4.7. Raw VBI Data Interface

VBI is an abbreviation of 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. Using an oscilloscope you will find here the vertical synchronization pulses and short data packages ASK modulated [1] onto the video signal. These are transmissions of services such as Teletext or Closed Caption.

Subject of this interface type is raw VBI data, as sampled off a video signal, or to be added to a signal for output. The data format is similar to uncompressed video images, a number of lines times a number of samples per line, we call this a VBI image.

Conventionally V4L2 VBI devices are accessed through character device special files named /dev/vbi and /dev/vbi0 to /dev/vbi31 with major number 81 and minor numbers 224 to 255. /dev/vbi is typically a symbolic link to the preferred VBI device. This convention applies to both input and output devices.

To address the problems of finding related video and VBI devices VBI capturing and output is also available as device function under /dev/video. To capture or output raw VBI data with these devices applications must call the VIDIOC_S_FMT ioctl. Accessed as /dev/vbi, raw VBI capturing or output is the default device function.

4.7.1. Querying Capabilities

Devices supporting the raw VBI capturing or output API set the V4L2_CAP_VBI_CAPTURE or V4L2_CAP_VBI_OUTPUT flags, 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. VBI devices may or may not have a tuner or modulator.

4.7.2. Supplemental Functions

VBI devices shall support video input or output, tuner or modulator, and controls ioctls as needed. The video standard ioctls provide information vital to program a VBI device, therefore must be supported.

4.7.3. Raw VBI Format Negotiation

Raw VBI sampling abilities can vary, in particular the sampling frequency. To properly interpret the data V4L2 specifies an ioctl to query the sampling parameters. Moreover, to allow for some flexibility applications can also suggest different parameters.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications should always ensure they really get what they want, requesting reasonable parameters and then checking if the actual parameters are suitable.

To query the current raw VBI capture parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VBI_CAPTURE or V4L2_BUF_TYPE_VBI_OUTPUT, and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_vbi_format vbi member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_vbi_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers return an EINVAL error code only when the given parameters are ambiguous, otherwise they modify the parameters according to the hardware capabilities and return the actual parameters. When the driver allocates resources at this point, it may return an EBUSY error code to indicate the returned parameters are valid but the required resources are currently not available. That may happen for instance when the video and VBI areas to capture would overlap, or when the driver supports multiple opens and another process already requested VBI capturing or output. Anyway, applications must expect other resource allocation points which may return EBUSY, at the ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF ioctl and the first read() , write() and select() calls.

VBI devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.

v4l2_vbi_format

struct v4l2_vbi_format

__u32	sampling_rate	Samples per second, i. e. unit 1 Hz.
__u32	offset	Horizontal offset of the VBI image, relative to the leading edge of the line synchronization pulse and counted in samples: The first sample in the VBI image will be located offset / sampling_rate seconds following the leading edge. See also Figure 4.1. Line synchronization.
__u32	samples_per_line
__u32	sample_format	Defines the sample format as in Image Formats, a four-character-code. [2] Usually this is V4L2_PIX_FMT_GREY, i. e. each sample consists of 8 bits with lower values oriented towards the black level. Do not assume any other correlation of values with the signal level. For example, the MSB does not necessarily indicate if the signal is ‘high’ or ‘low’ because 128 may not be the mean value of the signal. Drivers shall not convert the sample format by software.
__u32	start[2]	This is the scanning system line number associated with the first line of the VBI image, of the first and the second field respectively. 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. 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. VBI input drivers can return start values 0 if the hardware cannot reliable identify scanning lines, VBI acquisition may not require this information.
__u32	count[2]	The number of lines in the first and second field image, respectively.
Drivers should be as flexibility as possible. For example, it may be possible to extend or move the VBI capture window down to the picture area, implementing a ‘full field mode’ to capture data service transmissions embedded in the picture. An application can set the first or second count value to zero if no data is required from the respective field; count[1] if the scanning system is progressive, i. e. not interlaced. The corresponding start value shall be ignored by the application and driver. Anyway, drivers may not support single field capturing and return both count values non-zero. Both count values set to zero, or line numbers are outside the bounds depicted[4], or a field image covering lines of two fields, are invalid and shall not be returned by the driver. To initialize the start and count fields, applications must first determine the current video standard selection. The v4l2_std_id or the framelines field of struct v4l2_standard can be evaluated for this purpose.
__u32	flags	See Raw VBI Format Flags below. Currently only drivers set flags, applications must set this field to zero.
__u32	reserved[2]	This array is reserved for future extensions. Drivers and applications must set it to zero.

Raw VBI Format Flags

V4L2_VBI_UNSYNC	0x0001	This flag indicates hardware which does not properly distinguish between fields. Normally the VBI image stores the first field (lower scanning line numbers) first in memory. This may be a top or bottom field depending on the video standard. When this flag is set the first or second field may be stored first, however the fields are still in correct temporal order with the older field first in memory. [3]
V4L2_VBI_INTERLACED	0x0002	By default the two field images will be passed sequentially; all lines of the first field followed by all lines of the second field (compare Field Order V4L2_FIELD_SEQ_TB and V4L2_FIELD_SEQ_BT, whether the top or bottom field is first in memory depends on the video standard). When this flag is set, the two fields are interlaced (cf. V4L2_FIELD_INTERLACED). The first line of the first field followed by the first line of the second field, then the two second lines, and so on. Such a layout may be necessary when the hardware has been programmed to capture or output interlaced video images and is unable to separate the fields for VBI capturing at the same time. For simplicity setting this flag implies that both count values are equal and non-zero.

Figure 4.1. Line synchronization

Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL)

Figure 4.3. ITU-R 625 line numbering

Remember the VBI image format depends on the selected video standard, therefore the application must choose a new standard or query the current standard first. Attempts to read or write data ahead of format negotiation, or after switching the video standard which may invalidate the negotiated VBI parameters, should be refused by the driver. A format change during active I/O is not permitted.

4.7.4. Reading and writing VBI images

To assure synchronization with the field number and easier implementation, the smallest unit of data passed at a time is one frame, consisting of two fields of VBI images immediately following in memory.

The total size of a frame computes as follows:

(count[0] + count[1]) * samples_per_line * sample size in bytes

The sample size is most likely always one byte, applications must check the sample_format field though, to function properly with other drivers.

A VBI device may support read/write and/or streaming (memory mapping or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.

Remember the VIDIOC_STREAMON ioctl and the first read(), write() and select() call can be resource allocation points returning an EBUSY error code if the required hardware resources are temporarily unavailable, for example the device is already in use by another process.

[1]	ASK: Amplitude-Shift Keying. A high signal level represents a ‘1’ bit, a low level a ‘0’ bit.

[2]	(1, 2, 3, 4) A few devices may be unable to sample VBI data at all but can extend the video capture window to the VBI region.

[3]	Most VBI services transmit on both fields, but some have different semantics depending on the field number. These cannot be reliable decoded or encoded when V4L2_VBI_UNSYNC is set.

[4]	The valid values ar shown at Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL) and Figure 4.3. ITU-R 625 line numbering.