Ch07 Data Comm

Chapter Seven:Digital Communication

Chapter 7: Digital Communication

IntroductionMany signals in modern communication systems are digital Additionally, analog signals are transmitted digitallyDigitizing a signal results in reduced distortion and improvement in signal-to-noise ratios


Types of Signal Transmission


Channels and Information CapacityAll practical communication channels are band-limitedThere are theoretical limits to the rate at which data may be transmittedThe relationship between time, information capacity, and channel bandwidth is given by Hartleys Law:


Shannon-Hartley TheoremThere is a limit to the amount of data that can be sent in a given bandwidth:


Pulse ModulationNyquist showed that it is possible to reconstruct a band-limited signal from periodic samples, as long as the sampling rate is at least twice the frequency of the of highest frequency component of the signalSeveral types of sampling are available for pulse modulation


Sampling Rate ErrorsSampling rates that are too low result in aliasing or foldoverThe figures illustrate correct and incorrect sampling rates:


SamplingSampling alone is not a digital techniqueThe immediate result of sampling is a pulse-amplitude modulation (PAM) signalPAM is an analog scheme in which the amplitude of the pulse is proportional to the amplitude of the signal at the instant of samplingAnother analog pulse-forming technique is known as pulse-duration modulation (PDM). This is also known as pulse-width modulation (PWM)Pulse-position modulation is closely related to PDM


Analog Pulse-Modulation Techniques


Pulse-Code ModulationPulse-Code Modulation (PCM) is the most commonly used digital modulation schemeIn PCM, the available range of signal voltages is divided into levels and each is assigned a binary numberEach sample is represented by a binary number and transmitted seriallyThe number of levels available depends upon the number of bits used to express the sample valueThe number of levels is given by: N = 2m


QuantizingThe process of converting analog signals to PCM is called quantizingSince the original signal can have an infinite number of signal levels, the quantizing process will produce errors called quantizing errors or quantizing noiseThe dynamic range of a system is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signalIn a linear PCM system, the maximum dynamic range is found by:DR = (1.76 + 6.02m) dB


CompandingCompanding is used to improve dynamic rangeCompression is used on the transmitting end and expanding is used on the receiving end, hence companding


Coding and DecodingThe process of converting an analog signal into PCM is called coding, the inverse operation is called decodingBoth procedures are accomplished in a CODEC


PCM Coding


Delta ModulationIn Delta Modulation, only one bit is transmitted per sample That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negativeSince so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction


Line CodesLine codes are methods of converting binary numbers back into analog voltages or currentsThe simplest line code is to use the presence or absence of a voltage/current to indicate the logic stateUnipolar NRZ (non-return-to-zero) means that there is no requirement for a signal to return to zero at the end of each elementRZ (return-to-zero) methods are used to eliminate low-frequency ac components and dc components


Bipolar NRZ Code


Time-Division MultiplexingThere are two basic types of multiplexing:Frequency-division multiplexing (FDMTime-division multiplexingIn TDM, each information signal is allowed to use all available bandwidthIn theory, it is possible to to divide the bandwidth or the time among the users of a channelContinuously variable signals, such as analog, are not well adapted to TDM because the signal is present all the time


TDM in TelephonyTDM is used extensively in telephonyThe most common standard is the DS-1 signal, which consists of 24 PCM voice channels, multiplexed using TDMEach channel is sampled at 8 kHz with 8 bits per sample, which gives a bit rate of 64 kb/s for each voice channelThe samples must be transmitted at the rate they were obtained to be reconstructed The overall bit rate is 1.544 Mb/sThe whole system is known as a T1 Carrier


Digital Signal Hierarchy


Data CompressionData compression is a technique used to reduce the bandwidth to transmit an analog signal in a digital formThe exact bandwidth necessary is dependent upon the modulation scheme


Lossy and Lossless CompressionThere are two main categories of data compression:Lossless compression involves transmitting all of the data in the original signal but using fewer bits. Lossless compression generally looks for redundancies in the dataLossy compression allows for some reduction in the quality of the transmitted signal. Lossy compression involves reducing the number of bits per sample or reducing the sampling rate


VocodersA vocoder (voice coder) is an example of lossy compression applied to human speechA typical vocoder reduces the amount of data that needs to be transmitted by constructing a model of the human vocal system


Vocoder TypesThere are two main ways of generating the excitation signal in a linear predictive vocoder:Pulse Excited Linear Predictive (PELP)Residual Excited Linear Predictive (RELP)


Ch07 Data Comm

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