Sliced VBI Data Interface

Video for Linux Two API Specification: Revision 0.24
Prev | Chapter 4. Interfaces | Next

# 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 `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 control 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 `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 returns an EINVAL error code only when the given parameters are ambiguous, otherwise it 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 `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 fieldsa 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 Table 4-7. 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
|  | Drivers must set `service_lines`[0][0] and `service_lines`[1][0] to zero.
__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.
Notes: 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 | 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 | 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).

## 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 `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 2 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 and Figure 4-3 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 2 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.

Prev | Home | Next
Raw VBI Data Interface | Up | Teletext Interface