VBI stands for Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen.
Sliced VBI devices use hardware to demodulate data transmitted in the VBI. V4L2 drivers shall not do this by software, see also the raw VBI interface. The data is passed as short packets of fixed size, covering one scan line each. The number of packets per video frame is variable.
Sliced VBI capture and output devices are accessed through the
same character special files as raw VBI devices. When a driver
supports both interfaces, the default function of a
/dev/vbi device is raw VBI
capturing or output, and the sliced VBI function is only available
after calling the VIDIOC_S_FMT ioctl as defined below. Likewise a
/dev/video device may support the sliced VBI API,
however the default function here is video capturing or output.
Different file descriptors must be used to pass raw and sliced VBI
data simultaneously, if this is supported by the driver.
Devices supporting the sliced VBI capturing or output API
set the V4L2_CAP_SLICED_VBI_CAPTURE or
V4L2_CAP_SLICED_VBI_OUTPUT flag respectively, in
the capabilities field of struct v4l2_capability
returned by the VIDIOC_QUERYCAP ioctl. At least one of the
read/write, streaming or asynchronous I/O
methods must be supported. Sliced VBI devices may have a tuner
or modulator.
Sliced VBI devices shall support video input or output and tuner or modulator ioctls if they have these capabilities, and they may support control ioctls. The video standard ioctls provide information vital to program a sliced VBI device, therefore must be supported.
To find out which data services are supported by the
hardware applications can call the VIDIOC_G_SLICED_VBI_CAP ioctl.
All drivers implementing the sliced VBI interface must support this
ioctl. The results may differ from those of the VIDIOC_S_FMT ioctl
when 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 are
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.
To determine the currently selected services applications
set the type field of struct v4l2_format to
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE or
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT, and the VIDIOC_G_FMT
ioctl fills the fmt.sliced member, a
struct v4l2_sliced_vbi_format.
Applications can request different parameters by
initializing or modifying the fmt.sliced
member and calling the VIDIOC_S_FMT ioctl with a pointer to the
v4l2_format structure.
The sliced VBI API is more complicated than the raw VBI API
because the hardware must be told which VBI service to expect on each
scan line. Not all services may be supported by the hardware on all
lines (this is especially true for VBI output where Teletext is often
unsupported and other services can only be inserted in one specific
line). In many cases, however, it is sufficient to just set the
service_set field to the required services
and let the driver fill the service_lines
array according to hardware capabilities. Only if more precise control
is needed should the programmer set the
service_lines array explicitly.
The VIDIOC_S_FMT ioctl modifies the parameters
according to hardware capabilities. When the driver allocates
resources at this point, it may return an EBUSY error code if the required
resources are temporarily unavailable. Other resource allocation
points which may return EBUSY can be the
VIDIOC_STREAMON ioctl and the first read(), write() and
select() call.
Table 4.6. struct v4l2_sliced_vbi_format
| __u32 | service_set | If
On return the driver sets this field to the union of all
elements of the returned | ||
| __u16 | service_lines[2][24] | Applications initialize this array with sets of data services the driver shall look for or insert on the respective scan line. Subject to hardware capabilities drivers return the requested set, a subset, which may be just a single service, or an empty set. When the hardware cannot handle multiple services on the same line the driver shall choose one. No assumptions can be made on which service the driver chooses. Data services are defined in Table 4.7, “Sliced VBI services”. Array indices map to ITU-R line numbers (see also Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering”) 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 | ||
Drivers must set
service_lines[0][0] and
service_lines[1][0] to zero.
The V4L2_VBI_ITU_525_F1_START,
V4L2_VBI_ITU_525_F2_START,
V4L2_VBI_ITU_625_F1_START and
V4L2_VBI_ITU_625_F2_START defines give the start
line numbers for each field for each 525 or 625 line format as a
convenience. Don't forget that ITU line numbering starts at 1, not 0.
| ||||
| __u32 | io_size | Maximum number of bytes passed by
one read() or write() call, and the buffer size in bytes for
the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. Drivers set this field to
the size of struct v4l2_sliced_vbi_data times the number of non-zero
elements in the returned service_lines
array (that is the number of lines potentially carrying data). | ||
| __u32 | reserved[2] | This array is reserved for future extensions. Applications and drivers must set it to zero. | ||
[a] According to ETS 300 706 lines 6-22 of the first field and lines 5-22 of the second field may carry Teletext data. | ||||
Table 4.7. 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 | [CEA 608-E] | NTSC line 21, 284 (second field 21) | Two bytes in transmission order, including parity bit, lsb first transmitted. |
V4L2_SLICED_WSS_625 | 0x4000 | [ITU BT.1119], [EN 300 294] | 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. | ||
Drivers may return an EINVAL error code when applications attempt to
read or write data without prior format negotiation, after switching
the video standard (which may invalidate the negotiated VBI
parameters) and after switching the video input (which may change the
video standard as a side effect). The VIDIOC_S_FMT ioctl may return
an EBUSY error code when applications attempt to change the format while i/o is
in progress (between a VIDIOC_STREAMON and VIDIOC_STREAMOFF call,
and after the first read() or write() call).
A single read() or write() call must pass all data
belonging to one video frame. That is an array of
v4l2_sliced_vbi_data structures with one or
more elements and a total size not exceeding
io_size bytes. Likewise in streaming I/O
mode one buffer of io_size bytes must
contain data of one video frame. The id of
unused v4l2_sliced_vbi_data elements must be
zero.
Table 4.8. struct v4l2_sliced_vbi_data
| __u32 | id | A flag from Table A.86, “Sliced VBI services”
identifying the type of data in this packet. Only a single bit must be
set. When the id of a captured packet is
zero, the packet is empty and the contents of other fields are
undefined. Applications shall ignore empty packets. When the
id of a packet for output is zero the
contents of the data field are undefined
and the driver must no longer insert data on the requested
field and
line. |
| __u32 | field | The video field number this data has been captured
from, or shall be inserted at. 0 for the first
field, 1 for the second field. |
| __u32 | line | The field (as opposed to frame) line number this
data has been captured from, or shall be inserted at. See Figure 4.2, “ITU-R 525 line numbering (M/NTSC and M/PAL)” and Figure 4.3, “ITU-R 625 line numbering” for valid
values. Sliced VBI capture devices can set the line number of all
packets to 0 if the hardware cannot reliably
identify scan lines. The field number must always be valid. |
| __u32 | reserved | This field is reserved for future extensions. Applications and drivers must set it to zero. |
| __u8 | data[48] | The packet payload. See Table A.86, “Sliced VBI services” for the contents and number of bytes passed for each data type. The contents of padding bytes at the end of this array are undefined, drivers and applications shall ignore them. |
Packets are always passed in ascending line number order,
without duplicate line numbers. The write() function and the
VIDIOC_QBUF ioctl must return an EINVAL error code when applications violate
this rule. They must also return an EINVAL error code when applications pass an
incorrect field or line number, or a combination of
field, line and
id which has not been negotiated with the
VIDIOC_G_FMT or VIDIOC_S_FMT ioctl. When the line numbers are
unknown the driver must pass the packets in transmitted order. The
driver can insert empty packets with id set
to zero anywhere in the packet array.
To assure synchronization and to distinguish from frame dropping, when a captured frame does not carry any of the requested data services drivers must pass one or more empty packets. When an application fails to pass VBI data in time for output, the driver must output the last VPS and WSS packet again, and disable the output of Closed Caption and Teletext data, or output data which is ignored by Closed Caption and Teletext decoders.
A sliced VBI device may support read/write and/or streaming (memory mapping and/or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.
If a device can produce an MPEG output stream, it may be capable of providing negotiated sliced VBI services as data embedded in the MPEG stream. Users or applications control this sliced VBI data insertion with the V4L2_CID_MPEG_STREAM_VBI_FMT control.
If the driver does not provide the V4L2_CID_MPEG_STREAM_VBI_FMT
control, or only allows that control to be set to
V4L2_MPEG_STREAM_VBI_FMT_NONE, then the device
cannot embed sliced VBI data in the MPEG stream.
The V4L2_CID_MPEG_STREAM_VBI_FMT control does not implicitly set the device driver to capture nor cease capturing sliced VBI data. The control only indicates to embed sliced VBI data in the MPEG stream, if an application has negotiated sliced VBI service be captured.
It may also be the case that a device can embed sliced VBI data in only certain types of MPEG streams: for example in an MPEG-2 PS but not an MPEG-2 TS. In this situation, if sliced VBI data insertion is requested, the sliced VBI data will be embedded in MPEG stream types when supported, and silently omitted from MPEG stream types where sliced VBI data insertion is not supported by the device.
The following subsections specify the format of the embedded sliced VBI data.
The
V4L2_MPEG_STREAM_VBI_FMT_NONE embedded sliced VBI
format shall be interpreted by drivers as a control to cease
embedding sliced VBI data in MPEG streams. Neither the device nor
driver shall insert "empty" embedded sliced VBI data packets in the
MPEG stream when this format is set. No MPEG stream data structures
are specified for this format.
The
V4L2_MPEG_STREAM_VBI_FMT_IVTV embedded sliced VBI
format, when supported, indicates to the driver to embed up to 36
lines of sliced VBI data per frame in an MPEG-2 Private
Stream 1 PES packet encapsulated in an MPEG-2
Program Pack in the MPEG stream.
Historical context: This format
specification originates from a custom, embedded, sliced VBI data
format used by the ivtv driver. This format
has already been informally specified in the kernel sources in the
file Documentation/video4linux/cx2341x/README.vbi
. The maximum size of the payload and other aspects of this format
are driven by the CX23415 MPEG decoder's capabilities and limitations
with respect to extracting, decoding, and displaying sliced VBI data
embedded within an MPEG stream.
This format's use is not exclusive to
the ivtv driver nor
exclusive to CX2341x devices, as the sliced VBI data packet insertion
into the MPEG stream is implemented in driver software. At least the
cx18 driver provides sliced VBI data insertion
into an MPEG-2 PS in this format as well.
The following definitions specify the payload of the
MPEG-2 Private Stream 1 PES packets that contain
sliced VBI data when
V4L2_MPEG_STREAM_VBI_FMT_IVTV is set.
(The MPEG-2 Private Stream 1 PES packet header
and encapsulating MPEG-2 Program Pack header are
not detailed here. Please refer to the MPEG-2 specifications for
details on those packet headers.)
The payload of the MPEG-2 Private Stream 1 PES packets that contain sliced VBI data is specified by struct v4l2_mpeg_vbi_fmt_ivtv. The payload is variable length, depending on the actual number of lines of sliced VBI data present in a video frame. The payload may be padded at the end with unspecified fill bytes to align the end of the payload to a 4-byte boundary. The payload shall never exceed 1552 bytes (2 fields with 18 lines/field with 43 bytes of data/line and a 4 byte magic number).
Table 4.9. struct v4l2_mpeg_vbi_fmt_ivtv
| __u8 | magic[4] | A "magic" constant from Table 4.10, “Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv
magic field” that indicates
this is a valid sliced VBI data payload and also indicates which
member of the anonymous union, itv0 or
ITV0, to use for the payload data. | |
| union | (anonymous) | ||
| struct v4l2_mpeg_vbi_itv0 | itv0 | The primary form of the sliced VBI data payload that contains anywhere from 1 to 35 lines of sliced VBI data. Line masks are provided in this form of the payload indicating which VBI lines are provided. | |
| struct v4l2_mpeg_vbi_ITV0 | ITV0 | An alternate form of the sliced VBI data payload used when 36 lines of sliced VBI data are present. No line masks are provided in this form of the payload; all valid line mask bits are implcitly set. |
Table 4.10. Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv
magic field
| Defined Symbol | Value | Description |
|---|---|---|
V4L2_MPEG_VBI_IVTV_MAGIC0
| "itv0" | Indicates the itv0
member of the union in struct v4l2_mpeg_vbi_fmt_ivtv is valid. |
V4L2_MPEG_VBI_IVTV_MAGIC1
| "ITV0" | Indicates the ITV0
member of the union in struct v4l2_mpeg_vbi_fmt_ivtv is valid and
that 36 lines of sliced VBI data are present. |
Table 4.11. struct v4l2_mpeg_vbi_itv0
| __le32 | linemask[2] | Bitmasks indicating the VBI service lines
present. These
|
| struct v4l2_mpeg_vbi_itv0_line | line[35] | This is a variable length array that holds from 1
to 35 lines of sliced VBI data. The sliced VBI data lines present
correspond to the bits set in the linemask
array, starting from b0 of
linemask[0] up through b31 of
linemask[0], and from b0
of linemask[1] up through b
3 of linemask[1].
line[0] corresponds to the first bit
found set in the linemask array,
line[1] corresponds to the second bit
found set in the linemask array, etc.
If no linemask array bits are set, then
line[0] may contain one line of
unspecified data that should be ignored by applications. |
Table 4.12. struct v4l2_mpeg_vbi_ITV0
| struct v4l2_mpeg_vbi_itv0_line | line[36] | A fixed length array of 36 lines of sliced VBI
data. line[0] through line
[17] correspond to lines 6 through 23 of the
first field. line[18] through
line[35] corresponds to lines 6
through 23 of the second field. |
Table 4.13. struct v4l2_mpeg_vbi_itv0_line
| __u8 | id | A line identifier value from
Table 4.14, “Line Identifiers for struct
v4l2_mpeg_vbi_itv0_line id
field” that indicates
the type of sliced VBI data stored on this line. |
| __u8 | data[42] | The sliced VBI data for the line. |
Table 4.14. Line Identifiers for struct
v4l2_mpeg_vbi_itv0_line id
field
| Defined Symbol | Value | Description |
|---|---|---|
V4L2_MPEG_VBI_IVTV_TELETEXT_B
| 1 | Refer to Sliced VBI services for a description of the line payload. |
V4L2_MPEG_VBI_IVTV_CAPTION_525
| 4 | Refer to Sliced VBI services for a description of the line payload. |
V4L2_MPEG_VBI_IVTV_WSS_625
| 5 | Refer to Sliced VBI services for a description of the line payload. |
V4L2_MPEG_VBI_IVTV_VPS
| 7 | Refer to Sliced VBI services for a description of the line payload. |