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| Format of embedded V4L2_MPEG_STREAM_VBI_FMT_IVTV VBI data |
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| 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. |
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| 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. |
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| 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: |
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| '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. |
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| 'ITV0': This magic number assumes all VBI lines are captured, i.e. it implicitly |
| implies that the bitmasks are 0xffffffff and 0xf. |
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| After these magic cookies (and the 8 byte bitmask in case of cookie 'itv0') the |
| captured VBI lines start: |
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| 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. |
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| Here is the list of possible data types: |
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| #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) |
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| Hans Verkuil <hverkuil@xs4all.nl> |
