Modulation

In electronics and telecommunications, accentuation is the action of capricious one or added backdrop of a high-frequency alternate waveform, alleged the carrier signal, with a modulating arresting which about contains advice to be transmitted. This is done in a agnate appearance to a artist modulating a accent (a alternate waveform) from a agreeable apparatus by capricious its volume, timing and pitch. The three key ambit of a alternate waveform are its amplitude ("volume"), its appearance ("timing") and its abundance ("pitch"). Any of these backdrop can be adapted in accordance with a low abundance arresting to access the articulate signal. About a high-frequency sinusoid waveform is acclimated as carrier signal, but a aboveboard beachcomber beating alternation may aswell be used.

In telecommunications, accentuation is the action of carrying a bulletin signal, for archetype a agenda bit beck or an analog audio signal, central addition arresting that can be physically transmitted. Accentuation of a sine waveform is acclimated to transform a baseband bulletin arresting into a passband signal, for archetype low-frequency audio arresting into a radio-frequency arresting (RF signal). In radio communications, cable TV systems or the accessible switched blast arrangement for instance, electrical signals can alone be transferred over a bound passband abundance spectrum, with specific (non-zero) lower and high blow frequencies. Modulating a sine-wave carrier makes it accessible to accumulate the abundance agreeable of the transferred arresting as abutting as accessible to the centre abundance (typically the carrier frequency) of the passband.

A accessory that performs accentuation is accepted as a modulator and a accessory that performs the changed operation of accentuation is accepted as a demodulator (sometimes detector or demod). A accessory that can do both operations is a modem (from "modulator–demodulator").

Aim

The aim of agenda accentuation is to alteration a agenda bit beck over an analog bandpass channel, for archetype over the accessible switched blast arrangement (where a bandpass clarify banned the abundance ambit to amid 300 and 3400 Hz), or over a bound radio abundance band.

The aim of analog accentuation is to alteration an analog baseband (or lowpass) signal, for archetype an audio arresting or TV signal, over an analog bandpass approach at a altered frequency, for archetype over a bound radio abundance bandage or a cable TV arrangement channel.

Analog and agenda accentuation facilitate abundance analysis multiplexing (FDM), breadth several low canyon advice signals are transferred accompanying over the aforementioned aggregate concrete medium, application abstracted passband channels (several altered carrier frequencies).

The aim of agenda baseband accentuation methods, aswell accepted as band coding, is to alteration a agenda bit beck over a baseband channel, about a non-filtered chestnut wire such as a consecutive bus or a active bounded breadth network.

The aim of beating accentuation methods is to alteration a narrowband analog signal, for archetype a buzz alarm over a wideband baseband approach or, in some of the schemes, as a bit beck over addition agenda manual system.

In music synthesizers, accentuation may be acclimated to synthesise waveforms with an all-encompassing association spectrum application a baby amount of oscillators. In this case the carrier abundance is about in the aforementioned adjustment or abundant lower than the modulating waveform. See for archetype abundance accentuation amalgam or ring accentuation synthesis.

Analog modulation methods

In analog modulation, the accentuation is activated continuously in acknowledgment to the analog advice signal. Common analog accentuation techniques are:1

Amplitude accentuation (AM) (here the amplitude of the carrier arresting is assorted in accordance to the direct amplitude of the modulating signal)

Double-sideband accentuation (DSB)

Double-sideband accentuation with carrier (DSB-WC) (used on the AM radio broadcasting band)

Double-sideband suppressed-carrier manual (DSB-SC)

Double-sideband bargain carrier manual (DSB-RC)

Single-sideband accentuation (SSB, or SSB-AM)

SSB with carrier (SSB-WC)

SSB suppressed carrier accentuation (SSB-SC)

Vestigial sideband accentuation (VSB, or VSB-AM)

Quadrature amplitude accentuation (QAM)

Angle modulation

Abundance accentuation (FM) (here the abundance of the carrier arresting is assorted in accordance to the direct amplitude of the modulating signal)

Appearance accentuation (PM) (here the appearance about-face of the carrier arresting is assorted in accordance to the direct amplitude of the modulating signal)

Digital modulation methods

In agenda modulation, an analog carrier arresting is articulate by a detached signal. Agenda accentuation methods can be advised as digital-to-analog conversion, and the agnate demodulation or apprehension as analog-to-digital conversion. The changes in the carrier arresting are alleged from a bound amount of M addition symbols (the accentuation alphabet).

Schematic of 4 baud (8 bit/s) abstracts hotlink absolute arbitraily alleged values.

A simple example: A blast band is advised for appointment aural sounds, for archetype tones, and not agenda $.25 (zeros and ones). Computers may about acquaint over a blast band by agency of modems, which are apery the agenda $.25 by tones, alleged symbols. If there are four addition symbols (corresponding to a agreeable apparatus that can accomplish four altered tones, one at a time), the aboriginal attribute may represent the bit arrangement 00, the additional 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the attribute amount is 1000 symbols/second, or baud. Since anniversary accent (i.e., symbol) represents a bulletin consisting of two agenda $.25 in this example, the bit amount is alert the attribute rate, i.e. 2000 $.25 per second. This is agnate to the address acclimated by dialup modems as against to DSL modems.

According to one analogue of agenda signal, the articulate arresting is a agenda signal, and according to addition definition, the accentuation is a anatomy of digital-to-analog conversion. A lot of textbooks would accede agenda accentuation schemes as a anatomy of agenda transmission, alike to abstracts transmission; actual few would accede it as analog transmission.

edit Axiological agenda accentuation methods

The a lot of axiological agenda accentuation techniques are based on keying:

In the case of PSK (phase-shift keying), a bound amount of phases are used.

In the case of FSK (frequency-shift keying), a bound amount of frequencies are used.

In the case of ASK (amplitude-shift keying), a bound amount of amplitudes are used.

In the case of QAM (quadrature amplitude modulation), a bound amount of at atomic two phases, and at atomic two amplitudes are used.

In QAM, an inphase arresting (the I signal, for archetype a cosine waveform) and a quadrature appearance arresting (the Q signal, for archetype a sine wave) are amplitude articulate with a bound amount of amplitudes, and summed. It can be apparent as a two-channel system, anniversary approach application ASK. The consistent arresting is agnate to a aggregate of PSK and ASK.

In all of the aloft methods, anniversary of these phases, frequencies or amplitudes are assigned a different arrangement of bifold bits. Usually, anniversary phase, abundance or amplitude encodes an according amount of bits. This amount of $.25 comprises the attribute that is represented by the accurate phase, abundance or amplitude.

If the alphabet consists of M = 2N addition symbols, anniversary attribute represents a bulletin consisting of N bits. If the attribute amount (also accepted as the baud rate) is fS symbols/second (or baud), the abstracts amount is NfS bit/second.

For example, with an alphabet consisting of 16 addition symbols, anniversary attribute represents 4 bits. Thus, the abstracts amount is four times the baud rate.

In the case of PSK, ASK or QAM, area the carrier abundance of the articulate arresting is constant, the accentuation alphabet is generally calmly represented on a afterlife diagram, assuming the amplitude of the I arresting at the x-axis, and the amplitude of the Q arresting at the y-axis, for anniversary symbol.

Modulator and detector principles of operation

PSK and ASK, and sometimes aswell FSK, are generally generated and detected application the assumption of QAM. The I and Q signals can be accumulated into a complex-valued arresting I+jQ (where j is the abstract unit). The consistent so alleged agnate lowpass arresting or agnate baseband arresting is a complex-valued representation of the real-valued articulate concrete arresting (the so alleged passband arresting or RF signal).

These are the accepted accomplish acclimated by the modulator to address data:

Group the admission abstracts $.25 into codewords, one for anniversary attribute that will be transmitted.

Map the codewords to attributes, for archetype amplitudes of the I and Q signals (the agnate low canyon signal), or abundance or appearance values.

Adapt beating abstraction or some added clarification to absolute the bandwidth and anatomy the spectrum of the agnate low canyon signal, about application agenda arresting processing.

Accomplish digital-to-analog about-face (DAC) of the I and Q signals (since today all of the aloft is commonly accomplished application agenda arresting processing, DSP).

Generate a high-frequency sine beachcomber carrier waveform, and conceivably aswell a cosine quadrature component. Carry out the modulation, for archetype by adding the sine and cosine beachcomber anatomy with the I and Q signals, consistent in that the agnate low canyon arresting is abundance confused into a articulate passband arresting or RF signal. Sometimes this is accomplished application DSP technology, for archetype absolute agenda amalgam application a waveform table, instead of analog arresting processing. In that case the aloft DAC footfall should be done afterwards this step.

Amplification and analog bandpass clarification to abstain harmonic baloney and alternate spectrum

At the receiver side, the demodulator about performs:

Bandpass filtering.

Automatic accretion control, AGC (to atone for attenuation, for archetype fading).

Abundance alive of the RF arresting to the agnate baseband I and Q signals, or to an average abundance (IF) signal, by adding the RF arresting with a bounded oscillator sinewave and cosine beachcomber abundance (see the superheterodyne receiver principle).

Sampling and analog-to-digital about-face (ADC) (Sometimes afore or instead of the aloft point, for archetype by agency of undersampling).

Equalization filtering, for archetype a akin filter, advantage for multipath propagation, time spreading, appearance baloney and abundance careful fading, to abstain intersymbol arrest and attribute distortion.

Detection of the amplitudes of the I and Q signals, or the abundance or appearance of the IF signal.

Quantization of the amplitudes, frequencies or phases to the abutting accustomed attribute values.

Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups).

Parallel-to-serial about-face of the codewords into a bit stream.

Canyon the resultant bit beck on for added processing such as abatement of any error-correcting codes.

As is accepted to all agenda advice systems, the architecture of both the modulator and demodulator accept to be done simultaneously. Agenda accentuation schemes are accessible because the transmitter-receiver brace accept above-mentioned ability of how abstracts is encoded and represented in the communications system. In all agenda advice systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they accomplish changed operations.

Non-coherent accentuation methods do not crave a receiver advertence alarm arresting that is appearance synchronized with the sender carrier wave. In this case, accentuation symbols (rather than bits, characters, or abstracts packets) are asynchronously transferred. The adverse is articular modulation.

List of common digital modulation techniques

The a lot of accepted agenda accentuation techniques are:

Phase-shift keying (PSK):

Binary PSK (BPSK), application M=2 symbols

Quadrature PSK (QPSK), application M=4 symbols

8PSK, application M=8 symbols

16PSK, application M=16 symbols

Differential PSK (DPSK)

Differential QPSK (DQPSK)

Offset QPSK (OQPSK)

π/4–QPSK

Frequency-shift keying (FSK):

Audio frequency-shift keying (AFSK)

Multi-frequency about-face keying (M-ary FSK or MFSK)

Dual-tone multi-frequency (DTMF)

Continuous-phase frequency-shift keying (CPFSK)

Amplitude-shift keying (ASK)

On-off keying (OOK), the a lot of accepted ASK form

M-ary abeyant sideband modulation, for archetype 8VSB

Quadrature amplitude accentuation (QAM) - a aggregate of PSK and ASK:

Polar accentuation like QAM a aggregate of PSK and ASK.citation needed

Connected appearance accentuation (CPM) methods:

Minimum-shift keying (MSK)

Gaussian minimum-shift keying (GMSK)

Erect frequency-division multiplexing (OFDM) modulation:

detached multitone (DMT) - including adaptive accentuation and bit-loading.

Wavelet modulation

Filigree coded accentuation (TCM), aswell accepted as filigree modulation

Spread-spectrum techniques:

Direct-sequence advance spectrum (DSSS)

Chirp advance spectrum (CSS) according to IEEE 802.15.4a CSS uses pseudo-stochastic coding

Frequency-hopping advance spectrum (FHSS) applies a appropriate arrangement for approach release

MSK and GMSK are accurate cases of connected appearance modulation. Indeed, MSK is a accurate case of the sub-family of CPM accepted as continuous-phase frequency-shift keying (CPFSK) which is authentic by a ellipsoidal abundance beating (i.e. a linearly accretion appearance pulse) of one symbol-time continuance (total acknowledgment signaling).

OFDM is based on the abstraction of frequency-division multiplexing (FDM), but is activated as a agenda accentuation scheme. The bit beck is breach into several alongside abstracts streams, anniversary transferred over its own sub-carrier application some accepted agenda accentuation scheme. The articulate sub-carriers are summed to anatomy an OFDM signal. OFDM is advised as a accentuation address rather than a circuitous technique, back it transfers one bit beck over one advice approach application one arrangement of alleged OFDM symbols. OFDM can be continued to multi-user approach admission adjustment in the erect frequency-division assorted admission (OFDMA) and multi-carrier cipher analysis assorted admission (MC-CDMA) schemes, acceptance several users to allotment the aforementioned concrete average by giving altered sub-carriers or overextension codes to altered users.

Of the two kinds of RF ability amplifier, switching amplifiers (Class C amplifiers) amount beneath and use beneath array ability than beeline amplifiers of the aforementioned achievement power. However, they alone plan with almost constant-amplitude-modulation signals such as bend accentuation (FSK or PSK) and CDMA, but not with QAM and OFDM. Nevertheless, even admitting switching amplifiers are absolutely clashing for accustomed QAM constellations, generally the QAM accentuation assumption are acclimated to drive switching amplifiers with these FM and added waveforms, and sometimes QAM demodulators are acclimated to accept the signals put out by these switching amplifiers.

Digital baseband modulation or line coding

The appellation agenda baseband accentuation (or agenda baseband transmission) is alike to band codes. These are methods to alteration a agenda bit beck over an analog baseband approach (a.k.a. lowpass channel) application a beating train, i.e. a detached amount of arresting levels, by anon modulating the voltage or accepted on a cable. Common examples are unipolar, non-return-to-zero (NRZ), Manchester and alternating mark antagonism (AMI) codings.2

Pulse modulation methods

Pulse accentuation schemes aim at appointment a narrowband analog arresting over an analog baseband approach as a two-level arresting by modulating a beating wave. Some beating accentuation schemes aswell acquiesce the narrowband analog arresting to be transferred as a agenda arresting (i.e. as a quantized discrete-time signal) with a anchored bit rate, which can be transferred over an basal agenda manual system, for archetype some band code. These are not accentuation schemes in the accepted faculty back they are not approach coding schemes, but should be advised as antecedent coding schemes, and in some cases analog-to-digital about-face techniques.

Analog-over-analog methods:

Pulse-amplitude accentuation (PAM)

Pulse-width accentuation (PWM)

Pulse-position accentuation (PPM)

Analog-over-digital methods:

Pulse-code accentuation (PCM)

Differential PCM (DPCM)

Adaptive DPCM (ADPCM)

Basin accentuation (DM or Δ-modulation)

Delta-sigma accentuation (∑Δ)

Continuously capricious abruptness basin accentuation (CVSDM), aswell alleged Adaptive-delta accentuation (ADM)

Pulse-density accentuation (PDM)

Miscellaneous modulation techniques

The use of on-off keying to address Morse cipher at radio frequencies is accepted as connected beachcomber (CW) operation.

Adaptive modulation

Space accentuation A adjustment whereby signals are articulate aural airspace, such as that acclimated in Instrument landing systems.