GB2124460A - Editing of teletext - Google Patents

Abstract

To transmit a number of magazines, each normally made up of multiple pages, by Teletext, e.g. from a central station by satellite, on a dedicated channel in such a way that one or more magazines may be provided locally, each raster line interval of a video signal carries digital codes for characters to make up the rows on a page. The same raster lines in successive fields are allotted to the same magazine, thus ensuring that a group of lines allotted to one magazine carry successive blocks of characters of the rows making up the pages of this magazine. To insert locally such a group, which may be left blank in the central transmission or may carry unwanted data, the signal is retransmitted with the raster line spaces of specified number filled with data from the local source.

Description

SPECIFICATION Transmission of digital information in a video teletext network The transmission of digitally encoded textual, pictorial or other information by means of television signals is well known. In a typical teletext service, encoded information might be prepared at a central location, broadcast via satellite to local broadcasters, and retransmitted by them via cable to subscribers.

The basic unit of teletext information is the page, which typically comprises 8,000 bits of information. The pages are grouped into units called magazines, each of which usually contains a single category of information, e.g., general news, sports, weather, etc. This information may, for example, be encoded in eight-bit bytes, of which one video signal scan line can carry about twenty-seven. A considerable number of scan lines is thus required to transmit each page. Each scan line (also called "data packet" herein) begins with information (the "packet address") identifying the page and magazine to which the teletext information in the data packet beiongs.

Teletext information is often transmitted only on the scan lines of the video signal raster that are not used to carry video information, viz. the scan lines of the vertical blanking interval, although video channels devoted entirely to teletext are also known. In both cases, it is conventional to transmit the entire body of teletext information available (several magazines) sequentially. By way of example, in the full-channel case, the first forty scan lines would carry the first page of the first magazine, lines 41-80 would carry the second page, etc. At the end of the first frame (525 lines in the United States), thirteen pages and five lines of the fourteenth have been transmitted. The second field transmitted carries the rest of page 14, all of the next twelve pages, and the first ten lines of the twenty-seventh page.Transmission continues in this manner until the first magazine has been sent. Each of the other magazines is then transmitted in the same way in turn. (It is assumed for the purpose of this example that the first magazine contains more than twenty-seven pages, although, of course, this need not be the case.) By transmitting the more popular magazines more than once in each cycle of transmission of the entire set of magazines, it is possible to reduce the average time a subscriber will have to wait to use the repeated magazines, Teletext information is typically transmitted from a central location, e.g., by satellite, to local broadcasters. A local broadcaster may desire to retransmit some or all of the information he receives from the central broadcaster and may also wish to add locally-provided teletext information to the data stream.

If a local broadcaster desires to transfer a particular magazine from the central broadcaster's video teletext signal to another video signal (a process known as "data bridging"), he must use a computer to monitor the incoming video signal and identify, by the packet address, the desired information. This information must be recorded as it is received, and the information is later inserted into the second video signal. Since the number of data packets may not be the same for each page, it is impossible for the local broadcaster to predict which scan lines, in which field, will contain the information that must be extracted.

If the local broadcaster wants to add a locallyproduced magazine, it is necessary to decode and store all the teletext information borne by the incoming signal, insert the new material at the proper place, and re-encode the data. Because of the data transmission speeds and the quantity of information typically involved, this is a complex, expensive process and requires a powerful minicomputer.

It is the object of the invention to make it possible to transmit teletext data without the above-described drawbacks of conventional systems.

Another object of the invention is to provide a method of transmitting teletext data that takes advantage of the built-in time division of a video signal to time-multiplex the data to be transmitted.

According to the invention, a full television channel is devoted to teletext transmission, and each of the one or more magazines which it is desired to transmit on that channel is assigned to a respective set of one or more specific, preferably contiguous, scan lines. For example, magazine no.

123 might be assigned to lines 17-19. The contents of each magazine are transmitted only on the scan lines assigned to that magazine.

The number of scan lines need not be the same for each magazine; the number of lines allocated to a given magazine can be determined by the central broadcaster as a function, for example, of that magazine's popularity or length. As a result, the time needed for a specific magazine to be transmitted can be set by the central broadcaster.

This establishes the maximum time it will take for a user's decoder to access a specific page in that magazine, i.e., different magazines in the service can have different cycle times.

If a magazine needs to be inserted by a local broadcaster, the central broadcaster can leave a hole (one or more contiguous blank scan lines) in the raster during the original process of encoding the teletext information for transmission in a video signal. The local broadcaster is then able to insert a new magazine into the data stream by inserting the new magazine into the scan lines that make up the hole. In this manner, the information already present on the signal need not be processed at all in order to insert new material. It will be understood that more than one hole can be left, according to the number of magazines to be added.

It may be desirable to extract a particular magazine from a video teletext signal and transfer it to another video signal. If this is attempted without the invention, an intelligent processor would be needed to detect the desired magazine and to store all of the pages thereof as they are received. The pages would then be repackaged for insertion on the second video signal. If, however, scan line assignment according to the invention is used, only the lines assigned to the magazine to be bridged need be detected, and it is not necessary to store the entire magazine in a buffer memory for bridging.A much smaller memory is therefore needed for data bridging according to the invention than would conventionally be necessary, since the data extracted from the pertinent scan lines in each field can be bridged (i.e., inserted into the second signal) and then erased from the buffer memory before it is necessary to store the data from the corresponding scan lines of the next field.

Thus, assigning data channels or magazines to specific video signal scan line numbers permits many processes to be implemented in a teletext service simply and inexpensively, and allows versatility in formatting the data for transmission.

It will be clear that the invention is applicable to any system in which encoded information is transmitted by means of a video signal.

Many variations and modifications of the process disclosed herein will be apparent to those skilled in the art. Accordingly, the scope of the invention is to be limited only by the terms of the appended claims.

Claims (6)

Claims

1. A process for transmitting information by means of a video signal, comprising transmitting each of n sets of data (n being a natural number greater than 1), in time-multiplexed manner, on a respective channel, each said channel being a respective set Aj (ibeing a natural number from 1 to n, inclusive) of at least one predetermined scan line of a video signal raster, and so transmitting each said set of data repeatedly and continuously.

2. The process of claim 1, wherein at least two said sets of data have different cycle times.

3. A method for processing information carried by means of a video signal, comprising the steps of: receiving a first data-carrying video signal that has been transmitted by means of the process of claim 1; storing the ith said set of data by storing only the portoin of said first signal corresponding to the scan lines included in said set Aj; and inserting the stored data into a second video signal.

4. The method of claim 3, wherein said inserting step is begun before the completion of said storing step.

5. A method for processing information carried by means of a video signal, comprising the steps of: receiving a video signal that has been transmitted by the process of claim 1, at least one set of one or more scan lines carrying no data information; and retransmitting said signal, inserting additional information into the portion of said signal corresponding to said one set of one or more scan lines that previously carried no data information.

6. A method for processing information carried by means of a video signal, comprising the steps of: receiving a video signal that has been transmitted by the process of claim 1; and retransmitting said signal, the portion of said signal corresponding to the scan lines of one said set Aj being deleted and additional information being inserted into said portion of said signal for the retransmission.