4.6. 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.
4.6.1. 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.
4.6.2. 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.
4.6.3. 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¶
| __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 Both To initialize the |
||
| __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. |
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.
4.6.4. 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. |