Modulation has defined as the process of combining an input signal m (t) and a frequency carrier FC to produce an S (t) signal whose bandwidth is (usually) centred around FC. In the case of digital data, the justification of the modulation must be clear: it is necessary when there is only the possibility of analogue transmission, to convert the digital data into analogues. However, when the data is analogue, the justification is not so apparent. After all, voice signals are transmitted through telephone lines using their original spectrum (this is called baseband transmission). There are two main reasons:
To carry out a more effective transmission, a higher frequency may require for unguided media; it is practically impossible to transmit baseband signals since the size of the antennas would have to be several kilometres in diameter.
Modulation allows multiplication by frequency division, a very important technique.
To convert analogue signals to digital and vice versa, several types of modulation are required.
Analog to digital conversion
Sometimes it is necessary to digitise an analogue signal. For example, to send the human voice over a long distance, it is necessary to digitise them since digital signals are less vulnerable to noise. It is called analogue to digital conversion or digitisation of an analogue signal. To carry it out, it is necessary to reduce the number of values, potentially infinite in an analogue message, so that they can represent as a digital flow with minimal loss of information. There are several methods for converting from analogue to digital. The figure shows an analogue to digital converter, called a codec (encoder and decoder).
Pulse amplitude modulation (PAM)
This technique takes an analogue signal, the sample and generates a series of pulses based on the sampling results. The term sampling means measuring the amplitude of the signal at equal intervals.
The method used in PAM is more useful for other engineering series than for data transmission. However, PAM is the foundation of a significant analogue to digital conversion method called pulse coding modulation (PCM).
In PAM, the original signal is shown at equal intervals of time, as shown in the figure. PAM uses a technique called sampling and retention. At a particular time, the signal level is read and briefly maintained.
The reason that PAM not used for data transmission is that, although it translates the original wave into a series of pulses, these pulses still do not have any amplitude (they are still an analogue signal, not digital). To convert them into a digital signal, it is necessary to code them using pulse coding modulation (PCM).
Pulse amplitude modulation (PAM) has some applications, but it not used in itself for data transmission. However, it is the first step for another popular conversion method called pulse coding modulation (PCM).
Pulse coding modulation (PCM)
PCM modifies the pulses created by PAM to create a fully digital signal. To do this, PCM first quantifies the PAM pulses. Quantification is the method of assigning integral values within a specific range of sampled instances — the result of the quantification presented in the figure.
The figure shows a simple method for assigning sign and magnitude values to quantified samples — each value translated in a binary equivalent of seven bits. The eighth bit indicates the sign.
The digital binaries are then transformed into a digital signal using one of the digital to digital coding techniques. The figure shows the results of the pulse code modulation of the original signal finally encoded within the unipolar signal. Only the three sampled values represented.
The PCM is composed of four different processes: PAM, quantification, binary quantification and digital to digital coding. This sampling method is used for digitising voice on the transmission lines T of the North American telecommunications system.
Digital to analog conversion
The conversion from digital to analogue, or analogue digital modulation, is the process of changing one of the characteristics of an analogue base signal into information based on a digital signal (zeros and ones). For example, when data is transmitted from one computer to another through a public telephone network, the original data is digital, but, because telephone cables carry similar signals, it is necessary to convert that data. The digital data must modulate on an analogue signal that has been manipulated to appear as two different values corresponding to 0 and 1 binary. The figure shows the relationship between the digital information, the digital to analogue modulation hardware and the resulting analogue signal value.
Of the many existing mechanisms for analogue digital modulation, only the most useful for data transmission will treat.
As seen in the previous topics, a sine wave defined by three characteristics: amplitude, frequency and phase. When any of these characteristics are changed, the second version of this wave created. If it said then that the original wave represents the binary 1, the variation might represent the binary 0, or vice versa. Therefore, changing the appearance of a pure electrical signal forward and backward can be used to represent digital data. Any of the three characteristics mentioned can be altered in this way, giving us at least three mechanisms to modulate digital data in analogue signals:
Amplitude shift modulation (ASK), Frequency shift modulation (FSK) and Phase shift modulation (PSK). Besides, there is a fourth mechanism (and better) that combines changes in phase and amplitude and is called quadrature amplitude modulation (QAM). QAM is the most efficient of these options and is the mechanism used in all modern modems.
Amplitude shift modulation (ASK)
In this modulation, the power of the carrier signal is changed to represent binary 1 and 0. Both the frequency and the phase remain constant while the amplitude changes. What voltage represents 1, and what voltage represents 0 is left for system designers. The duration of the bit is the period that defines a bit. The peak signal amplitude during each bit duration is constant, and its value depends on the bit (0 or 1). The transmission speed using ASK limited by the physical characteristics of the transmission medium. The figure shows a conceptual view of the ASK.
Unfortunately, ASK transmission is highly susceptible to noise interference. Recall the term noise refers to the intentional voltages introduced into a line by various phenomena such as heat or electromagnetic induction created by other sources.
These unintentional voltages combine with the signal and change their amplitude. A 0 can change to a 1 and a 1 to a 0. You can already see that noise is especially problematic for the ASK, which relies solely on the amplitude for recognition. Noise usually affects amplitude; therefore, ASK is the modulation method most affected by noise.
Frequency shift modulation (FSK)
In this type of modulation, the frequency of the carrier signal changes to represent binary 1 and 0. The frequency of the signal during the bit duration is constant, and its value depends on a bit (0 or 1): both the peak amplitude and the phase remain constant. The figure shows a conceptual view of the FSK.
FSK avoids most of the noise problems of the ASK. Because the receiving device is looking for specific frequency changes in a certain number of periods, ignore the voltage peaks. The factors that limit the FSK are the physical capabilities of the carrier.
Phase Shift Modulation (PSK)
In PSK modulation, the carrier phase changes to represent the binary 1 or 0. Both peak amplitude and frequency remain constant while the phase changes. For example, if you start with a phase of 0 degrees to represent a binary 0, you can change the phase to 180 degrees to send a binary 1. The phase of the signal during the duration of each bit is constant, and its value depends on the bit (0 or 1). The figure gives a conceptual view of PSK.
The above method is often called 2-PSK, 0 binary PSK because two different phases (0 and 180 degrees) used. The figure clarifies this point by showing the relationship between the phase and binary value. A second diagram, called a constellation or a phase-state diagram, shows the same relationship illustrating only the phases.
PSK is not susceptible to noise degradation that affects ASK or FSK band limitations. It means that small variations in the signal can be detected reliably in the receiver. In addition to using only two variations of a signal, each representing one bit, you can use four variations and let each phase shift represent two bits.
Quadrature amplitude modulation (QAM)
PSK limited by the ability of teams to distinguish small differences in phase. This factor limits your potential bit rate.
So far, only the three characteristics of a sine wave have been altered one at a time, but what happens if two altered. Bandwidth limitations make combinations of FSK with other changes virtually useless. However, why not combine ASK and PSK. In that case, we could have x variations in phase and variations in amplitude, giving us x times and possible variations and the corresponding number of bits per variation. Quadrature amplitude modulation (QAM) does just that. The term quadrature is derived from the restrictions necessary for minimum performance and is related to trigonometry.
Quadrature amplitude modulation (QAM) means combining ASK and PSK so that there is maximum contrast between each bit, debit, tibit, quadbit, etc. The figure shows two possible combinations, 4-QAM and 8-QAM.