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Friday, 27 February 2009 By: Steve Warner

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Teletext Information

 

What is Teletext


 

2 PAL Fields PAL Teletext Lines in VBI
Figure 1 - BBC1 2 Fields of Bill Clinton Walking down the
Step of the White House
Figure 2 - 12 Teletext lines in the field blanking interval
3 PAL lines
Figure 3 - BBC1 Teletext Line at 4-JUN-1995 17:18:08

Teletext is transmitted during the unused lines of a Television picture, which may be seen at the top of the picture of a television operating with reduced height. The signal, if seen look like a series of white small dots that flicker as each field is scanned. Each line is capable of transmitting 40 useful bytes of data, another 5 bytes are used for synchronising the clock of the receiver.

The time at which the teletext information is transmitted is called the blanking interval. This is an interval that is unused due to the problem of depleting the stored emf within the scan coils. This will be discussed further within this report.
The television picture transmitted here in the UK is interlaced, this means that for a complete picture to be displayed there has to be two fields, this is for the odd and even lines that make up the picture. Two fields can be seen in figure 1, 2 fields make one frame. At the beginning of every field there are the teletext data lines, this can be seen in figure 2 and an example of the teletext line can be seen in figure 3.
Teletext is becoming a viable method of distributing data from one point to multiple points with relatively cheap receivers.

Teletext arose because of a historical accident. When Alan Blumlein layed down the standards for electronically scanned television in the 1930's, he decided to have a relatively long vertical flyback time - the time it takes for the electron beam to return from the bottom of the screen to the top after the picture had been scanned. This was because the electron beam was deflected, as it is today, by an electromagnet, and a rapid change in the current through this magnet would induce a large back EMF in it. Blumlein was worried that this high voltage would arc through the glass of the cathode ray tube, and make a hole in it.


CRT Flyback

During the vertical flyback time, obviously no picture information could be transmitted. It was a waste of transmission time. This time was eventually put to a variety of uses. Engineering test signals were added to the signal to check the performance of the broadcast television equipment. Then, in October 1972, the BBC announced the idea of a system of Text screens, which could be selected by the viewer, and displayed on the television screen. In September 1974, the Teletext service (as it became known) was started as an experimental service. The standard, however, is almost unchanged today, although several so-called Extension Packets have been defined.

Video Waveform

The teletext line consists of a row of characters. In the European version of teletext, there are 40 characters per line. These characters occupy an area of 40 centre of the 52 active video region of the teletext line. There are two 6 borders on each side of the character block. As there are 40 characters per 40 , each character occupies 1 .
The rows of the teletext picture are transmitted sequentially. This means that the time needed to transmit and therefore display a teletext page is dependent on the number of teletext lines transmitted with every field of the television signal. The teletext page is 24 rows in height. It uses the centre 240 lines of the screen. Each row is ten lines high on the television screen. This is why it is more reminiscent of a home computer chunky graphics display than an ordinary television picture. The display technology of the home computer is similar in structure and this similarity is utilised by some of the teletext services. Datacast lines are not used by ordinary teletext decoders but it would not be impossible to receive these lines. These services, how they are received and their relative security are covered in a later section dealing specifically with "BBC Datacasting".

Bytes in Header and Display Teletext Line
1-2 3 4-5 6-7 8-11 12-13 14-45
Header CRI
10101010
Framing
11100100
Magazine & Row
Address
Page Number Time Control Group Display Data
Display Line CRI
10101010
Framing
11100100
Magazine & Row
Address
Display Data









The first two bytes in each line are 10101010. This format is known as a Clock Run In, CRI. They synchronise the system clock to the data rate of the teletext data. The frequency of these bytes is 6.9375 MHz. The lock up of the system clock with the data is essential.

The third byte is the framing code, 11100100. The purpose of this code is to allow the decoder to recognise the beginning and end of each byte of data.

The fourth and fifth bytes are the magazine and row address codes. These codes are used to ensure that the rows of each page of teletext are displayed in the correct order on the television screen.

From this byte onwards the Display line carries only display data

The sixth & seventh byte carries the page number.

The eighth to the eleventh bytes contain information about timed teletext pages.

The twelfth and thirteenth bytes are the control Group

The fourteenth byte onwards contains display data.



Video Signal Width (64µS-12.05µS-2*6µS) = 40µS
The number of bits transmitted per line = 360

Data Pulse Width = = 111nS
Transmitted Bit Rate = 444 * Line Frequency
Line Frequency = = 15.625 Hz
Burst Bit Rate = = 6.9375 Mbit/s

The Data rate is actually a burst rate, meaning that when data are being transmitted, they go at that speed, but because there are also moments when no data are being transmitted, the effective data rate is considerably less.

The transmitted waveform for a teletext line is shown in figure 10. It can be seen that the peak-to-peak voltage is 0% to 66% video modulation.

Teletext Waveform

Teletext Transmissions

 

Data Pulse Train

 

NRZ Data Pulse Train

Figure 11 - A NRZ Formatted Byte

The transmission used by teletext is a NRZ (Non-Return to Zero) signal, with suitably phase corrected shaping filters is as shown in the figure 12. This also reduces the intersymbol interference as there is minimal energy above 5MHz.

Data Pulse Spectrum

Figure 12 - Data Pulse Spectrum

The corresponding one-bit pulse is indicated in figure 13.

One-Bit Data Pulse

Figure 13 - Data Pulse


Teletext Line Format

Bytes in the Teletext Transmission
2 1 2 2 2 2 2 40
Header Clock Run-In Magazine & Row Address Page
Number
Address
Time
Minutes
Time
Hours
Control
Bits
Display
Characters
10101010 11100100









The data line in table 1 comprises of 360 bits which may be considered as 45 eight-bit bytes. The first three bytes, which have even parity serve to synchronise the bit and byte recovery operation in the receiver. The remaining 42 bytes have odd parity and carry address and control information. The use of odd parity during the variable part of the data line ensures that there are never more than 14 bit periods between the data level transitions in the waveform. This simplifies the recovery of the bit rate clock directly from the data waveform.
All the address and page control information is transmitted using Hamming Code Bytes to reduce the possibility of the
wrong character rows being stored in the receiver.

 

Teletext Packet Breakdown

 

Packet Chapter Definitions
X/24 Fastext / Page Extensions
X/25 Telesoftware
X/26 Enhanced Display Facilities and VPT
X/26/0 Display Enhancement
X/27 Linked Pages
X/27/0..3 Editorial
X/27/4..7 Composition
X/28 Page Related Redefinition
X/28/0 Data Designations & Colour Def
X/28/1 Character Set Designation
X/28/2 Conditional Access
X/29 Magazine Related Redefinition
X/29/0 Colour Redefinition
X/29/1 Character Set Designation
X/29/4
8/30 Broadcasting Service Data Packet / PDC
8/30/0 PDC - Unified Date & Time
8/30/1 PDC - Unified Date & Time
8/30/2 VPT - VCR Programming
8/30/3 VPT - VCR Programming
8/30/4-30/15
/31 Independent Data Line




iTc Line Definitions

 

Line Description Line Description
6 Not Used 318-319
7-9 Commercial Teletext Licence 320-322 Commercial Licence
10-17 Public Teletext Service Licence 323-329 Public Teletext Service Licence
18 Programme Related Teletext 330-331 Programme Related Teletext
19-20 Television Test Signals 332-333 Television Test Signals
21 Schedule Information (Internal Use) 334 Schedule Information (Internal Use)
22 Quiet Line 335 Programme Subtitles
23 First Line of Field 1 Television Picture 336 First Line of Field 1 Television Picture





Teletext Levels

Teletext Level Description
1 The Initial specifications set out by the BBC, IBA, BREMA in September 1976 permitting the production of domestic television sets with Teletext. The specification continued to be developed as additional features became available.
2 Provided Multi-language text, and a wider range of display attributes that may be non-spacing. There is a wider range of colours and an extended mosaic pictorial set.
3 Introduced dynamically redefined character sets (DRCS) permitting the display of non-Roman characters, for example Arabic or Chinese. Pictorial Graphic characters may also be defined, allowing the composition of improved illustrations for the text compared with earlier versions.
4 Includes full geometric graphics, and requires computing power to generate the display from a sequence of drawing instructions. This permits graphic displays as good as the highest resolution mode of the BBC-B computer. This level offers a colour palette of over 250,000 shades.
5 Is Full-definition still pictures, permitting an image of a better quality than achievable from a video camera. It has no losses due to modulating onto a carrier, and no noise added to the picture during transmission.



Levels 4 & 5 exist as specifications although level 4 was transmitted by the IBA as long ago as 1981. There appears to be no television sets able to handle these levels, and until the editors of CEEFAX etc. use it, the extra cost would not be worth while.

Currently Transmitted Level

The currently transmitted level is Level 1. Teletext Ltd (Main independent Teletext Transmitter) has plans to introduce a Level 2 service from Autumn 1995. After the new Level 2.5 & 3.5 specifications have been set.

An Early Experimental Mullard decoder utilising TTL technology


Experimental Mullard decoder

 

Teletext Character Set


Teletext Character Set

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