Television – Synchronization – Sync separation
Patent
1995-02-09
1997-04-08
Kostak, Victor R.
Television
Synchronization
Sync separation
348533, 348468, H04N 7087
Patent
active
056192754
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to detection of information that may be present in a video signal during vertical blanking intervals.
BACKGROUND
A video signal typically includes vertical display intervals or fields, having a plurality of horizontal line intervals, e.g. 262.5 lines per field in NTSC video systems. The beginning of each vertical and horizontal interval is identified by respective vertical and horizontal sync pulses that are included in a composite video signal. During a portion of each vertical interval, information in the video signal may not be intended for display. For example, a vertical blanking interval spans approximately the first 20 horizontal line intervals in each field. In addition, several line intervals adjacent to the vertical blanking period, e.g. line 21, may be within an overscan region of a video display and will not be visible.
The lack of displayed image information during blanking and overscan intervals makes it possible to insert an auxiliary information component, e.g. teletext or closed caption data, into these intervals. Standards such as Federal Communications Commissions (FCC) Regulations define the format for each type of auxiliary information including the positioning of the information within a vertical interval. For example, the present closed captioning standard (see e.g. 47 CFR .sctn..sctn.15.119 and 73.682) specifies that digital data corresponding to ASCII characters for closed captioning must be in line 21 of field 1. Future modifications to the standard may permit auxiliary information such as closed caption data to be located in other lines, e.g. line 21 of every field.
Auxiliary video information is extracted from the video signal using a decoder. Acceptable performance of the decoder requires reliable identification of the particular lines and fields of the video signal that include auxiliary video data. Examples of approaches to identifying particular video line intervals are disclosed in JP-A-57 052 280 and U.S. Pat. No. 4,172,262. If video signal strength decreases (e.g. caused by poor reception), the probability of errors in the decoder output increases. For example, detection of synchronizing events in a video signal, such as vertical sync pulses, may provide a timing reference point for identifying video lines (e.g. line 21) that may contain auxiliary video information. However a decrease in video signal strength causes a corresponding decrease in sync signal amplitude. Attempting to decode auxiliary video information from a weak video signal increases the likelihood either that the desired synchronizing pulses may not be detected or that noise pulses may be erroneously interpreted as the desired synchronizing pulses. As a result, a decoder may operate incorrectly in response to a weak video signal.
The error rate of the decoder should remain at an acceptable level as long as the strength of the video signal is sufficient to produce a "viewable" picture. The point at which the error rate becomes unacceptable depends on the content of the auxiliary information. For example, an error rate causing loss of several characters of closed caption text during an interval spanning several minutes may not be objectionable. It is desirable, however, to minimize the error rate over a broad range of video signal strength.
SUMMARY OF THE INVENTION
In accordance with aspects of the present invention, apparatus comprises first detecting means for detecting a first pulse included in a synchronization component of a video signal for indicating the beginning of a vertical display interval of the video signal; a counter responsive to a clock signal having a period representative of a horizontal display interval of the video signal for generating a count representing the number of horizontal display intervals within the vertical display interval after the first detecting means detects the first pulse; means responsive to the clock signal for generating an enable signal defining an enable interval beginning in response to the count being a
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EDN, vol. 35, No. 4, 15 Feb. 1990, Newton (US), p. 185, Andrew K Dart "Simple Circuit Extracts TV Sync Pulse".
Patent Abstracts of Japan, vol. 10, No. 87, 5 Apr. 1986, and Japan Pat. 60-229590 Nippon Denki KK, 14 Nov. 1985 (see above).
Patent Abstracts of Japan, vol. 6, No. 123, 8 Jul. 1982 and Japan Pat. 57-052280 (Sanyo Electric Co., Ltd), 27 Mar. 1982 (see above).
IEEE Transactions on Consumer Electronics, vol. 38, No. 3, Aug. 1992, New York (US), pp. 261-267, N.F. Hurley "A Single Chip Line 21 Captioning Deoder".
Emanuel Peter M.
Kostak Victor R.
Shedd Robert D.
Thomson Consumer Electronics Inc.
Tripoli Joseph S.
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