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Chapter Seven: Digital Communication. Introduction. Many signals in modern communication systems are digital Additionally, analog signals are transmitted digitally Digitizing a signal results in reduced distortion and improvement in signal-to-noise ratios. Types of Signal Transmission.

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Presentation Transcript
introduction
Introduction
  • Many signals in modern communication systems are digital
  • Additionally, analog signals are transmitted digitally
  • Digitizing a signal results in reduced distortion and improvement in signal-to-noise ratios
channels and information capacity
Channels and Information Capacity
  • All practical communication channels are band-limited
  • There are theoretical limits to the rate at which data may be transmitted
  • The relationship between time, information capacity, and channel bandwidth is given by Hartley’s Law:
shannon hartley theorem
Shannon-Hartley Theorem
  • There is a limit to the amount of data that can be sent in a given bandwidth:
pulse modulation
Pulse Modulation
  • Nyquist 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 signal
  • Several types of sampling are available for pulse modulation
sampling rate errors
Sampling Rate Errors
  • Sampling rates that are too low result in aliasing or foldover
  • The figures illustrate correct and incorrect sampling rates:
sampling
Sampling
  • Sampling alone is not a digital technique
  • The immediate result of sampling is a pulse-amplitude modulation (PAM) signal
  • PAM is an analog scheme in which the amplitude of the pulse is proportional to the amplitude of the signal at the instant of sampling
  • Another 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
pulse code modulation
Pulse-Code Modulation
  • Pulse-Code Modulation (PCM) is the most commonly used digital modulation scheme
  • In PCM, the available range of signal voltages is divided into levels and each is assigned a binary number
  • Each sample is represented by a binary number and transmitted serially
  • The number of levels available depends upon the number of bits used to express the sample value
  • The number of levels is given by: N = 2m
quantizing
Quantizing
  • The process of converting analog signals to PCM is called quantizing
  • Since the original signal can have an infinite number of signal levels, the quantizing process will produce errors called quantizing errors or quantizing noise
  • The dynamic range of a system is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signal
  • In a linear PCM system, the maximum dynamic range is found by:

DR = (1.76 + 6.02m) dB

companding
Companding
  • Companding is used to improve dynamic range
  • Compression is used on the transmitting end and expanding is used on the receiving end, hence companding
coding and decoding
Coding and Decoding
  • The process of converting an analog signal into PCM is called coding, the inverse operation is called decoding
  • Both procedures are accomplished in a CODEC
delta modulation
Delta Modulation
  • In 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 negative
  • Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction
line codes
Line Codes
  • Line codes are methods of converting binary numbers back into analog voltages or currents
  • The simplest line code is to use the presence or absence of a voltage/current to indicate the logic state
  • Unipolar NRZ (non-return-to-zero) means that there is no requirement for a signal to return to zero at the end of each element
  • RZ (return-to-zero) methods are used to eliminate low-frequency ac components and dc components
time division multiplexing
Time-Division Multiplexing
  • There are two basic types of multiplexing:
    • Frequency-division multiplexing (FDM
    • Time-division multiplexing
  • In TDM, each information signal is allowed to use all available bandwidth
  • In theory, it is possible to to divide the bandwidth or the time among the users of a channel
  • Continuously variable signals, such as analog, are not well adapted to TDM because the signal is present all the time
tdm in telephony
TDM in Telephony
  • TDM is used extensively in telephony
  • The most common standard is the DS-1 signal, which consists of 24 PCM voice channels, multiplexed using TDM
  • Each channel is sampled at 8 kHz with 8 bits per sample, which gives a bit rate of 64 kb/s for each voice channel
  • The samples must be transmitted at the rate they were obtained to be reconstructed
  • The overall bit rate is 1.544 Mb/s
  • The whole system is known as a T1 Carrier
digital signal hierarchy

Carrier

Signal

Voice

Channels

Bit Rate

(Mb/s)

Typical Medium

T1

DS-1

24

1.544

Twisted-pair

T1C

DS-1C

48

3.152

Twisted-pair

T2

DS-2

96

6.312

Low-capacitance twisted-pair

microwave

T3

DS-3

672

44.736

Coax, microwave

T4

DS-4

4032

274.176

Coax, fiber-optic

T5

DS-5

8064

560.16

Fiber optics

Digital Signal Hierarchy
data compression
Data Compression
  • Data compression is a technique used to reduce the bandwidth to transmit an analog signal in a digital form
  • The exact bandwidth necessary is dependent upon the modulation scheme
lossy and lossless compression
Lossy and Lossless Compression
  • There 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 data
    • Lossy 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
vocoders
Vocoders
  • A vocoder (voice coder) is an example of lossy compression applied to human speech
  • A typical vocoder reduces the amount of data that needs to be transmitted by constructing a model of the human vocal system
vocoder types
Vocoder Types
  • There are two main ways of generating the excitation signal in a linear predictive vocoder:
    • Pulse Excited Linear Predictive (PELP)
    • Residual Excited Linear Predictive (RELP)
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