meaning, block diagram, data format

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VideotextVideotext
Contents:Contents:
Introduction
Working of Videotext
Block diagram
Signal Modulation and Decoding
Serial data format
Advantages
Disadvantages
Applications
Dr. M V Raghavendra,
Dept of Electronics & Communication

2. Annie Besant Women’s College
Videotex (or "interactive videotex") was one of the earliest implementations of an
end-user information system.
From the late 1970s to early 2010s, it was used to deliver information (usually
pages of text) to a user in computer-like format, typically to be displayed on
a television or a dumb terminal.
In a strict definition, videotex is any system that provides interactive content and
displays it on a video monitor such as a television, typically using modems to send
data in both directions.
A close relative is teletext, which sends data in one direction only, typically encoded
in a television signal.
All such systems are occasionally referred to as viewdata.
Unlike the modern Internet, traditional videotex services were highly centralized.
Videotex in its broader definition can be used to refer to any such service, including
• The Internet,
• Bulletin board systems,
• Online service providers, and even
• The arrival/departure displays at an airport. This usage is no longer
common.
Introduction
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•Videotext or view data are the terms used to describe
computer based information systems which organize and
display text and graphics in the form of electronic book.
•The information is held on frames or pages each one
numbered and linked to other pages.
•A videotext system is basically a time share computer
system which is specifically designed to hold large volumes
of data, and can cope with large number of simultaneous
users.
•For a large system, the computer may consist of network
of computers around the country which are connected by
high speed data lines.
•Users access the computer via the normal telephone lines
using full duplex serial data transmission and there is a
two way communication between the user and computer
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•The data is displayed on a normal Television screen
using same display format.
•Data is input to the computer system from
1. Information providers using a modified
videotex decoder or
2. By a special task bulk update method from
information provider’s local computer.
•Videotext systems are three types
•Viewdata
•Teletext
•Open channel teletext
•Figure shows the simple block diagram of a basic
videotex system.
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Videotex
Computer
Videotex
Computer
Videotex
Computer
Videotex
Computer
High speed data links
Public Switched Telephone System
Informationprovider
Informationprovider
High
speed
data
links
High
speed
data
links
User terminal User terminal User terminal
Block diagram of a basic videotex system
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Videotex serial data format
In the data format, each frame consists of 10 bits.
• One start bit
• One stop bit
• Eight data bits(Parity is eighth bit)
In the above example total number of 1’s are even, so we have even
parity i.e. parity bit is ‘0’
Continuous stream of logic ‘1’ is treated as idle.
Information gets started by a start bit which is logic ‘0’.
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Signal Modulation
•The logic signal cannot be sent directly down the telephone line, so it
is modulated by FSK which involves allocating one carrier frequency
for logic one level and different carrier frequency for logic zero level.
•The system can work simultaneously in both the directions in full
duplex operation and so in order that the two signals do not interfere,
the carrier frequencies (data rates) are different.
Decoding the serial signal
•At the receiving end, the signal is scanned for a logic zero, indicating a start
bit, the signal is tested again after the time interval corresponding to half the
time of one bit and if the signal is still logic zero then it is reasonably assumed
as logic zero and is not due to interference or noise in fact a start bit.
•The signal is then tested at regular time intervals corresponding to bit time,
and resulting logic levels should give the required seven character bits, the
parity bit and the stop bit.
•If the stop bit sample is at logic one, and the parity check is correct, it is likely
that the character has been correctly received.
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Viewdata
View data is a two way communication system that allows transmission and
reception of information through the same centralized computer.
Teletext
Teletext is a broadcasted or a pseudo interacted one way communication
system.
It allows the user to choose the desired information to be viewed, the time at
which the information is displayed and the display sequence.
Open Channel Teletext
It is a non interactive one-way communication system which has no control
over the pre selected information, the time and sequence of displays.
It is of three types
I. Dedicated open channel
II. Open captioning
III. Closed captioning.
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•The system can be used by any one who has access to
telephone line and a suitable terminal
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Applications:
•Electronic banking
•Shopping
•Messaging
•Teleservices
•Commercial and personal transactions
•Educational Services
•Computing and
•Gaming
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