Television – Format – Including additional information
Patent
1994-11-28
1996-10-01
Peng, John K.
Television
Format
Including additional information
348478, 348465, H04N 708
Patent
active
055614698
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to detection of information that may be present in a video signal during vertical blanking or overscan intervals. 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.
The first step in extracting auxiliary video information is to locate the auxiliary information. Various approaches may be used depending on the type of information involved. For example, recognition of teletext data characteristics such as the framing code pattern is a method of locating teletext data. Closed caption information in line 21 may be located by counting video lines, e.g. counting horizontal sync pulses.
After the auxiliary video information is located, the information must be extracted. In the case of digital data, a "data slicer" may be used to convert the video signal into binary data. A data slicer typically operates by comparing the video signal level to a reference level known as the slicing level. For video levels that exceed the slicing level, the comparison produces a logic 1. Video levels that are less than the slicing level produce a logic 0. As an example, closed caption data in line 21 of the video signal may exhibit a signal amplitude range of 0 IRE to 50 IRE. For a signal range of 0 IRE to 50 IRE, a slicing level of 25 IRE would be appropriate.
A constant slicing level may not be adequate for all video signals video signal levels may vary depending on the source of the video signal. Utilizing a constant slicing level with varying video signal levels may bias the extracted data undesirably toward logic 0 or logic 1 resulting in erroneous data extraction. For example, a typical NTSC video signal of 1 V peak-to-peak corresponds to an amplitude range of -40 IRE (sync tip) to 100 IRE (peak video). Under these conditions, the peak 50 IRE and 25 IRE slicing levels associated with an auxiliary video signal correspond to 350 mV and 175 mV respectively. However, if the video signal is 0.5 V peak-to-peak rather than 1 V, the 50 IRE peak of the auxiliary video signal would correspond to approximately 175 mV rather than 350 mV. If the slicing level were fixed at 175 mV for the 1 V peak-to-peak signal, 50 IRE auxiliary video signal levels associated with the 0.5 V peak-to-peak signal would not exceed the slicing level. A logic 1 within the auxiliary video data would never be extracted. Thus, it is desirable to adapt the slicing level to the amplitude of the input video signal.
NTSC video signals typically exhibit a black level at 0 IRE, a negative-going sync pulse peaking at -40 IRE, and a peak picture level of 100 IRE. Although the absolute peak-to-peak amplitude range of typical video signals may not be constant, the ratio in IRE units between peak sync (-40 IRE) and peak picture (100 IRE) of typical signals is constant. If
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Emanuel Peter M.
Peng John K.
Shedd Robert D.
Thomson Consumer Electronics S.A.
Tripoli Joseph S.
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