EET260Introduction to digital communication

Digital signals Binary digital signals use two discrete voltage

levels to represent binary 1 or 0. Combining multiple bits into words permits us to

represent larger values. Digital circuits operate on digital signals

performing logic and arithmetic functions.

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1 0 1 01 1

Analog systems Analog systems use electrical signals that

vary continuously, not having discrete values Analog signals are electrical representations of

signals from nature (pressure, light, sound, etc.)

Examples of analog systems: AM/FM radio, cassettes, telephone, VCR, standard television

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Analog examples

Digital systems Digital systems use electrical signals that

represent discrete, binary values. Digital signals are not representative of signals

that occur in nature (pressure, light, sound, etc.).

Natural signals must be converted into digital format.

Historically, signals in communications systems have been analog but a migration to digital systems has been underway for the last 25 years.

Digital examples

Advantages of digital signals The most important advantage of digital

communications is noise immunity. Receiver circuitry can distinguish between a

binary 0 and 1 with a significant amount of noise.

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1 0 1 01 1

digital signal with noiseanalog signal with noise

Advantages of digital signals Digital signals can be stripped of any noise in a

process called signal regeneration. Consider a network of relay stations.

microwave relay stations

Advantages of digital signals An analog signal is received, amplified and

retransmitted at each station. However, the noise is also amplified each link.

microwave relay stations

original analog signal

signal at repeater 1

signal at repeater 2

signal at repeater 3

Advantages of digital signals An digital signal is received, regenerated, then

retransmitted at each station. The noise can be eliminated at each repeater.

microwave relay stations

original analog signal

signal at repeater 1

signal at repeater 2

signal at repeater 3

Advantages of digital signals Even if a digital signal does contain bit errors,

many of these errors can be fixed at the receiver through the use of error correcting codes. Error correcting codes allows CDs with minor

scratches to be played without errors. We will discuss such codes later.

scratched CD

Advantages of digital signals Digital signals are easier to multiplex. Multiplexing is the process of allowing multiple

signals to share the same transmission channel.

Advantages of digital signals Digital is the native format for computers. Computers permit the digital signal processing

(DSP) and digital storage of communication signals. DSP allows operations such as filtering, equalization

and mixing to be done numerically without the use of analog circuits.

DSP also permits data compression.

Transmission of digital data There are two ways to move bits from one place

to another: Transmit all bits of a word simultaneously (parallel

transfer). Send only 1 bit at a time (serial transfer).

Serial transmission In serial transmission, each bit of a word is

transmitted sequentially, one after another. The least significant bit (LSB) is transmitted first, and

the most significant bit (MSB) last. Each bit is transmitted for a fixed interval of time.

Serial data can be transmitted faster and over longer distances than parallel data.

Serial buses are now replacing parallel buses in equipment where very high speeds are required.

Parallel transmission Parallel data transmission is extremely fast

because all the bits of the data word are transferred simultaneously. There must be one wire for each bit of information to

be transmitted. Multi-wire cable must be used.

Parallel transmission Parallel data transmission is impractical for long-distance

communication because of: Cost of laying multi-wire cables. Signal attenuation over long distances. At high speeds, capacitance and inductance of multiple wires

distorts the pulse signal. To reduce this, line lengths must be severely shortened.

To achieve clock speeds up to 400 MHz, line lengths must limited to a few inches

Parallel data transmission by radio would be complex and expensive.

One transmitter and one receiver for each bit.

Serial-parallel conversion Because both parallel and serial transmission

occur in computers and other equipment, there must be techniques for converting between parallel and serial and vice versa. Such data conversions are usually taken care of by

shift registers, sequential logic circuits made up of a number of flip-flops connected in cascade.

Q0D0

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Q1D1

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Q2D2

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Q4D4

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Q0 Q1 Q2 Q3

Data input

Clock input

Parallel outputs

Serial-parallel conversion

Conversion from analog to digital Before we can use digital transmission, we must

convert the signal of interest into a digital format. Translating an analog signal into a digital signal

is called analog-to-digital (A/D) conversion, digitizing a signal, or encoding. The device used to perform this translation is known

as an analog-to-digital converter or ADC.

Conversion from analog to digital An analog signal is a smooth or continuous

voltage or current variation. Through A/D conversion these continuously variable

signals are changed into a series of binary numbers.

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01101010100111001101010101111

A/D conversion The first step in A/D conversion is a process of

sampling the analog signal at regular time intervals.

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sample points

sampling period (T)

sampling frequency

1fT

A/D conversion How often do we need to sample the signal?

How large does our sampling frequency f need to be in order to accurately represent the signal?

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high sampling rate

low sampling rate

Minimum sampling frequency The minimum sampling rate required in order to

accurately reconstruct the analog input is given by the Nyquist sampling rate fN given

where fm is the highest frequency of the analog input. The Nyquist rate is a theoretical minimum. In practice, sampling rates are typically 2.5 to 3 times

the Nyquist rate fN.

2N mf f

Consider the signal from the oboe depicted below in time and frequency domain representations.

a. What is the maximum frequency present in the oboe signal?

b. Based upon this, what would be the minimum sampling rate according to Nyquist?

c. What would be practical sampling rate?

Example Problem 1

1 1.0005 1.001 1.0015 1.002 1.0025 1.003 1.0035 1.004 1.0045 1.005-1

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0 1000 2000 3000 4000 5000 60000

0.05

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0.15

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A/D conversion The actual analog signal is smooth and

continuous and represents an infinite number of actual voltage values. It is not possible to convert all analog samples to a

precise binary number. Therefore, samples are converted to a binary number

whose value is close to the actual sample value.

A/D conversion The A/D converter can represent only a finite

number of voltage values over a specific range. An A/D converter divides a voltage range into

discrete increments, each of which is represented by a binary number.

A/D conversion The process of mapping the sampled

analog voltage levels to these discrete, binary values is called quantization.

Quantizers are characterized by their number of output levels.

An N-bit quantizer has 2N levels and outputs binary numbers of length N. Telephones use 8-bit encoding 28 = 256 levels CD audio use 16-bit encoding 216 = 65,536 levels