readme.vbi

linux 内核源代码

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.

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 <[e-mail]>