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## PowerPoint Slideshow about 'Chapter Seven: Digital Communication' - guillermina

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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

- 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

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

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

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

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 low-frequency ac components and dc components

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

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 HierarchyData 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)

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