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- Format of embedded V4L2_MPEG_STREAM_VBI_FMT_IVTV VBI data
- =========================================================
- This document describes the V4L2_MPEG_STREAM_VBI_FMT_IVTV format of the VBI data
- embedded in an MPEG-2 program stream. This format is in part dictated by some
- hardware limitations of the ivtv driver (the driver for the Conexant cx23415/6
- chips), in particular a maximum size for the VBI data. Anything longer is cut
- off when the MPEG stream is played back through the cx23415.
- The advantage of this format is it is very compact and that all VBI data for
- all lines can be stored while still fitting within the maximum allowed size.
- The stream ID of the VBI data is 0xBD. The maximum size of the embedded data is
- 4 + 43 * 36, which is 4 bytes for a header and 2 * 18 VBI lines with a 1 byte
- header and a 42 bytes payload each. Anything beyond this limit is cut off by
- the cx23415/6 firmware. Besides the data for the VBI lines we also need 36 bits
- for a bitmask determining which lines are captured and 4 bytes for a magic cookie,
- signifying that this data package contains V4L2_MPEG_STREAM_VBI_FMT_IVTV VBI data.
- If all lines are used, then there is no longer room for the bitmask. To solve this
- two different magic numbers were introduced:
- 'itv0': After this magic number two unsigned longs follow. Bits 0-17 of the first
- unsigned long denote which lines of the first field are captured. Bits 18-31 of
- the first unsigned long and bits 0-3 of the second unsigned long are used for the
- second field.
- 'ITV0': This magic number assumes all VBI lines are captured, i.e. it implicitly
- implies that the bitmasks are 0xffffffff and 0xf.
- After these magic cookies (and the 8 byte bitmask in case of cookie 'itv0') the
- captured VBI lines start:
- For each line the least significant 4 bits of the first byte contain the data type.
- Possible values are shown in the table below. The payload is in the following 42
- bytes.
- Here is the list of possible data types:
- #define IVTV_SLICED_TYPE_TELETEXT 0x1 // Teletext (uses lines 6-22 for PAL)
- #define IVTV_SLICED_TYPE_CC 0x4 // Closed Captions (line 21 NTSC)
- #define IVTV_SLICED_TYPE_WSS 0x5 // Wide Screen Signal (line 23 PAL)
- #define IVTV_SLICED_TYPE_VPS 0x7 // Video Programming System (PAL) (line 16)
- Hans Verkuil <hverkuil@xs4all.nl>
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