Analog and Digital Communication Systems

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The signal can be an image, audio or video message that must be transmitted from an end to another. Here, the figure clearly shows that it consists of an input transducer with a modulator and an output transducer with demodulator between a communication channel. The analog signal containing information is first provided to the input transducer where the analog message signal is first converted into an equivalent electrical signal. This signal is then provided to the modulator circuit where modulation of the applied signal is performed in order to have long distance and proper signal transmission.

Communication Systems/Analog vs. Digital

The modulated signal is then transmitted through the communication channel where noise components also get added in it. On reaching the receiver end, the transmitted signal undergoes demodulation by which originally transmitted signal is recovered from the modulated one. This signal is then fed to the output transducer where the actually transmitted analog signal is received.

A way of communication in which the message is transmitted in the form of digital pulses or non-continuous form is known as digital communication. Here, the information is sent in digital format. Here, the above figure clearly represents a digital communication system comprising of information source along with source encoder and a modulator.

At the other end of the communication channel, a source decoder and demodulator is present. A discrete information source generates a message signal that does not show continuous variation with time. This signal is provided to the source encoder. This unit provides a digitized message signal i.

The binary sequence is then fed to the modulator, where the modulation of the digital signal takes place. This modulated digital signal is then sent via communication channel. During communication, the signal undergoes some distortion due to interference caused by additional noise components. At the receiver, the digital signal is first demodulated in order to get the originally transmitted digital signal. This digitized signal is then fed to the source decoder which converts the digital signal into user understandable format. It finds applications majorly in television broadcasting and radio broadcasting.

Also, at the time of telephonic conversion, analog communication is taken into consideration.

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It is used in computers and satellite communication system. As digital communication supports data encryption thus is widely used for military purpose and long-distance communication. So, from the above discussion, we can conclude that an analog signal is nothing but a composition of continuous values. However, a digital signal is a composition of discrete pulses.

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Both find applications in different fields according to the properties exhibited by them. Your email address will not be published. Skip to content. The result is a double-sideband signal, centered on the carrier frequency, with bandwidth twice that of the bandlimited PAM signal. As with the PAM case, components in the signal chain must be selected to maintain amplitude integrity within the band centered around the carrier frequency, f C. In this case, analog components may be evaluated based on their linearity, THD total harmonic distortion or SFDR spurious free dynamic range performance at f C.

For multi-bit symbols with numerous distinct amplitude levels, noise may be an important consideration in component specification.

It is important to note that the transmitted signal bandwidth actually spreads over a larger bandwidth than just the span between f 0 and f 1 , because the speed of transitioning between the two frequencies generates additional spectral content. To simplify receiver design, it is desirable that the symbol rate be substantially less than the difference between f 0 and f 1 ; this makes changes in frequency easier to detect. Frequency modulation significantly reduces the sensitivity to amplitude errors in the signal path.

Communication: Analog vs. Digital

Since all the useful information is held in the frequency domain, many FSK receivers feature a limiter , a high-gain circuit designed to convert a variable-amplitude sinusoidal signal to a more nearly constant-amplitude square wave, desensitizing the circuit to component non-linearities and making it easier for subsequent processing circuitry to detect the frequency of the signal even by counting crossings within a given time interval.

Signal bandwidth is at least as important as with AM: intersymbol interference still results from insufficient processing bandwidth. Because a carrier frequency must be processed, the required bandwidth is probably significantly larger than PAM modulation of the same data. These systems are typically more sensitive to timing errors, such as jitter, than to voltage noise. In PM, the signal is encoded in the phase of a fixed-frequency carrier signal, f c. This can be accomplished with a direct digital synthesizer DDS that generates a digital sine wave, whose phase is modulated by a control word.

click Systems embodying this principle of phase modulation are often referred to as quadrature phase-shift keying QPSK. As with FM, the relationship between the bandwidth of the transmitted spectrum and the symbol rate is fairly complicated.

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  • These types of modulation schemes are popular in difficult environments such as cellular telephony, because the phase information can be maintained in the presence of noise and the distortion introduced by power amplifiers. As with FSK, components for PSK systems are typically selected based on bandwidth and other frequency domain specifications. Limiters may be used to eliminate amplitude noise. Here amplitude- and phase match between the I and Q paths are important specifications, since any mismatches map to an effective phase error.

    More bits per symbol could be transmitted if, instead of just two levels for I and Q, they were further quantized; then, by adding the differing amounts of sine I axis and cosine Q axis together, the combination in vector sums would modulate both amplitude and phase.

    Content: Analog Vs Digital Communication

    Figure 2a shows the use of 2-bit quantization of both I and Q to realize 16 unique states of the carrier in each symbol, allowing transmission of 4 bits per symbol. This modulation could be produced by varying the phase and amplitude of the generated carrier directly using, for example, direct digital synthesis. More commonly, amplitude-modulated I and Q sine and cosine versions of the carrier are combined. Hence the term quadrature amplitude modulation QAM : the two quadrature versions of the carrier are separately amplitude modulated, then combined to form the amplitude- and phase-modulated resultant.

    The points of the constellation represent the transmitted signal and the expected value of the received signal; but noise or distortion will displace the received signal from its ideal position; it can be misinterpreted as a different constellation point if the error is large. Figures 2a and 2b compare the point constellation 2 bits I and Q to a point constellation 3 bits I and Q. In this way, misinterpreting a constellation point for one of its neighbors would corrupt only 1 or 2 bits of a multi-bit symbol. Here are some of the important specifications for components selected for QAM signal processing.

    Bandwidth should be sufficient to handle the carrier frequency, plus enough frequencies within the band to avoid introducing intersymbol interference. Total harmonic distortion THD at the carrier frequency is an important consideration, since distortion will tend to corrupt the amplitude information in the carrier. Jitter should be minimized to ensure that the phase information can be properly recovered.

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    Matching of amplitude and phase between the I and Q processing blocks is important.