4.8. Sliced VBI Data Interface

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.

4.8.1. Querying Capabilities

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 ioctl 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.

4.8.2. Supplemental Functions

Sliced VBI devices shall support video input or output and tuner or modulator ioctls if they have these capabilities, and they may support User Controls ioctls. The video standard ioctls provide information vital to program a sliced VBI device, therefore must be supported.

4.8.3. Sliced VBI Format Negotiation

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 struct 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 ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF ioctl and the first read(), write() and select() call.

v4l2_sliced_vbi_format struct v4l2_sliced_vbi_format

__u32 service_set

If service_set is non-zero when passed with VIDIOC_S_FMT or VIDIOC_TRY_FMT, the service_lines array will be filled by the driver according to the services specified in this field. For example, if service_set is initialized with V4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625, a driver for the cx25840 video decoder sets lines 7-22 of both fields [1] to V4L2_SLICED_TELETEXT_B and line 23 of the first field to V4L2_SLICED_WSS_625. If service_set is set to zero, then the values of service_lines will be used instead.

On return the driver sets this field to the union of all elements of the returned service_lines array. It may contain less services than requested, perhaps just one, if the hardware cannot handle more services simultaneously. It may be empty (zero) if none of the requested services are supported by the hardware.

__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 Sliced VBI services. Array indices map to ITU-R line numbers[2] 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 ioctl VIDIOC_QBUF, VIDIOC_DQBUF 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. 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 ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF and VIDIOC_STREAMOFF call, and after the first read() or write() call).

4.8.4. Reading and writing sliced VBI data

A single read() or write() call must pass all data belonging to one video frame. That is an array of struct 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 struct v4l2_sliced_vbi_data elements must be zero.

v4l2_sliced_vbi_data struct v4l2_sliced_vbi_data

__u32 id A flag from 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 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 ioctl VIDIOC_QBUF, VIDIOC_DQBUF 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.

4.8.5. Sliced VBI Data in MPEG Streams

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. MPEG Stream Embedded, Sliced VBI Data Format: NONE

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. MPEG Stream Embedded, Sliced VBI Data Format: IVTV

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/media/v4l-drivers/cx2341x.rst . 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).

v4l2_mpeg_vbi_fmt_ivtv struct v4l2_mpeg_vbi_fmt_ivtv

__u8 magic[4]   A “magic” constant from 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. 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.
v4l2_mpeg_vbi_ITV0 structs v4l2_mpeg_vbi_itv0 and v4l2_mpeg_vbi_ITV0

__le32 linemask[2]

Bitmasks indicating the VBI service lines present. These linemask values are stored in little endian byte order in the MPEG stream. Some reference linemask bit positions with their corresponding VBI line number and video field are given below. b0 indicates the least significant bit of a linemask value:

linemask[0] b0:     line  6     first field
linemask[0] b17:    line 23     first field
linemask[0] b18:    line  6     second field
linemask[0] b31:    line 19     second field
linemask[1] b0:     line 20     second field
linemask[1] b3:     line 23     second field
linemask[1] b4-b31: unused and set to 0
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 b3 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. 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.
v4l2_mpeg_vbi_itv0_line struct v4l2_mpeg_vbi_itv0_line

__u8 id A line identifier value from 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. 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.
[1]According to ETS 300 706 lines 6-22 of the first field and lines 5-22 of the second field may carry Teletext data.
[2]See also Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL) and Figure 4.3. ITU-R 625 line numbering.