To find out which data services are supported by a sliced
VBI capture or output device, applications initialize the
type field of a struct v4l2_sliced_vbi_cap,
clear the reserved array and
call the VIDIOC_G_SLICED_VBI_CAP ioctl. The
driver fills in the remaining fields or returns an EINVAL error code if the
sliced VBI API is unsupported or type
is invalid.
Note the type field was added,
and the ioctl changed from read-only to write-read, in Linux 2.6.19.
Table 1. struct v4l2_sliced_vbi_cap
| __u16 | service_set | A set of all data services
supported by the driver. Equal to the union of all elements of the
service_lines array. | ||
| __u16 | service_lines[2][24] | Each element of this array contains a set of data services the hardware can look for or insert into a particular scan line. Data services are defined in Table 2. Array indices map to ITU-R line numbers (see also Figure 4-2 and Figure 4-3) as follows: | ||
| Element | 525 line systems | 625 line systems | ||
service_lines[0][1] | 1 | 1 | ||
service_lines[0][23] | 23 | 23 | ||
service_lines[1][1] | 264 | 314 | ||
service_lines[1][23] | 286 | 336 | ||
The number of VBI lines the
hardware can capture or output per frame, or the number of services it
can identify on a given line may be limited. For example on PAL line
16 the hardware may be able to look for a VPS or Teletext signal, but
not both at the same time. Applications can learn about these limits
using the VIDIOC_S_FMT ioctl as described in Section 4.8. | ||||
Drivers must set
service_lines[0][0] and
service_lines[1][0] to zero. | ||||
| enum v4l2_buf_type | type | Type of the data stream, see Table 3-2. Should be
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE or
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT. | ||
| __u32 | reserved[3] | This array is reserved for future extensions. Applications and drivers must set it to zero. | ||
Table 2. Sliced VBI services
| Symbol | Value | Reference | Lines, usually | Payload |
|---|---|---|---|---|
V4L2_SLICED_TELETEXT_B (Teletext
System B) | 0x0001 | ETS 300 706, ITU BT.653 | PAL/SECAM line 7-22, 320-335 (second field 7-22) | Last 42 of the 45 byte Teletext packet, that is without clock run-in and framing code, lsb first transmitted. |
V4L2_SLICED_VPS | 0x0400 | ETS 300 231 | PAL line 16 | Byte number 3 to 15 according to Figure 9 of ETS 300 231, lsb first transmitted. |
V4L2_SLICED_CAPTION_525 | 0x1000 | EIA 608-B | NTSC line 21, 284 (second field 21) | Two bytes in transmission order, including parity bit, lsb first transmitted. |
V4L2_SLICED_WSS_625 | 0x4000 | EN 300 294, ITU BT.1119 | PAL/SECAM line 23 | Byte 0 1
msb lsb msb lsb
Bit 7 6 5 4 3 2 1 0 x x 13 12 11 10 9 |
V4L2_SLICED_VBI_525 | 0x1000 | Set of services applicable to 525 line systems. | ||
V4L2_SLICED_VBI_625 | 0x4401 | Set of services applicable to 625 line systems. | ||