Modulation (technology)

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The term modulation (from Latin modulatio = clock, rhythm) describes a process in communications engineering in which a useful signal to be transmitted (e.g. music, speech, data) changes (modulates) a so-called carrier . This enables high-frequency transmission of the low-frequency useful signal. In the carrier frequency range, the transmitted signal occupies a bandwidth that is dependent on the useful signal . The message is recovered at the receiving end by a demodulator .

The carrier signal itself is irrelevant to the transmitted message; it is only necessary to adapt to the physical properties of the transmission channel and can be suppressed (with certain types of modulation ).

analog amplitude modulation (AM) and frequency modulation (FM) of a low-frequency signal

Need for modulation, examples

  • If you wanted to broadcast voice or music directly, there would be only one “program” nationwide, because it would take up the entire low-frequency range . Every other program would occupy the same frequency range and interfere. Because of the low frequency, huge antennas would be required on both the transmit and receive sides.
  • By modulation, a significantly higher frequency carrier is changed and sent in the rhythm of the information. Other transmitters do the same with carriers of different frequencies. If these frequency ranges can be separated from each other in the receiver with the help of filters ( resonant circuits ), you can choose from different programs. The antennas are also becoming more manageable.
  • Several pieces of information can be modulated independently of one another on a carrier frequency in such a way that they can be separated on the receiver side. For example, with VHF radio, in addition to the "foreground music", other information such as RDS or the station identification is transmitted.
  • Simultaneous transmission of the two stereo channels (left and right) would also be impossible without modulation. In color television , the brightness and color information for the individual pixels is so cleverly modulated onto a carrier that they do not interfere with one another. A black and white television simply ignores the color modulation.
  • The multiple use of a cable, fiber optic or radio link ( directional radio , satellite television ) for several hundred simultaneous telephone calls or several television programs would be unthinkable without sophisticated modulation methods.
  • With DSL , you can send and receive digital Internet data simultaneously and independently of one another, and you can also make calls.
  • With a remote control , different commands such as changing channels, changing the volume or switching off can be transmitted wirelessly by modulating the infrared radiation .

meaning

Modulation has many advantages over direct transmission of the useful signal. Both analog and digital signals can be transmitted in this way. The modulation method can, however, be both analog and digital, regardless of the type of useful signal.

Devices that can demodulate both modulate as are often just as modem ( Mo dulator The designated odulator).

Closely related to digital modulation is line coding , which has the task of adapting a digital useful signal to a transmission channel , in this case a line, but does not convert it from baseband to a higher carrier frequency band.

Modulation is one of the most important methods in communications engineering. Their use enabled higher frequency ranges to be used in the transmission of messages and thus opened up new transmission paths (for example radio , radio relay , satellite radio ).

We encounter modulation everywhere in everyday life:

Recently, the importance of modulation in everyday life has become particularly clear. By developing new modulation methods such as ADSL , the data transmission rate over the existing telephone connection lines could be increased drastically.

history

Speech and music themselves are based on modulation. The movement of the tongue and lips alone produces snaps and smackles that cannot be heard far. The sound from the larynx carries much further and already serves the infant as a carrier, through whose continuous (analog) changes in volume and pitch he informs about his condition. Modulation is used deliberately in whistled languages .

Guglielmo Marconi achieved the first radio transmission over the English Channel in 1899 by turning a pop-spark transmitter on and off. Morse code could be transmitted with this digital modulation . A coherer in the receiver responded to the high frequency signal and the unsteady output of the transmitter was heard as a croak. At that time, the selectivity did not play a role as the number of channels was still quite limited. While Marconi was striving for higher transmission power, Reginald Fessenden improved the sensitivity of the receivers at the same time .

Until 1913, transmitters could only be switched on and off, which at best can be described as very rudimentary modulation. Modulation with a wide variety of signals, the subtle nuances of which also have to be transmitted, requires an oscillator circuit that initially generates a constant signal - this was only possible after the invention of the Meissner circuit . That was the beginning of broadcasting .

technical description

With modulation, the useful signal is converted into a different frequency range. Parameters such as the amplitude , frequency and / or phase of the carriers are varied by the useful signal. In the case of analog amplitude modulation, there is only a variation in the amplitude of the carriers. The signal, which is spectrally offset by this modulation, can then be transmitted via a transmission channel to the receiver, which uses demodulation to recover the original useful signal. The transmission can be wired via electrical cables and fiber optics or by means of antennas in the form of free space propagation (“ radio ”).

Time-continuous and time-discrete methods

Continuous-time modulation methods use a continuous signal such as a sine wave as a carrier . The information signal to be modulated does not have to represent the information continuously. It is essential that the modulated signal at the modulator output is continuous in time. The time-continuous methods are divided into value-continuous and value-discrete modulation methods. The continuous-value and continuous-time processes are imprecisely referred to as analog modulation , while the discrete-value and continuous-time processes are referred to as digital modulation .

Discrete-time modulation methods, on the other hand, only deliver a defined carrier signal at the output of the modulator at certain times. Typical representatives of this class are the pulse carrier processes. The time-discrete methods are also divided into value-continuous and value-discrete modulation methods. Pulse amplitude modulation (PAM) is a value-continuous and time-discrete modulation method. A typical representative of a value-discrete and time-discrete modulation method is pulse code modulation (PCM).

Linear and non-linear modulation methods

Modulation techniques can be divided into linear and non-linear modulation methods. A modulation method is linear if the mathematical function between the useful signal and the transmitted signal, which describes the modulation process, is a linear function . This is the case, for example, with amplitude modulation , which represents a multiplication in the time domain.

In the case of non-linear modulations, on the other hand, which have a non-linear function as the relationship between the useful signal and the transmitted signal, the mapping is dependent on the instantaneous values ​​of the useful signal. Their analysis is associated with greater effort, often no closed solutions are available and approximation methods such as for estimating the necessary bandwidth of the transmission signal must be used. An example of non-linear modulation is frequency modulation , in which the link between the useful signal and the transmitted signal is formed by angle functions such as the cosine function .

Modulation and multiplex technology

The term modulation is closely linked to the term multiplex technology . Multiplex technology deals with the transmission of several useful signals in parallel and ideally without mutual interference over a shared channel, for example a cable or a radio frequency range. The practical implementation of the various multiplex techniques such as time division multiplex , frequency division multiplex or code division multiplex takes place through the use of suitable modulation methods.

Physical modulation

Modulation methods are not only used in the frequency ranges up to a few 100 GHz that are directly accessible to electronic circuit technology, but there are also modulators that are based directly on material-specific, physical principles. In these cases, the modulator or demodulator is no longer formed by an electronic circuit made up of individual components and their interaction in a circuit. The essential properties of these modulators are significantly higher carrier frequencies, which can extend into the visible range of the electromagnetic spectrum (light) and beyond (ultraviolet). The disadvantage here is the small number of possible variations, since the material properties, in contrast to the modulators constructed as electronic circuits, cannot be changed so easily. In most cases, only simple amplitude modulations are therefore used.

A modulator in this context would be, for example, a light-emitting diode , a laser or, for low useful signal frequencies, an incandescent lamp whose brightness is controlled. This amplitude modulation, since the brightness is varied, takes place in the internal structure through physical processes and the transmission signal can be in the range of optical frequencies and above. These modulators are used, for example, to control fiber optics or optocouplers .

Modulation method

Analog modulation methods

Analog useful signals are, for example, speech, music or image signals. An essential property of analog modulation techniques is the continuity of the modulation both in the time domain and in the value domain. I.e. Analog modulations process the useful signal continuously, there is no digitization of the transmitted signal values. Analog modulated signals are also called Analog Spectrum Modulation (ASM for short).

The analog modulation methods can be divided into two main groups: amplitude modulation and angle modulation . All other analog modulation techniques can be derived from these two modulation techniques.

With amplitude modulation, the information in the useful signal is continuously mapped in the amplitude of the transmitted signal. There are special modifications of the amplitude modulation, such as the amplitude modulation with suppressed carrier , the single sideband modulation (SSB) or the residual sideband modulation . The amplitude modulation is used, for example, in analog broadcasting on medium wave and analog television technology . Single sideband modulation is technically more complex, but uses the frequency band more efficiently and is used, for example, in the field of amateur radio .

In the group of angle modulations, which mainly include frequency modulation (FM) and phase modulation (PM), the useful signal is mapped in the phase angle of the carrier signal. This leads to a change in the carrier frequency or the phase position of the carrier signal. These techniques are used, for example, in analogue VHF radio .

The combination of amplitude and angle modulation is also called vector modulation . In this case, the information of the useful signal is accommodated both in the amplitude and in the phase angle of the carrier oscillations. In the analog domain, the best known application is expected to transfer the color information in the PAL - and NTSC - color image (CVBS) signal to be. The color saturation determines the amplitude and the color type ( hue ) determines the phase angle of a carrier signal, the so-called color subcarrier .

Digital modulation methods

Digital modulation methods transmit symbols that are clearly defined for both sender and receiver. This is called Digital Spectrum Modulation ( DSM for short ). The time course of these symbols or the superposition of the emitted symbols form a continuous course. The shape of the symbols must be chosen so that their spectrum remains within the prescribed bandwidth of the transmission channel. Analog signals such as speech or music must therefore be digitized before digital modulation . These digital samples are then mapped onto the symbols to be transmitted . These modulation methods are therefore implemented with the help of digital signal processing .

For didactic reasons, also because it is easier to represent graphically, the symbols are often shown in a rectangular shape, i.e. without rounding. However, this easily leads to an incorrect idea of ​​the problem.

The digital modulations only deliver valid values ​​at certain times, the so-called sampling times. This is referred to as time-discrete. The time interval between the sampling times is called the symbol rate . In the time intervals between two sampling times, the information of the transmission signal is undefined. This is why so-called clock recovery plays a central role in digital demodulation : the receiver or demodulator must be able to use suitable methods to identify the times at which valid information is available.

With digital modulations, only a finite number of different values ​​can be transmitted. This is called discrete value. With a suitable choice of the discrete-value transmission symbols, minor deviations, which occur, for example, due to transmission errors, can be recognized and compensated for. This is the reason why digital modulation methods are usually more immune to interference than analog methods. The interference of some digital modulation methods can be assessed, for example by means of an eye diagram or in the form of the representation of transmission symbols in the complex plane .

More precisely, the digital modulations are time and value discrete modulation methods, based on the transmitted information signal. The time curve of the modulation signal, on the other hand, is continuous in time and value. However, the not quite fitting term of digital modulation has already established itself in the literature. However , under certain conditions, channel coding can also be viewed as a form of digital modulation. The term “coded modulation” is used for this in the literature.

Some of the digital modulation techniques have counterparts or are derived from analog modulation techniques. However, there are also a large number of digital modulations that have no direct analog equivalents, such as pulse width modulation , which is a special digital angle modulation and can also be used for temporal sampling (discrete time sampling) of an analog signal.

Digital modulation method with one carrier

One of the simplest digital modulations is digital amplitude modulation or amplitude shift keying (ASK), in which the amplitude of the transmission signal is switched in discrete steps depending on the user data sequence. If there are only two transmission symbols, a choice is made between two different amplitude values, one of which can also be zero. However, several amplitude values ​​(levels) can also be selected.

The digital angle modulations cover a large field and in their simplest form are also known as Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). The frequency or the phase angle of the carrier signal is switched in discrete steps. A special form of FSK is Gaussian Minimum Shift Keying (GMSK), in which the modulation index is exactly 0.5. Typical applications of these modulations are the first telephone modems from the 1980s, which were able to transmit up to 1200 bits per second over a telephone line using FSK in the ITU-T standards V.21 or V.23. Analog fax machines that are common today also use this modulation method.

Digital phase modulations such as QPSK only transmit the user data in the phase position of the carrier. These modulations are also used primarily in the telecommunications sector, for example in digital cellular networks such as GSM .

In the digital sector, combinations of amplitude and angle modulations are also used. The information (user data sequence) is accommodated in both the amplitude and the phase position of the carrier. A common modulation method is quadrature amplitude modulation , abbreviated as QAM, 16QAM, 32QAM, 64QAM, etc. The numbers indicate the discrete data points (transmission symbols) in the complex level: the more transmission symbols there are, the more bits can be transmitted per symbol, the more difficult it is but it is also important to distinguish between the individual symbols on the recipient's side. For this reason, methods with few transmission symbols are used in robust transmissions and in the case of more disturbed transmissions.

Multi-carrier digital modulations

In the case of digital modulations, it is also possible to divide the useful data stream onto several different carriers. This creates an additional possibility of adapting to the properties of the transmission channel as optimally as possible: If, for example, certain carriers cannot be used for data transmission due to interference, this only reduces the overall data throughput , since the other carriers can still be used. A typical method is Discrete Multitone (DMT), which is used in the area of ADSL . This also includes Orthogonal Frequency Division Multiplex (OFDM) and Coded Orthogonal Frequency Division Multiplex (COFDM), which are used in the field of terrestrial digital television DVB-T .

Digital modulations such as 16QAM are used as narrow-banded as possible on the individual carriers. Due to the large number of carriers - these can be up to a few 10,000 carriers - the transmission properties of the transmission channel can be addressed very selectively. This means that up to a few tens of kbit of useful data are transmitted in parallel in just one clock step. Due to significant technological advancements in the field of high-performance and inexpensive digital signal processors and application-specific integrated circuits , these complex modulation methods have become widespread in the consumer electronics sector in recent years.

Coded modulation

With coded modulation, the channel coding that is separate from other modulation methods , which offers protection against transmission errors by adding redundancy, is inseparably combined with a digital modulation method. The additional code gain of the channel coding that can be achieved by combining is then not based on the minimum Hamming distance , as in the separate method , but on the Euclidean distance of the transmission symbols of the modulation method , which are spanned in the complex plane.

An example of coded modulation is trellis code modulation (TCM), which is based on a convolutional code in combination with a modulation method such as QAM. Related Block Code Modulation (BCM) uses a block code instead of the convolutional code . Both methods can be split up (partitioned) into partial codings, from which the group of multilevel code modulation methods (MLCM) are derived. Methods such as Binary Offset Carrier (BOC), some of which are still the subject of current research, also belong to the area of ​​coded modulation.

Special modulations

Spread spectrum modulations

These include various types of spread spectrum modulations such as Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). These modulations are the basis of code division multiplexing methods and extremely expand the transmission spectrum compared to the useful data spectrum. The reception by means of correlation is characterized by special code sequences, which mostly have properties similar to chance and which differentiate the individual channels from one another.

This means that transmissions are also possible whose transmission signal is below the background noise level , so that the existence of a transmission cannot even be recognized. As with all other modulations, the message transmission can also be encrypted if required. A detection of whether a transmission is taking place is only possible if the corresponding spread spectrum code sequences are known and by means of correlation . Applications of these techniques are therefore mainly found in the military sector for message transmission or in the field of espionage for eavesdropping devices that are very difficult to detect . In recent years, these techniques have also been used in civil areas, such as in the GPS or Galileo navigation systems and in third-generation mobile communications using CDMA and in steganography applications , in order to be able to detect copyright infringements using electronic watermarks, for example on pieces of music or video films.

Pulse modulations

With these modulations, a continuous analog signal is converted into a time-discrete signal sequence consisting of individual pulses which, as in the cases of pulse width modulation (PWM), pulse amplitude modulation (PAM), pulse frequency modulation (PFM) and pulse phase modulation (PPM), are continuous in amplitude. There are also amplitude-discrete versions of these methods, with PAM the discrete-value version is then referred to as pulse code modulation (PCM). The PWM occurs both value discrete and value continuous. PWM applications are, for example, the power control of electric motors or in the audio sector for class D amplifiers . Pulse-step modulation (PSM) is a method used for amplitude-modulated transmission output stages with higher power .

With pulse code modulation (PCM), a pulse comb , a periodic sequence of short individual pulses, is multiplied by the input signal in order to obtain the output signal ("transmit signal"). The individual output values ​​are then quantized, i.e. converted into a finite number of levels. This modulation is used in some analog-to-digital converters , especially when a signal sequence is to be continuously obtained, as is the case with the digitization of voice and music signals.

See also

literature

  • Karl Dirk Kammeyer: message transmission . Teubner, Stuttgart 1996, ISBN 3-519-16142-7 .
  • Martin Bossert: Channel Coding . Teubner, Stuttgart 1998, ISBN 3-519-16143-5 .
  • Carsten Roppel: Basics of digital communication technology . Hanser, Leipzig 2006, ISBN 3-446-22857-8 .
  • Roger L. Freeman: Radio System Design for Telecommunications . 3. Edition. IEEE, The Institute of Electrical and Electronics Engineers, New York 2007, ISBN 978-0-471-75713-9 .

Web links

Commons : Modulation  - collection of images, videos and audio files
Technical modulation methods
Overview: modulator


Analog modulation methods

AM  | FM  | PM  | VM  | SSB  | SSBSC  | DSBSC 

Digital modulation methods

ASK  | FSK  | GFSK  | PSK  | QPSK  | QAM  | APSK  | OFDM  | DMT  | TCM  | VSB 

Pulse modulation method

PWM  | PAM  | PFM  | PPM (1)  | PPM (2)  | PCM 

Frequency spreading modulation methods

FHSS  | DSSS  | THSS  | CSS