It is well known that many services are provided which enable users to display on screen digital data provided to subscribers. For example, subscribers can receive services related to business and stock market quotations, where the stock market prices are transmitted over telephone lines and are received by a modem at the subscriber's terminal. Having a computer and keyboard entry device enables the subscriber to selectively access digital data which is sent over the telephone lines in order to display this data on the computer screen.
Many services of the type described in the previous paragraph are available, except that the expense of the service is not trivial. Also, the subscriber generally has to have a computer and a modem in order to be able to fully participate in the range of service offered to the subscribers.
Many tv channels transmit programs including digital data for the purpose of close-captioning. The digital data is transmitted with the video signals and is stored in the vertical blanking interval (VBI). This data can be extracted using a decoder which then re-integrates the translated digital data to a regular video signal that can be displayed on the tv screen. This displays the captions on the screen simultaneously with the video picture.
In more detail, an existing system is a teletext system authorized by the FCC in lines 10-18 of the transmission band. Teletext is a one way data transmision system that is sent out as part of a tv signal, whether the signal is sent out via satellite, cable, or regular broadcast tv. This teletext information is available free of charge to viewers, in contrast with the digital data sent to subscribers of various services of the type mentioned hereinabove. There are many types of teletext services being offered at this time covering topics such as cultural affairs, home and catalog shopping, sports, news, financial information, weather, and other types of statistics. Such teletext information is regularly offered by the major networks.
As noted, teletext text and graphics are transmitted as digital data squeezed into a broadcast television signal in the vertical blanking interval. This interval is the time at the end of each television field when the cathode-ray beam is cut off while it returns to start the next field. The VBI is also used for the transmission of information other than teletext information. Such other uses include closed-captioned information, automatic color-balance information and broadcast test data. Teletext is sent as an endless loop of pages where the data for the pages are transmitted serially at the rate of 13,500 bits per second per VBI line used. The total rate of transmission is dependent upon the number of lines (up to 8) used to transmit the data.
At the user end, a decoder is used to convert the teletext data to a regular video signal that can be displayed on a tv screen. Any of the pages in the loop can be accessed at random. However, because an endless-loop format is used, it takes time for each page to come around in the loop. This means that there is a slight delay between the time the page number is entered and the time that the page actually appears on the tv screen. In turn, this imposes a practical limit to the number of pages that a teletext service can offer. One way to alleviate this delay is to transmit the more important pages of information more than once within the endless loop, so that these pages will come up faster. For example, indices are transmitted several times in the loop, since these pages are more important to the users.
When using teletext services, it is not possible to access any page of information without the attendant delay in being able to extract and display the digital information. Further, there is no provision for permanently storing a page of information that is interesting to the user. Rather, the endless loop of information is continuously updated and is often changed so that a desired page is no longer part of the loop of information that is transmitted. Since the presently available teletext decoders are rather expensive and further since the ease of extracting information is limited, such systems have not found great popularity. On the other hand, the online services, while solving many of the teletext services problems, require expensive equipment and are expensive due to their high subscription rates.
Accordingly, it is an object of the present invention to provide an inexpensive apparatus for use with conventional tv sets which will economically enable consumers to utilize the digital data sent with video signals.
It is another object of the present invention to provide a system for use with a conventional tv set which enables one to extract and use digital data sent with tv signals in a manner wherein such information can be extracted, permanently stored, and retrieved for display on a tv screen at any time.
It is another object of the present invention to provide and apparatus enabling the ready extraction, storage, and retrieval for display on a tv screen of digital data sent along with video signals, where the restrictions of an endless-loop format are overcome.
It is another object of this invention to provide an apparatus for extracting and storing digital data sent along with video signals, where the updating of the digital data sent with the video signal does not preclude the display on the tv screen of digital data which is no longer being transmitted with the video signals.
It is another object of this invention to provide a system enabling expanded use of a conventional tv set wherein digital data sent with the tv signal can be selectively accessed and displayed on the tv screen.