Faced with new concepts in everyday life, many try to find answers to their questions. It is for this that it is necessary to describe any phenomena. One of them is such a thing as modulation. It will be discussed further.
General Description
Modulation is the process of changing one or a whole set of high-frequency oscillation parameters in accordance with the law of low-frequency information message. The result of this is the transfer of the spectrum of the control signal to the high frequency region, since effective broadcasting into space requires that all transceivers operate at different frequencies without interrupting each other. Thanks to this process, information oscillations are placed on a carrier known a priori. The control signal contains the transmitted information. High-frequency oscillation takes on the role of a carrier of information, due to which it acquires the status of a carrier. The control signal contains the transmitted data. There are different types of modulation, which depend on which waveform is used: rectangular, triangular, or some other. With a discrete signal, it is customary to talk about manipulation. So,modulation is a process involving oscillations, so it can be frequency, amplitude, phase, etc.
Varieties
Now we can consider what kinds of this phenomenon exist. In essence, modulation is the process by which a low frequency wave is carried by a high frequency wave. The following types are most often used: frequency, amplitude and phase. With frequency modulation, a change in frequency occurs, with amplitude modulation, the amplitude, and with phase modulation, the phase. There are also mixed species. Pulse modulation and modification are separate types. In this case, the parameters of the high-frequency oscillation change discretely.
Amplitude modulation
In systems with this kind of change, the amplitude of the carrier wave changes at a high frequency with the help of a modulating wave. When analyzing the frequencies at the output, not only the input frequencies are revealed, but also their sum and difference. In this case, if the modulation is a complex wave, such as speech signals consisting of many frequencies, then the sum and difference of frequencies will require two bands, one below the carrier and one above. They are called lateral: upper and lower. The first is a copy of the original audio signal shifted by a certain frequency. The lower band is a copy of the original signal that has been inverted, that is, the original high frequencies are the lower frequencies in the lower side.
The lower sideband is a mirror image of the upper sideband relative to the carrier frequency. A system using amplitude modulation,transmitting the carrier and both sides is called two-way. The carrier contains no useful information, so it can be removed, but in any case, the signal bandwidth will be twice the original. The narrowing of the band is achieved by replacing not only the carrier, but also one of the side ones, since they contain one information. This type is known as SSB modulation with suppressed carrier.
Demodulation
This process requires the modulated signal to be mixed with a carrier of the same frequency as that emitted by the modulator. After that, the original signal is obtained as a separate frequency or frequency band, and then filtered from other signals. Sometimes the generation of the carrier for demodulation takes place in situ, and it does not always coincide with the carrier frequency on the modulator itself. Due to the small difference between frequencies, mismatches appear, which is typical for telephone circuits.
Pulse modulation
This uses a digital baseband signal, meaning it allows more than one bit per baud to be encoded by encoding a binary data signal into a multi-level signal. Bits of binary signals are sometimes split into pairs. For a pair of bits, four combinations can be used, with each pair being represented by one of four amplitude levels. Such an encoded signal is characterized by the fact that the modulation baud rate is half that of the original data signal, so it can be used toamplitude modulation in the usual way. She found her application in radio communications.
Frequency modulation
Systems with this modulation assume that the frequency of the carrier will change according to the shape of the modulating signal. This type is superior to the amplitude type in terms of resistance to certain influences available on the telephone network, so it should be used at low speeds where there is no need to attract a large frequency band.
Phase-amplitude modulation
To increase the number of bits per baud, you can combine phase and amplitude modulation.
One of the modern methods of amplitude-phase modulation can be called the one that is based on the transmission of several carriers. For example, in some application, 48 carriers are used, separated by a bandwidth of 45 Hz. By combining AM and PM, up to 32 discrete states are allocated per carrier for each individual baud period, so that 5 bits per baud can be carried. It turns out that this whole set allows you to transfer 240 bits per baud. When operating at 9600 bps, the modulation rate requires only 40 baud. Such a low figure is quite tolerant of the amplitude and phase jumps inherent in the telephone network.
PCM
This type is usually considered as a system for broadcasting analog signals, such as voice in digital form. This modulation technique is not used in modems. Here is the gating of the analog signal withat twice the highest frequency of the analog signal component. When using such systems on telephone networks, strobe occurs 8000 times per second. Each sample is a voltage level encoded with a seven-bit code. To best represent spoken language, logarithmic coding is used. Seven bits, together with the eighth, which indicates the presence of a signal, form an octet.
Modulation and detection are required to restore the message signal, that is, the reverse process. In this case, the signal is converted in a non-linear way. Nonlinear elements enrich the output signal spectrum with new spectrum components, and filters are used to isolate low-frequency components. Modulation and detection can be carried out using vacuum diodes, transistors, semiconductor diodes as non-linear elements. Traditionally, point semiconductor diodes are used, since planar input capacitance is noticeably larger.
Modern Views
Digital modulation provides much more information capacity and ensures compatibility with a variety of digital data services. In addition, it increases the security of information, improves the quality of communication systems, and speeds up access to them.
There are a number of limitations that designers of any systems face: the allowable power and bandwidth, the given noise level of communication systems. The number of users is increasing every daycommunication systems, and the demand for them is also growing, which requires an increase in the radio resource. Digital modulation differs markedly from analog in that the carrier in it transmits large amounts of information.
Difficulty of use
The developers of digital radio communication systems face such a main task - to find a compromise between the bandwidth of data transmission and the complexity of the system in technical terms. To do this, it is appropriate to use different modulation methods to get the desired result. Radio communication can also be organized using the simplest transmitter and receiver circuits, but for such communication a frequency spectrum proportional to the number of users will be used. More complex receivers and transmitters require less bandwidth to broadcast the same amount of information. In order to move to spectrally efficient transmission methods, it is necessary to complicate the equipment accordingly. This problem does not depend on the type of communication.
Alternative options
Pulse width modulation is characterized by the fact that its carrier signal is a sequence of pulses, while the pulse frequency is constant. The changes concern only the duration of each pulse according to the modulating signal.
Pulse-width modulation is different from frequency-phase modulation. The latter involves the modulation of the signal in the form of a sinusoid. It is characterized by constant amplitude and variable frequency or phase. Pulse signals can also be modulated in frequency. May be durationpulses is fixed, and their frequency is in some average value, but their instantaneous value will vary depending on the modulating signals.
Conclusions
Simple modulations can be used, with only one parameter changing according to the modulating information. The combined modulation scheme that is used in modern communication equipment is when both the amplitude and the phase of the carrier change simultaneously. In modern systems, several subcarriers can be used, each of which uses a certain type of modulation. In this case, we are talking about signal modulation schemes. This term is also used for complex multi-level views, when an additional description of the characteristics is required for comprehensive information.
Modern communication systems use the most efficient modulation types to minimize bandwidth to free up frequency space for other types of signals. The quality of communication only benefits from this, but the complexity of the equipment in this case is very high. Ultimately, the modulation frequency gives a result that is visible to the end user only in terms of ease of use of technical means.
