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Transcript
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
Channels and Information Capacity
• All practical communication channels are bandlimited
• 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:
I  ktB
Shannon-Hartley Theorem
• There is a limit to the amount of data that can be
sent in a given bandwidth:
C  2B log 2 M
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 rates that
are too low result
in aliasing or
foldover
• The figures
illustrate correct
and incorrect
sampling rates:
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
Analog Pulse-Modulation Techniques
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
• 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 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
• 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
PCM Coding
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 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 lowfrequency ac components and dc components
Bipolar NRZ Code
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 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
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
• 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
• 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
• 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)