Overview of Broadcast Teletext Systems for NTSC Television Standards
|Author(s):||J. R. Storey ; H. G. Bown ; C. D. O'Brien ; W. Sawchuk|
|Publisher:||Society of Motion Picture and Television Engineers|
|Publication Date:||1 February 1980|
|Conference Location:||Toronto, ON, Canada|
|Conference Date:||1 February 1980|
|Page(s):||116 - 129|
The development of the British teletext and viewdata systems has sparked a new interest in information services for the general public. In this paper, the term broadcast teletext is used to refer to one-way broadcast information services whereas the generic term, videotex, is used to refer to interactive services carried over public telephone networks. Broadcast teletext refers to the transmission of digital data for direct reception by the public. It is intended to provide a limited one-way information service through the cyclic repetition of pages of information transmitted as ancillary signals on normal television programs. The information contained in both videotex and broadcast teletext signals is received by suitably supplemented television receivers and displayed on the television screen as textual messages or graphic images. Two spare lines in the Vertical Blanking Interval (VBI) of each field of the television signal are used in the United Kingdom to transmit teletext signals, but more lines could be dedicated to this use if they became available. The use of only two lines in the VBI provides a strictly limited service with the public only having access to a few hundred pages of information. However, as it is a broadcast service, all members of the public with teletext decoders and within range of the television station can access the information at the same time. Of course, a much improved one-way broadcast service could be provided by using the full bandwidth of a television channel to provide public access to a few thousand pages of information. This could easily be provided on cable television networks in North America which often have sufficient bandwidth for up to 35 television channels. On the other hand, people with videotex decoders can have access to data bases containing over 100,000 pages by communicating interactively over the public telephone network: but in this case, the service may be limited to only a few hundred users at a time. Cable television operators with two-way networks could also provide an interactive service of this kind. Other configurations are feasible and include hybrid configurations in which page requests are transmitted over the telephone network to an information supplier who then inserts data in the broadcast teletext cycle of the local television station.