How teletext works

Find out how the hundreds of pages of Ceefax get on to your screen.

Volume 5

Number 3

May 1987

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 information.

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 line.

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 digit.

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 . . .