46 lines
2.3 KiB
Plaintext
46 lines
2.3 KiB
Plaintext
|
|
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>
|