A series by John J. Smith looking
at aspects of communication
FIRST TELETEXT
When BBC and ITV teletext software
was being experimentally transmitted during Christmas 1981 I stumbled
across it accidentally on a decoder I had built at that time.
The first telesoftware transmissions
were made in February 1977 using simple programs in the form of
machine code, employing pages of ASCII characters. From 1977 to
1980 further development was carried out and with the co-operation
of Mullard, who built 12 specially converted TV sets supplied to
Brighton Polytechnic and others around the country, the refined
system of Teletext and Telesoftware appeared. Unfortunately for
those who do not have a BBC/ACORN computer it cannot be used. ITV
seemed to have abandoned the idea and CHANNEL 4 now seems to have
opted for Commodore and Sinclair for their photon light pen type
downloader. Alas it would seem that once again an opportunity of
standardisation has been missed.
A similar thing happened with the
original 1974 Kansas City cassette interface which was based on
years of hobbyists' experiences and discussions and which several
manufacturers started using but then other manufacturers came up
with different methods. Even the speed of recording the data on
cassette varied from 300/600/1100/1200 baud depending on which micro
computer you had and this often meant that you had to buy a specially
adapted cassette recorder from the makers of the computer. The MSX
idea might have worked but it has come too late.
DATA ON THE RADIO?
A few years ago the BBC did some
tests with a system called S.C.A (known as Storecasting in the U.S.A)
which is a system thought of long before Teletext existed. This
is a method of putting a subcarrier onto a radio signal (VHF/FM)
which sits on the sideband where space is available and can contain
additional background music. In America, and increasingly here,
shops or stores, hotels, restaurants, even buses, have background
music. Sometimes tapes are used but it is often cheaper and certainly
more convenient to use the radio. For background music in, say,
a restaurant or hotel you do not want to be blasted with the top
40 hits and have adverts and so on disturbing you, but nice gentle
background music is acceptable. The hotel or restaurant rents or
buys a decoder to separate part of the signal so they only hear
the background music. You or I, who may be tuned to exactly the
same radio station, will hear the usual non stop top 40 with jingles
adverts and all.
The system is similar in some respects
to Teletext. It is there all the time buried in the signal but you
don't know it's there unless you have a decoder. Many of the American
radio stations have been doing this for about 25 years so why hasn't
anybody thought of using this to broadcast database type information?
I asked BBC if they had carried out any "teletext like" transmissions
on radio using this SCA technique and received a brief reply saying
that they had not but they gave some brief details of 1975/1976
tests which used music. I understand that these tests were tried
using the normal 41KHS mono and 67KHS stereo sidebands as used in
U.S.A. but in U.K. our de-emphasis network inside the sets is different
so people who tune to the normal signal had a variety of problems
such as crosstalk and sideband splatter (nasty!). It soon became
apparent that with even a low injection rate of signal it was not
workable in U.K. Maybe we missed the opportunity of suggesting improved
modifications to set manufacturers before the numbers of VHF sets
grew to today's proportions. I am disappointed especially as the
system of decoding was so simple.
A NEW BBC SYSTEM
Things may however be looking up
and recent discussions with the BBC reveal that they have not been
idle and a system called RDS (RADIO DATA SYSTEMS) has not only been
built and tested using a microprocessor built into a radio but was
displayed at the Ideal Home Exhibition in March 1985. I understand
that only 2 receivers have been built but tests have been carried
out in various parts of the country and are currently continuing
on Radio London and BBC Radio 2, 3 and 4. The BBC are said to be
pleased with how things are going.
The RDS system operates on a carrier
of 57 KHz embedded in the main body of the signal similar to SCA
but the similarity ends there. RDS can be likened to Teletext because
it contains a digital signal which cannot carry extra music (only
SCA can do that) but which has many other uses. Amongst those that
have proved to be workable are signals that contain information
to switch your car radio to another transmitter as you drive from
one end of the country to the other. You would still hear uninterrupted
Radio 4 for example even though that station is transmitted on different
frequencies in different areas. As you get further away from one
transmitter and the signal gets weak the receiver is automatically
switched to another transmitter broadcasting the same signal. This
is similar to the cellular radio concept now being set up for 2
way communications. Another thing that it can do is to receive instant
news of, say, an accident on the motorway or sudden bad weather.
At the moment the information appears on a small screen like a calculator
type display and appears no matter what station you are listening
to but it could also actually switch the radio on if important news
is being broadcast.
A WORLD STANDARD?
One very important piece of news
is that this system has in fact been accepted as the European standard
and maybe, one day, it will become a world standard. Congratulations
to the BBC and I hope that those that have the prototypes will not
take too long to get them to the market place.
Finally now that 625 line UHF are
the only TV transmissions in U.K. and the old TV 405 1ine band 1
(49MHZ) and band 3 (approx. 200MHZ) have at long last been closed
down, what, if anything, is being done with these available frequencies?
Maybe we could have a DATA channel to pick up information on our
micros?
Beam me down the latest issue of
PAGE 6 magazine Scottie!
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