation techniques:

• In the case of PSK, a finite number of phases are used.
• In the case of FSK, a finite number of frequencies are used.
• In the case of ASK, a finite number of amplitudes are used.
• In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.

In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and 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. Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). ... It has been suggested that this article or section be merged with audio frequency-shift keying. ... Amplitude-shift keying (ASK) is a form of modulation which represents digital data as variations in the amplitude of a carrier wave. ... â€œQAMâ€ redirects here. ...

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 the symbol that is represented by the particular phase. The binary numeral system, or base-2 number system, is a numeral system that represents numeric values using two symbols, usually 0 and 1. ... This article is about the unit of information. ...

If the alphabet consists of M = 2N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is fS symbols/second (or baud), the data rate is NfS bit/second. In digital communications, the symbol rate is the bit rate divided by the number of bits transmitted in each symbol. ... For the town in France, see Baud, Morbihan. ... For the town in France, see Baud, Morbihan. ...

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 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. A constellation diagram is a representation of a digital modulation scheme in the complex plane. ...

Modulator and detector principles of operation

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 is a representation of the real-valued modulated physical signal (the so called passband signal or RF signal). In mathematics, the imaginary unit (or sometimes the Latin or the Greek iota, see below) allows the real number system to be extended to the complex number system . ... Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0. ... Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0. ... Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0. ... Rough plot of Earths atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves. ...

These are the general steps used by the modulator to transmit data: For the musical use of modulation, see modulation (music). ...

1. Group the incoming data bits into codewords, one for each symbol that will be transmitted.
2. Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values.
3. Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum of the equivalent low pass signal, typically using digital signal processing.
4. Perform digital-to-analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using digital signal processing, DSP).
5. Generate a high-frequency sine wave carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine wave form with the I and Q signals, resulting in that the equivalent low pass signal is frequency shifted into a modulated passband signal or RF signal. Sometimes this is achieved using DSP technology, for example direct digital synthesis using a waveform table, instead of analog signal processing. In that case the above DAC step should be done after this step.
6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum

At the receiver side, the demodulator typically performs: In optics, Femtosecond pulse shaping refers to various techniques to modify the time profile of an ultrashort pulse from a laser. ... Digital signal processing (DSP) is the study of signals in a digital representation and the processing methods of these signals. ... Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0. ... Rough plot of Earths atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves. ... Direct Digital Synthesis (DDS) is an electronic method for digitally creating arbitrary waveforms and frequencies from a single, fixed source frequency. ... A demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. ...

1. Bandpass filtering.
2. Automatic gain control, AGC (to compensate for attenuation, for example fading).
3. Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator sinewave and cosine wave frequency (see the superheterodyne receiver principle).
4. Sampling and analog-to-digital conversion (ADC) (Sometimes before or instead of the above point, for example by means of undersampling).
5. Equalization filtering, for example a matched filter, compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoid intersymbol interference and symbol distortion.
6. Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal.
7. Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.
8. Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups);.
9. Parallel-to-serial conversion of the codewords into a bit stream.
10. Pass the resultant bit stream on for further processing such as removal of any error-correcting codes.

Non-coherent modulation methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier wave. In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred. The opposite is coherent modulation. Phase synchronization is the process by which two or more cyclic signals tend to oscillate with a repeating sequence of relative phase angles. ... This article or section does not adequately cite its references or sources. ... Asynchronous communication can be non-electronic or electronic. ...

List of common digital modulation techniques

The most common digital modulation techniques are:

MSK and GMSK are particular cases of continuous phase modulation (CPM). 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). Like the OQPSK modulator (i. ... Gaussian minimum shift keying or GMSK is a kind of continuous phase modulation. ... Continuous phase modulation (CPM) is a method for modulation of data commonly used in wireless modems. ... Continuous-phase frequency-shift keying (CPFSK) is a commonly-used variation of frequency-shift keying (FSK), which is itself a special case of analog frequency modulation. ...

OFDM is based on the idea of frequency division multiplexing (FDM), but is utilized as a digital modulation scheme. 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. OFDM 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 MC-OFDM schemes, allowing several users to share the same physical medium by giving different sub-carriers or spreading codes to different users. Orthogonal frequency division modulation (OFDM, also called orthogonal frequency division multiplexing) is a technique for the modulation of digital information onto an analog carrier electromagnetic (e. ... Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal. ... In communications, a channel access method is used to share a communications channel or physical communications medium between multiple users. ... Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme for OFDM systems. ... Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of the popular OFDM digital modulation scheme. ...

Of the two kinds of RF power amplifier, switching amplifiers (Class C 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 the QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers. It has been suggested that this article or section be merged into Electronic amplifier. ... Block diagram of a basic switching or PWM (Class-D) amplifier. ... A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. ... General Information Generically (as a multiplexing scheme), code division multiple access (CDMA) is any use of any form of spread spectrum by multiple transmitters to send to the same receiver on the same frequency channel at the same time without harmful interference. ...

Digital baseband modulation or line coding

The term digital baseband modulation is synonymous to line codes, which are methods to transfer a digital bit stream over an analog lowpass channel using a pulse train, i.e. a discrete number of signal levels, by directly modulating the voltage or current on a cable. Common examples are unipolar, non-return-to-zero (NRZ), Manchester and alternate mark inversion (AMI) coding. An example of coding a binary signal using rectangular pulse amplitude modulation with polar non-return-to-zero code An example of Bipolar encoding, or AMI. Encoding of 11011000100 in Manchester encoding An example of Differential Manchester encoding An example of Biphase mark code An example of MLT-3 encoding. ... A low-pass filter passes low frequencies fairly well, but attenuates high frequencies. ... The introduction of this article does not provide enough context for readers unfamiliar with the subject. ... Contrast with: return-to-zero. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Alternate Mark Inversion (AMI) is a line encoding technique (line code) for T1s which uses bipolar pulses to represent logical 1 values. ...

Pulse modulation methods

Pulse modulation schemes aim at transferring a narrowband analog signal over an analog lowpass channel as a two-level quantized signal, by modulating a pulse train. 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. They are not modulation schemes in 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. Generally, quantization is the state of being constrained to a set of discrete values, rather than varying continuously. ... Generally, quantization is the state of being constrained to a set of discrete values, rather than varying continuously. ... Discrete sampled signal Digital signal A discrete signal or discrete-time signal is a time series, perhaps a signal that has been sampled from a continuous-time signal. ... An example of coding a binary signal using rectangular pulse amplitude modulation with polar non-return-to-zero code An example of Bipolar encoding, or AMI. Encoding of 11011000100 in Manchester encoding An example of Differential Manchester encoding An example of Biphase mark code An example of MLT-3 encoding. ... In digital telecommunications, channel coding is a pre-transmission mapping applied to a digital signal or data file, usually designed to make error-correction possible. ... It has been suggested that this article or section be merged with Entropy encoding. ...

Direct-sequence spread spectrum (DSSS) is based on pulse-amplitude modulation. PCM redirects here. ... An example of PWM: the supply voltage (blue) modulated as a series of pulses results in a sine-like flux density waveform (red) in a magnetic circuit of electromagnetic actuator. ... Principle of PAM; (1) original Signal, (2) PAM-Signal, (a) Amplitude of Signal, (b) Time Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. ... Pulse-position modulation is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of possible time-shifts. ... Pulse-density modulation, or PDM, is a form of modulation used to represent an analog signal in the digital domain. ... In information theory, oversampling is the process of sampling a signal with a sampling frequency higher than the nyquist frequency. ... In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. ... Principle of PAM; (1) original Signal, (2) PAM-Signal, (a) Amplitude of Signal, (b) Time Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. ...

Miscellaneous modulation techniques

On-off keying (OOK) is a type of modulation that represents digital data as the presence or absence of a carrier wave. ... 1922 Chart of the Morse Code Letters and Numerals Morse code is a method for transmitting telegraphic information, using standardized sequences of short and long elements to represent the letters, numerals, punctuation and special characters of a message. ... It has been suggested that this article or section be merged with Radio waves. ... A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. ... Adaptive modulation is a transmission scheme in digital communications where the transmitter adapts its transmission mode in accordance with the channel. ... The Localizer station at Hanover/Langenhagen International Airport in Hanover, Germany. ... Results from FactBites:

 modulation, in communications. The Columbia Encyclopedia, Sixth Edition. 2001-07 (690 words) When the carrier is thus modulated, a fraction of the power is converted to sidebands extending above and below the carrier frequency by an amount equal to the highest modulating frequency. In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time. The modulation might vary the amplitude (PAM or pulse amplitude modulation), the duration (PDM or pulse duration modulation), or the presence of the pulses (PCM or pulse code modulation).
 Nikon MicroscopyU: Modulation Transfer Function (4055 words) Modulation of the output signal, the intensity of light waves forming an image of the specimen, corresponds to the formation of image contrast in microscopy. Modulation is typically less in the image than in the specimen and there is often a slight phase displacement of the image relative to the specimen. The modulation transfer function is also related to the point spread function, which is the image of a point source of light (commonly referred to as the Airy disk) from the specimen projected by the microscope objective onto the intermediate image plane.
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