Teletext is a system of transmitting information on previously unus...
Whereas nowadays most TV remotes use infrared signals to communicat...
**Viewdata** was an interactive system by which a viewer retrieved ...
It’s important to stress how early 1977 was in the development of h...
In teletext, pages are broadcasted serially along with television c...
"Secure" here is more about "error resistance" than the modern unde...
In Teletext, textual characters and control characters are represen...
A page of Teletext consists of 24 rows of 40 characters. As explain...
Telesoftware: adding intelligence to teletext
J.
Hedger and R. Eason
Indexing
terms:
Television
applications,
Data-communication
systems,
Computer software
Abstract: The authors seek to show how, by using a microprocessor, teletext may be extended to provide
many new and useful facilities, while remaining compatible with existing teletext transmissions. The design
and implementation of an experimental telesoftware terminal are described, and a summary of current
thinking is given, indicating the probable future software and hardware constraints of the system and its
potential applications.
1
Introduction
Independent Television has been broadcasting Oracle as a
service of broadcast teletext since 1975.
1
It was soon
realised that teletext, as well as being an efficient means of
disseminating textual and simple graphic data, offered a
very convenient and cost-effective method of distributing
software programs for home computer systems. Further-
more, an existing teletext decoder already contains many of
the 'building blocks' necessary for a simple home computer,
such as memory, colour visual display and numeric data
entry. With the addition of a microprocessor, such a de-
coder may be transformed into a small, but potentially
powerful, personal computer inside the television set
itself.
By utilising the Oracle system to broadcast programs for
such an enhanced teletext decoder (with the programs
presented as pages of standard text), a user would need
only to select the pages containing his desired program.
Once received, this software could then be executed by the
microprocessor, utilising the existing colour display and
keypad, without the need for expensive storage peripherals
or telephone lines.
Thus telesoftware
2
represents a very inexpensive form of
personal computing, since once a user has purchased the
necessary 'intelligent' television set, telesoftware, like
teletext, is a free service.
2 An experimental terminal for telesoftware
To stimulate the interest of decoder manufacturers and
broadcasters in the concept of telesoftware, Independent
Television and Mullard Ltd. embarked, towards the end of
1977,
on a joint project to design and construct a simple
telesoftware terminal for evaluation, using the Oracle service
to broadcast the trial software. The terminal hardware was
built at the Mullard Applications Laboratory, Mitcham.
The aim of the project was to produce a prototype terminal
in a relatively short space of time, and so the design was
intentionally made somewhat simple, far from optimum
and not intended in any way to represent a definitive means
of implementation. However, this simplicity was far out-
weighed by the subsequent value of the terminal for demon-
stration purposes.
2.1 Compatibility with teletext
At this early stage there existed no specification or guide-
lines for the broadcasting of telesoftware, nor for its
implementation in hardware. It was realised therefore that
Paper 8451 E, first received 13th July and in revised form 9th
October 1979
Mr. Hedger is with Independent Television, South Bank Television
Centre, Kent House, Upper Ground, London, and Mr. Eason is with
the Mullard Applications Laboratory, Mitcham, Surrey, England
1412
0020-3270/79/121412
+
05 $01-50/0
parameters governing the experimental terminal, although
fairly arbitrary in their choice, could not be permitted to
interfere in any way with the existing teletext system. For
this reason, design work was based entirely upon the use of
normal teletext transmission techniques, using ordinary
characters to represent bytes of computer software.
2.2 Overall
design
The prototype terminal itself consists of three main parts: a
domestic colour television receiver with ultrasonic remote-
control; a teletext decoder card and purpose-built micro-
processor card. For convenience, the two latter items were
mounted outside the receiver together with their power
supplies; although ideally the whole terminal would be
housed within the receiver cabinet. Fig. 1 shows a block
diagram of the experimental terminal. For clarity, only the
bus lines and major interconnections are shown.
The teletext card contains a standard Mullard production
chipset, and is of the viewdata-compatible type. This
implies that the data and address lines to its page store are
accessible via a connector, a facility which is essential for
telesoftware operation. The microprocessor card contains a
Signetics 2650 microprocessor and associated memory
which together form the local microcomputer.
2.3 The telesoftware
page
For the experimental project, telesoftware programs were
broadcast as sequences of Oracle pages of text, using up to
seven to form a complete program. When teletext pages are
used in this way there is no means of determining which
pages of the sequence will be received first, and so the
terminal is designed to be able to load the pages in any
order. Fig. 2 shows the format of the telesoftware page, the
contents of which are as follows:
Row 00 is the Oracle page header and is not currently
affected by telesoftware transmission.
Row 01 contains a special sequence of characters which
identify the page as carrying telesoftware data. Since this
sequence is short, and the remainder of the row is not
scanned by the microprocessor, the space is used to carry
the program title.
Row 02 contains control data for the program itself and
must be highly secure. This data is protected by Hamming
code,
3
with each byte of data being represented by two
contiguous teletext characters. To avoid the use of teletext
control characters, only six of the possible seven bits per
character are used, giving a total of twelve usable bits. Eight
of these make up the data byte, while the remaining four
are used for error detection.
The control data are further protected by a block-check
character (b.c.c.) and use 16 teletext characters in total. As
a further security measure, the control data block is repeated
PROC.
IEE, Vol. 126, No. 12,
DECEMBER
1979