Ceefax line by line
Jeremy Brayshaw explains the secrets of teletext transmissions
CEEFAX is broadcast along with the BBC television picture. But
have you ever wondered how? How can both a TV signal and the Ceefax
pages be transmitted without interfering with each other?
The secret lies in the FBI. No, not the American G men but a
section of the TV picture.
A normal TV picture is made up of a series of horizontal lines.
If you look closely at a screen you may be able to see the individual
rows of the picture. Some 625 lines are used to build up the complete
frame. But where is teletext?
Each frame of the picture is drawn by an electron beam. This
starts drawing the picture at the top lefthand corner. By the
time it completes painting in one frame, the beam ends up pointing
at the bottom righthand corner of the tube.
Before starting on the next frame of the picture, it has to
move itself from the bottom right to the top left. During this
time, it must not draw anything, otherwise a diagonal stripe would
appear across the screen.
The process of moving the beam back to its starting position
is called the Field Blanking Interval (FBI). During this time,
the action of the TV tube must be suspended, or blanked, so as
not to draw a stripe across the screen, even if the TV signal
is still sending information.
In fact, the time taken during the FBI is about equivalent to
that taken to send up to 25 TV picture lines. As these lines are
not used for the TV picture, they are an ideal place to add teletext
Some of these extra lines (about six on each channel at the
moment) are used to carry the teletext signal which is transmitted
in binary form.
As the lines were designed to carry a picture, it is a simple
process to code a line to carry binary data.
If the line is divided into equal length sections, each unit
can represent one binary bit. If the section is black, this represents
a zero, if white it represents a one.
In this way, a single television picture line is split up into
360 sections, which represent 45 eight bit bytes.
Most TV sets take less than 25 lines for the FBI so you can
actually see these teletext data lines at the top of the normal
TV picture by adjusting the vertical height control to pull the
top of the screen down.
How, then, does a TV set cope with this? Well, a normal set
without teletext capability simply treats them as any other picture
Any lines received during the FBI are effectively ignored and
any received between the end of the FBI and the top of the picture
are adjusted off the top of the screen.
If this is a teletext set, circuitry inside the receiver also
detects and decodes these rows.
This circuitry is set to scan all the 25 lines transmitted during
the FBI. It can detect which lines carry a teletext signal by
the first three bytes on the row. If this produces rubbish, it
ignores the rest of the line.
If, however, it detects a specific predefined pattern of white
and black (representing the binary sequence 10101010 10101010
11100100) then it knows it has found a teletext signal and must
decode the rest of the line.
The reason for the alternating 1 and 0 pattern of the first
two bytes is simple. These bytes are used by the decoder to determine
the size of each section of the line representing a single binary
It measures the time taken for the section of the line to change
from white (binary 1) to black (binary 0). Given a preset sequence
of 1s and 0s it can work out the time taken for each bit to be
sent, and apply this timing to the rest of the line.
These initial codes are used to set the internal clock of the
decoder and are known as the clock run in sequence.
The problem of determining where each bit starts and ends may
be solved with the clock run-in sequence, but how does the decoder
know where each byte starts?
This is the purpose of the next byte, binary 11100100. As this
is always the same number, and it always appears immediately after
the clock run-in bytes, this sequence can be used to determine
the start and end points of each byte.
All it does is to break the rest of the line into chunks equivalent
in length to a single bit. It then groups these in units of eight,
starting immediately after this 11100100 binary sequence, known
as the framing code.
This leaves 42 bytes on each TV line, which are used by the
decoder to form the teletext display. The first two are used to
identify the row as belonging to a particular Ceefax page, and
each of the remaining 40 bytes represents a character for display.
There are just 40 bytes of data on each television line. This
directly relates to the length of a single row of text on a teletext
display (40 characters).
As a teletext page requires 24 rows of text, this will take
24 television lines for its transmission. As only six TV lines
can be used per transmitted picture frame, a single teletext page
can be transmitted with every four frames.
This may seem a long time until we realise that a complete TV
picture frame is transmitted every 1/50th of a second, so the
Ceefax magazine can be broadcast at the rate of 12.5 teletext
pages per second.
It takes on average around 15 seconds to transmit every available
page. The speed of transmission is nearly seven million bits per
second (actually 6.9375 Mbits/s), but as we have seen, the system
can only use six of the 625 television picture lines.
Now if we could forget about the TV picture and use all 625
lines for teletext . . .