Teletext
Teletext is an information service in the form of text that is broadcast along with the television signal. It was created in the 1970s by the British John Adams of the BBC. It was then known as Ceefax. It consists of inserting text and color blocks in the television signal, displaying the information as long as the television is compatible with this technology (in Television Settings if you have the option "Closed Captions: CC1, CC2, Text1, etc&# 3. 4;).
It was very popular in Europe but over time it has been disappearing from some European stations. On October 12, 2012, the BBC permanently removed the Ceefax/Teletext service, leaving Germany, Spain and Italy as countries where Teletext is still active and successful.
Introduction to Teletext
Teletext consists of a set (more or less extensive) of pages, which are chosen through the remote control. The page number (three digits) is written and after a while, it is displayed on the television screen. It only allows the transmission of written information and graphics.
There may also be "links", jumps or shortcuts between pages in the last row of the screen, which are activated through the colored buttons on the remote control. Some pages are of the rolling type, which means that different "subpages" from time to time, although the page number is the same. This is indicated (for example) like this: 2/4, which means that we are looking at the "subpage" 2 of 4 that exist in total.
Teletext bears certain similarities with videotex, a system that also offers information pages on television. Unlike teletext, videotex requires a decoder and allows the user to not only receive but also send information.
History
The pioneer of these services was the one launched by the British Post Office. The first channel to incorporate teletext was the BBC in 1972. Later, other countries such as Austria, Germany and the Netherlands also began to use this unique system.
In Spain, TVE began broadcasting uninterrupted teletext on May 16, 1988, although test broadcasts began during the 1982 Soccer World Cup. The main purpose of this system, in addition to informing, was to provide a service to people with hearing disabilities. In this way they began to create subtitles for the main programs of the stations.
Among the queries made, the viewer could receive breaking news, sports results or even the winning numbers of draws dependent on the Lottery or ONCE. In short, it offered services similar to those of any newspaper and, therefore, teletext came to reduce readers to the printed press.[citation required]
In the radio there is a similar system called radio text or RDS. In Digital Terrestrial Television, in addition to teletext, information about the channels and their programming is sent through EPG (electronic program guide) and MHP and HbbTV applications.
Transmission
The television signal is made up of lines grouped into fields and frames. Each frame (the equivalent of what a frame is in cinema) has two fields and each field is made up of a specific number of lines, which depends on the television system used. Each line incorporates the light and color information, luminance and chrominance, and the necessary synchronisms for its subsequent presentation in the TV receiver. These syncs are called horizontal sync and color salvo. The even and odd fields that form a square also have their synchronism system, which is called vertical synchronism. This synchronism is formed by the equalization pulses, before and after, synchronism pulses and recovery lines. These last lines do not carry image information, they are empty, since their duration time was used to stabilize the vertical deflection circuits that the existing technology at the time the television signal was defined (thermionic valve) required a long time for its stability to recover from the oscillation.
The new solid-state technologies have left these vertical synchronism lines without reason and have been used for the transmission of other information and services, such as VIT signals (VITS) and teletext, which uses a digital coding system. In this way, unprepared receivers simply ignore these lines, and prepared ones can pick up this signal without affecting the image quality for other people.
Teletext levels
Level 1 (basic alphamosaic)
It is the basic initial system created in 1976, whose basic repertoire is:
- 96 alphanumeric characters.
- 64 graphic characters of mosaic type (continuous or separated).
- 30 control characters.
Attributes are in serial mode, and take up space on screen and in memory. Characters are displayed with the current attribute until a new one is received.
Level 1.5
It is the one used in Spain. Created by TVE in 1981 as an extension of level 1 to support up to 128 alphanumeric characters, with a black background and 4-color code that allows faster access to the most consulted pages.
Level 2 (improved alphamosaic)
Created in 1995. It has a more extensive repertoire of characters, and it stands out that the attributes are in parallel mode, without taking up space on the screen. The generator checks if the character is affected by any attribute before displaying it on the screen. It is used by the French ANTIOPE system.
Level 2.5 (hi-text)
Improvement of level 2 created in the year 2000, with the following characteristics:
- Extended language repertoire.
- The attributes do not occupy any space.
- Increase the palette with up to 4016 colors, with colors and redefinable characters.
- It has additional on-screen space reserved for two side panels with more information.
Level 3 (alpha-DRCS)
Includes movable or dynamically redefinable characters (DRCS). The graphic characters are mosaics of 6x4 cells, which is why it presents a higher quality and resolution.
Level 3.5 (Extended DRCS)
Extends the number of redefinable characters and their complexity. Also, support is introduced to be able to use different font styles and proportional spacing.
Level 4 (alphageometric)
It has a greater number of characters, where a high quality of graphics is already appreciated. The receivers to display this teletext are more complex and more expensive. The color palette extends to 250,000 different shades.
Level 5 (alphaphotographic)
Allows transmission of still photographic images at television resolution. Each image is digitized and transmitted point by point, to be stored in the receiver. Despite its numerous drawbacks (longer waiting time, requirement of a large memory capacity...) it is the system adopted in Japan due to the complexity of the characters.