408 lines
14 KiB
Plaintext
408 lines
14 KiB
Plaintext
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\ / |_
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/ \ e n o n | o u n d a t i o n
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presents:
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\ /
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*------ the \ /
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/ \ files ------*
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/ \
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Fall/1993 Issue: 1
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INTRODUCTION TO BASIC DIGITAL TECHNOLOGY
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Written by: Erik Turbo
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File: 1 of 6
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1. Analog to Digital Conversion
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1.1 Introduction
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Data communication is simply sending information from one location to
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another by coded signals. There are three minimum components all
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communincations systems have regardless of how complex or simple the
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system is. Those three components are:
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1. a transmitter
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2. a receiver
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3. a transmission path between the transmitter and receiver
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Information is sent over communication systems from one location to another
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via data signals. There are two basic types of signals:
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> Analog
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> Digital
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On the next few pages you will read about analog and digital technology.
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Analog technology is used by customers that do not require sending infor-
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mation at high speed. Currently Analog services are cheaper than digital
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services but this will not always be the case. Analog technology does
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not offer the reproduction quality nor the vast services which are offered
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with digital technology instead of analog. Digital technology has only
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become available in the 80's.
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1.2 Overall Configuration And Digital Concepts
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Central Office 1 Central Office 2
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_______________ ________________
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| D4 | | D4 |
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| _____ | | _____ | __________
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| | |=====|=====================| | | | | |
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| | FXO | | | | FXS | |====== | TELPHONE |
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| | | | | | | | |__________|
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| |_____|=====|=====================| |_____| |
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|_______________| |________________|
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The FX Circuit configuration above contains groups of components that
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comprise most Special Service Circuits. These components include the
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following:
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> Switching Equipment
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> Facilities:
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- Exchange
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- Interoffice
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> Facility Terminal Equipment
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> All three component groups exist in either an Analog or Digital environment
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1.3 Analog Signal Characteristics
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An Analog signal is a continously varying voltage and current quantity
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representing the human voice. The amplitude represents the loudness and
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the number of cycles per second represents the voice frequency:
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Characteristics of the Analog signal are:
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1. The signal is continuous in time.
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2. All values are permitted in the positive maximum limits defined.
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3. In Analog transmission systems, the sound being transmitted is represented
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by the shape of the Analog signal.
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1.4 Analog Signal Generation
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__________________
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\ | | /
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\ | | /
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| | VARIABLE | |
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SOUND --> |======| RESISTOR |=====| ----> REPRODUCED
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WAVE | | | | SOUND
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/ | | \ WAVE
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1 / 2 | 3 | 4 \
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------------------
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Analog Signal Generation
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The generation of an Analog signal takes the following steps:
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1. When a person speaks into the transmitter of a telephone set, changes in
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the air pressure, sound waves, and sensed by the diaphragm (2)
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2. The diaphragm repsonds to changing air pressure and changes circuit resit-
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ance by compressing carbon in the transmitter.
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3. The change in resistance causes current flow to fluctuate, creating an
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electrical wave analogous to the sound wave.
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4. Fluctuating current flows through the coil and vibrates the receiver
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diaphragm, which reproduces the sound wave.
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AN ANALOG SIGNAL IS A CONTINOUSLY VARYING REPRESENTATION OF A SOUND WAVE.
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1.5 Analog Waves - Components
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An Analog signal is composed of amplitude and frequency. These components
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define the sound wave an Analog signal represents.
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The amplitude and frequency are two characterisitcs of the analog signal that
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can be varied to convey information.
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Amplitude is the measure of the volume or loudness of the Analog signal.
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Amplitude is the relative strength of the signal.
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Frequency is the number of cycles in a unit of time.
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1.6 Frequency
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Frequency is related to the pitch of a sound. Frequency is measured in
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Hertz (Hz) - The number of cycles or oscillations per second.
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Frequency and amplitude relationships:
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1. Low frequency, low amplitude - whispering at a low pitch.
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2. Low frequency, high amplitude - yelling at a low pitch.
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3. High frequency, low amplitude - whispering in a high pitch.
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4. High frequency, high amplitude - yelling at a high pitch.
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1.7 Analog Signal Impairments
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> Loss - Attenuation
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> Noise - Unwanted Electrical Signals
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> Distortion - Frequency Characteristic Changes
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Attenuation (Loss) and Amplification.
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The ideal transmission channel will deliver an accurate replica of the
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original signal to the receiving terminal. Three major problems affect
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the transmission of Analog signals:
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1. LOSS - Weakening of the signal
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2. NOISE - Unwanted electrical signals that interfere with the information
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signal.
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3. DISTORTION - Changing of the frequency characteristics of the signal.
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1.8 Analog Signal Attenuation and Amplificiation
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> The signal is continuously attenuated, or weakened, as it progresses along
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the transmission medium.
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> The signal is then amplified at intervals to compensate for the attenuation.
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> Line loss can be overcome by properly spacing amplifiers in the circuit.
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1.9 Amplifying Distorted Signals
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> The Analog signal is also affected by noise and distortion.
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> Analog signals pick up noise as they travel through the network.
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> Noise and distortion change the shape of the Analog signal.
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> Amplifiers are designed to reproduce all of the variation of the Analog
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signal, the amplifier cannot distinguish between the voice, noise, and
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distortion components of the Analog signal.
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> The amplifier amplifies the entire input signal, thus the noise is
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amplified along with the original signal.
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> As the signal path increases in length and more amplification is needed,
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more noise is introduced.
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> The effects of noise and distortion is cumulative along the Analog trans-
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mission system.
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1.10 Analog Signal Disadvantage
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> The major disadvantage of Analog transmission systems is the cumulative
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nature of transmission impairments.
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> Loss can be overcome by amplification to increase the sigmnal to its
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original value. Noise is also amplified.
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> Once introduced, the effects of noise and distortion cannot be eliminated.
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> Digital transmission systems solve the "Analog Problem."
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1.11 Digital Transmission Concepts
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Digital Signal Definition
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A Digital signal is a discontinuous signal in the form of pulses. Good
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examples would be flashes of light, telegraph clicks, and dialing pulses.
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A transmitted Digital signal generally represents a series of on/off
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pulses, transmitted at a steady rate and amplitude.
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1.12 Digital Signal Regeneration
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Digital transmission systems solve the basic "Analog Problem" of cumulative
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effects of noise and distoriton by regenerating rather than amplifying the
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transmitted signal.
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The regenerative repeater detects the presence of a pulse, (signal), and
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creates a new pulse, (signal), based on a sample of the existing signal.
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The regenerated signal duplicates exactly the signal originally transmitted.
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This eliminates the cumulative effects of noise and distortion inherent
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in Analog facilities. Distortion is not amplified as it is an Analog signal,
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it is omitted when the signal is regerated.
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1.13 Comparison of Analog and Digital Signals
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The Analog signal is a signal that varies in a continuous manner over a
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wide range of amplitude and time. As you know, in Analog transmission,
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amplifiers were used to boost the strenght of the signal. With Analog
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signal transmission, the line noise is amplified along with the signal
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at each repeater point. Thus, as the distance increases, so does the
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distortion.
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The Digital signal is a series of pulses, all having a specified amplitude
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and duration in time. A Digital signal has only a discrete number of states,
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0 or 1. This on/off state simplifies the process of detecing and regerating
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the digital bit stream.
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Instead of amplifying the signal, a rengenerator produces a fresh signal
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based on a sample of the existing signal. By using this method, noise does
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not accumulate. At each repeater location, the incoming Ditital signal
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is regenerated into the correct 0 or 1 signal. While the associated line
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noise is ignored.
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1.14 Analog to Digital Conversion - Overview
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1 1 0 1 0 1 1
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/\ ______ __________ ________ _ _ _ _ _
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/ \ /==| | | | | | | | | | | | | | | |
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\/ |SAMPLE|==| QUANTIZE |===| ENCODE |_| |_| |___| |___| |_| |__
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ANALOG |______| |__________| |________|
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SIGNAL
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Analog to Digital Conversion
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Converting an Analog signal to a Digital signal requires the steps of
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sampling, quanitizing, and encoding.
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> Sampling
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In the sampling process, portions of a signal are used to represent the
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whole signal. Each time the signal is sampled, a Pulse Amplitude
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Modulation (PAM) signal is generated. In order to accurately reproduce
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the Analog signal (speech), a sampling rate of at least twice the
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highest frequency to be reproduced is required. Because a majority of
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voice frequencies are less than 4 KHz, and 8 KHz sampling rate has
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been established as the standard.
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> Quantizing
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In order to obtain the Digital signal, the Pulse Amplitude Modulation (PAM)
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signal is measured and coded. The amplitude or height of the PAM is
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measured to derive a number that represents its amplitude level.
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> Encoding
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The decimal (Base 10) number derived in the quanitizing step is then
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converted to its equivalent 8 bit binary number. The output is an 8
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bit "word" in which each bit may be either a "1" (pulse) or a "0" (no
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pulse)
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This process is repeated 8,000 times a second for a telephone voice
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channel service.
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1.15 Filtering
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The range of frequencies in the human voice approximates 50 Hz to 20,000 Hz.
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Telephone transmission systems are arranged to transmit Analog signals
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between 200 Hz and 4,000 Hz. Extreme frequencies below 200 Hz and above
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4,000 Hz are removed by a process called Filtering.
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1.16 Sampling
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The sampler measures the filtered Analog signal 8,000 times a second, or
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once ever 125 microsecons (u sec.) The value of each of these samples is
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directly proportional to the amplititude of the Analog signal at the time
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of the sample.
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The sampling process is called Pulse Amplitude Modulation (PAM)
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1.17 Quantizing
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Quantizing is essentially matching the PAM signals to one of the 255 numbers
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on a segmented scale. The quantizer measures the amplitude or height of
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each PAM signal coming from the sampler and assigns it a value from
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-127 to plus 127.
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1.18 Pulse Code Modulation (PCM) Encoding
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Encoding involves the conversion of the number that was determined in the
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quantizing step, to a binary number. each quantized PAM signal is conerted
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ito an 8-bit binary "word" in which each bit may be either a "1" (pulse)
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or a "0" (no pulse). The 8-bit "word" represents the binary equivalent
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of the number from the quantizing step.
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1.19 PCM Encoding Example
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If the Pulse Amplitude Modualation (PAM) signal measures +45 on the
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quantizing scale, the output of the encoding step is the 8-bit word
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"10101101" (ie: the binary equivalent of +45.)
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1.20 Digital to Analog Conversion
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At the receiving terminal the following occurs:
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> The Digital pulses are converted back to the original Analog signal.
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> The Pulse Code Modulation (PCM) signals are decoded to the Pulse Amplitude
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Modulation (PAM) signals they represent.
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> The succession of PAM signals are passed through a filter, thereby
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reconstructing the orignal analog wave form.
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1.21 Conclusion
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Some customers are still using analog technology for services like FX lines,
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POT lines, WATTS lines and voice services. These customers feel they don't
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need the high speed of the quality of digital services. Currently the
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Analog services are cheaper than the Digital services, although this could
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change.
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In our fast paced environment many customners want higher speed communications
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with top quality. Digital technology provides this and allows our customers
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to send data and voice communications simulataneously. Our jobs will be
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influenced greatly by the new services our customers want, which only
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digital technology can provide. You will learn about the services which
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only digital technology can offer later on in this manual.
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NOTICE
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Not for use or disclosure outside the NYNEX Corporation
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or any of its subsidiaries except when rightfully stolen.
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------------------------------------------------------------------------------
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EOF ---------------- Xenon Foundation Productions 1993 -------------------EOF
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------------------------------------------------------------------------------
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