Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Patent
1980-12-30
1983-07-19
Richardson, Robert L.
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
H04N 708
Patent
active
043946879
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to television systems and more particularly to a method and apparatus for decoding digital information processed for inclusion in a wide band T.V. video signal.
BACKGROUND OF THE INVENTION
In any normal television system, the transmission of the wide band video signals which are to produce the actual picture elements on the screen of the receiver, is interrupted between the scanning periods for line and field synchronisation purposes. Consequently there are periods during which no video signals are being transmitted. It is now possible to use these periods for the transmission of data which is not necessarily concerned with the video transmission itself.
Basically, data representable by standard graphical symbols such as alpha-numeric symbols can be transmitted via a restricted channel provided that the rate of transmission is restricted. It is now possible to use periods as aforesaid especially the line times of the field blanking intervals (i.e. the times of the individual lines occurring between fields which correspond with the times occupied by video signals on active picture lines), for the transmission of pages of data. Typically, using 8-bit digital signals representing alpha-numeric characters at a bit rate of 2.5 M bit per second, 50 pages of data each consisting of 22 strips of 40 characters can be transmitted repeatedly in a total cycle time of 90 seconds using only a single line of the field blanking period per field of the 625 line system as operated in the United Kingdom.
Data transmission as described above is already commercially available in the United Kingdom under the name "Teletext."
It is generally accepted that teletext displays should consist of 40 characters per row and ideally for international compatibility 24 rows per page. The U.K. teletext transmission standard specified a data rate of 6.9375 Mbits per second (which has proven to be at the upper reasonable limit of transmission rate for system I, B/G system) so as just to achieve transmission of a complete row of text on one video line of the field blanking time.
The advantage of conveying one row of text on one video line is to achieve maximum economy in requirements for transmission of addressing information needed to correctly position the text information on the displayed page. Since whole rows of text are transmitted on each line, only a row number need be transmitted with each data line of text. Row zero which acts as the page demarcation signal requires additional page numbering information and also incorporates various display and interpretation codes appropriate to the particular page. In order to facilitate parallel magazine working every row of text also incorporates a 3 bit magazine number, being the most significant digit of the page number.
The above structure incorporating as it does one text row on every data line thus results in a very efficient utilization of the transmission facility. Inherent in this teletext transmission system, as just described, is its fixed format nature. Fixed format does not necessarily mean that a certain position of a code on a data line must correspond to a certain position of that character on the screen. When 40 characters are transmitted on a data line however, as in U.K. teletext, it does. Fixed format, more generally, means that each of the 40 character positions on the display screen have been "occupied" by the transmission of one and only one transmission code or alternatively every transmitted character or control code occupies one position on the display screen or equivalently in the display memory. The consequences of fixed format are economy in receiver storage and easy integration towards an error free display in the event of a moderate reception bit error probability.
A further attribute of Teletext is synchronism. By synchronism is meant the fact that control and address codes are synchronized in time to the horizontal sync pulses of the TV system. This means that the teletext decoder has prior knowledge of the p
REFERENCES:
patent: 3961137 (1976-06-01), Hutt et al.
patent: 3982065 (1976-09-01), Barnaby et al.
British Kinematography Sound and Television, vol. 55, No. 9, Sep. 1973.
Hutt Peter R.
Moyce John J.
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