Patent GB-1198658-A (GB)
Title: An Arrangement for Application of Text in Television Transmission.
IPC: H04N5/278
Filed: 1967-06-30
Abstract: 1,198,658. Television. SVERIGES RADIO A.B. 30 June, 1967 [20 July, 1966], No. 30216/67. Heading H4F. The invention relates to an arrangement for producing a composite picture in which a television picture 10, Fig. 4, is blanked over an area 11 to produce a dark background against which there is displayed a text formed of bright letters, the arrangement being characterized by the use of counters which respond to the text signals and clock pulses occurring at regular time intervals 50 throughout each line and serve to terminate the picture blanking until a position 45 before the text and from a position 47 after the text so that the dark background is confined to the text and has a length appropriate thereto. The arrangement is illustrated in Figs. 1 and 2, where the combination comprising a gate 17 for text signals on lead 16, counters 23, 40, text height sensor 25 (also a counter) and a further counter 35 and switch 37 functions in the manner described in Specification 1,198,657 to develop picture blanking signals which persist throughout the whole length of picture lines and recur for lines corresponding to the height 15 of the text and areas 13 and 14 above and below the text. The use of two counters 23 and 40 for controlling the text height sensor provides for two lines of text 11 and 111. The picture signal over lead 30 is blanked in a circuit 27 and is combined with the text signal from lead 16 over lead 28 in a mixer stage 52 to produce the composite picture signal on lead 72. In accordance with the invention the blanking signal from sensor 25, which appears on leads a and b for the upper and lower lines of the text, is applied to blanking stage 27 through gates 60-63 which are controlled so as to allow passage of the signal only between the limits 45 and 47 for the upper text line and limits 46 and 48 for the lower text line. Control is effected by memory counters 52-55 which respond to clock pulses defining the intervals 50 from source 48 and which are started and stopped by the text signals. The text signals appear on leads c and d for the upper and lower lines of text. Gates 60 and 61 determine the operation for the upper line of text. Counter 52 which controls gate 60 is started into operation by the commencement of text signals, produces a count corresponding to slightly less than the duration of a scan line and thereafter produces an output signal every time it counts a number of pulses corresponding to a whole line. Thus, at times corresponding to limit 45, gate 60 is enabled and the blanking signal sets blanking stage 27, which is a bi-stable element, to commence blanking of the picture signal. Counter 53 which controls gate 61 is started into operation at the end of the text line, produces an output at a time corresponding to limit 47 and thereafter, in the same manner as counter 52, produces an output every time it counts a number of pulses corresponding to a whole line. Blanking stage 27 is thereby reset and blanking ceases. Gates 62-63 under control of counters 54 and 55 function in an identical manner for the lower line of text. Counters 52- 55 are reset at the completion of the text signal by a pulse over lead f from gate 17. To ensure that the blanking is not interrupted in intervals between words of text, the signals from gate 17 are passed through a circuit 42 which prolongs the signal for a period sufficient to prevent this happening.