aving visited the excellent Teletext Then and Now site
created by Darren Meldrum and Mike Brown, I remembered that I had
some off-screen shots of early teletext
reception, so I dug them out and made up this page of
reminiscences around them. Later I came across a pile of
long-forgotten indexes, press releases and other items that I'd
received from BBC and IBA Engineering Information in the seventies.
I parcelled them all up and sent them off to Mike, who has
incorporated some of them into his site.
Long distance television reception was a hobby of mine from the
sixties, and when teletext arrived on the continent I was keen to
try to receive it. My first success was a Nederland 2 test transmission.
I also found some BBC Microcomputer
programmes that I'd copied down from CEEFAX pages into a
notebook, so I've included those, together with the photographs,
here on my own web site.
If you're interested in any aspect of teletext, you may wish to
subscribe to the UK
Teletext Mailing List.
our tv picture consists of a single spot of
light created by an electron beam which sweeps quickly from left to
right to form lines, and more slowly from top to bottom to form
frames. In the first half of the century, when the tv systems were
designed, it took receivers a relatively long time to force the
spot to "fly back" from the right-hand side of the screen to the
left, and from the bottom to the top. Gaps were left in the
transmitted signal that contained only black space and
"blacker-than-black" synchronising pulses.
Modern sets have much shorter flyback periods, and more
efficient blanking circuits, which means that the transmitted
blanking period no longer needs to be black. In the sixties
broadcasters started to use the "spare" lines for test signals, and
then began considering what else they might put there.
In the early seventies, the BBC was experimenting with optional
subtitles, and the IBA with source identification (SLICE), both of
which have found their way into the present teletext services. They
then realised the service would be more useful if it carried more
general news and information for viewers to read. After some
experiments the BBC (CEEFAX, first broadcast as a thirty-page
magazine on 23 September 1974) and IBA (ORACLE) in consultation
with BREMA, the set manufacturers' organisation, jointly produced
the 1974 Broadcast Teletext Specification. In 1976 a second
specification was published allowing for more display attributes
and a modified character set, among other things.
The key to teletext is that the receiver contains a memory, to
store data transmitted over a period of several frames, and a
character generator to present the data on the screen. That means
that data for a whole row of text (40 characters) can be
transmitted in one tv line, and data for a whole page spread over
several frames. At first only two lines per field (four lines per
frame) were allowed, and a 24-row page took about a quarter of a
second to send. Nowadays twelve lines per field is the norm, and
many more pages are transmitted.
n 1975/76 Wireless World published a design
for a teletext decoder that could be built by a home constructor
with little or no test equipment. The design used about a hundred
TTL (transistor-transistor logic) ICs on three printed circuit
boards, and a kit was made available. The intention was to tap off
a video signal from a colour tv, and feed the decoded RGB text
signal back to the set. Interfacing was quite tricky, especially as
sets of that period did not have the smooth video response they do
today. Later, other magazines published designs that took an aerial
signal and modulated the teletext to be fed into the aerial socket
of a tv set, like a vcr does. The displayed text quality was not as
good as with an integral decoder.
I built the WW decoder in 1976 and fitted it into a 1973 Pye 697
hybrid chassis. In those days of valves, it was not usual to apply
RGB to the cathodes of the display tube. Instead a powerful video
output valve fed luminance (Y) to the cathode, and smaller valves
fed the colour difference signals (R-Y, G-Y, B-Y) to the grids.
Because the colour difference signals were lower bandwidth than the
luminance this saved on wideband power output stages. It also saved
"matrixing" the colour signals to produce RGB - a process that's
all done on a chip these days.
Because the matrixing was done "in the tube" it meant there was
no RGB path in which to insert the teletext, so it was necessary to
apply an inverted signal to each colour difference circuit.
Unfortunately, the restricted bandwidth led to fuzzy characters,
and a circuit was included to widen the white parts of the letters
so they became brighter and more distinct. In 1978 I constructed a
Forgestone colour tv, which, while still being hybrid in the sense
that it had valves in the line output stage, had solid state RGB
drivers, and so was easier to interface, and gave sharper
results.
aving constructed the Wireless World
Teletext Decoder, and incorporated it into the Pye colour set, I
took some off-screen photographs. A couple of years later when I
moved the decoder to the Forgestone set I took some more. The
quality is not brilliant, but the shots give an idea of what was
being broadcast in 1976-78, so here
they are, together with some memories that they have jogged.
There's also a page of photographs of some Dutch teletext received here in Sheffield in
November 1979.
eletext and the BBC Computer Literacy
Project were closely linked in 1982 (the year the Project and the
associated BBC Microcomuter were launched). The intention was for
CEEFAX to broadcast pages of computer-readable data for downloading
via a teletext decoder attached to the BBC Micro. (Why do we not
call them "micros" any more? Presumably it's no longer "PC"!) At
first, the programmes were short, and transmitted as ASCII
listings, and so it was possible to copy them down in longhand.
Just as well, as there was no decoder available at the time, and
there was a six-month waiting list for the BBC Micro itself, even
for those (like me) who sent off their £400+ the day it was
launched in February 1982.
I jotted down five such programmes in my "How to learn BBC
BASIC" notebook, and they have survived both my BBC Micro and its
associated discs and tapes. I have typed them into my Archimedes
A440/1, and they work, so I present them here for your perusal, together with screen
shots.
HE BEAUTY of teletext is that text is transmitted as character codes which is economical with bandwidth and allows the decoder to deal with the rendering of the actual glyphs (character shapes) at whatever resolution the designer feels is appropriate.
However, because character generator chips are mass-produced, only a few styles of character have been used during the thirty years of decoder production.
Almost all use the dot-matrix method of rendering text, switching sharply between the foreground and background colours as determined by the character generator ROM.
Some advanced decoders use a system called 'anti-aliasing' whereby some of the pixels are a blend of the foreground and background colours which results in slightly blurred, but smoother, less jagged glyphs.
There are several basic ways of rendering text on an interlaced display like a television set.
The easiest is to use a 5 x 9 matrix and repeat the pattern on adjacent fields so that pairs of lines in the resulting 5 x 18 display are identical.
A refinement of this is to interpolate the 5 x 9 matrix and add half-sized 'fillets' to smooth the diagonals.
This results in very slight flicker because alternate fields are different.
Another way is to remove interlace altogether and superimpose the lines of alternate fields so that a 5 x 9 flicker-free display is produced.
Only the full-screen display can be treated that way.
Subtitles and mixed screens are rendered over the normal interlaced raster.
Many of the decoders using this technique have higher-resolution 10 x 9 glyphs.
Finally, it is possible for the decoder to rasterise fonts using anti-aliasing software which results in different patterns on alternate lines, giving higher definition glyphs that are subject to a degree of flicker, though this is minimised by the use of shades in between the foreground and background colours.
Digital tv receivers and of course modern computers use this method almost exclusively.
This is the basic character glyph found on the early decoders such as the Wireless World design that used standard computer character generator ROMs.
The actual space allocated to each character is 6 x 10 pixels (6 x 20 for both fields) and so the decoder inserts a row and column of blank pixels to keep the characters apart.
Here the same character generator is used but a character rounding circuit has been added to improve the diagonals.
This was how text in the later versions of the WW decoder and Mode 7 of the BBC Microcomputer looked.
In more modern decoders character generators with twice the horizontal resolution have been used, though with still only nine elements vertically.
The same character generator operated in non-interlaced mode, with alternate fields superimposed instead of interleaved.
In fact the character generator stores data for all 12 x 10 pixels so that large symbols and diacritical marks can be accommodated more easily, albeit with the danger of characters 'bleeding' into each other.
Character code 96 (_), the 'wide bar' or 'm-dash', is such a culprit, though a row of them may be used to make a dividing line so:
_________________________ which on an older decoder would appear dashed, so:
--------------------.
It is noticeable that Ceefax now uses this method of constructing horizontal lines in favour of using block graphics characters, presumably so that pages may be converted automatically to 'digital text' format, where block graphics are not available.
This is the sort of output you would expect to see from a character generator with anti-aliasing.
Not only is the horizontal resolution improved (there are about sixteen columns at teletext character size) but text looks smoother because shades in between the foreground and background colours are used.
In addition, the data on alternate lines is different, yielding eighteen rows of resolution at this text size at the expense of some flicker on thin horizontals.
People find this sort of text easier to read and quote it as the main reason why 'digital text' is better than FLOF teletext, though it would be quite possible to implement such a display in a standard teletext decoder.
Teletext has a basic graphics facility using a 2 x 3 matrix of blocks that is the same size as a text character.
On the left is a solid ('contiguous') character, and on the right the same character in separated ('non-contiguous' or 'mosaic') mode.
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Compiled by Alan Pemberton
Sheffield, South Yorkshire, England
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