1 .. -*- coding: utf-8; mode: rst -*-
9 VBI is an abbreviation of Vertical Blanking Interval, a gap in the
10 sequence of lines of an analog video signal. During VBI no picture
11 information is transmitted, allowing some time while the electron beam
12 of a cathode ray tube TV returns to the top of the screen. Using an
13 oscilloscope you will find here the vertical synchronization pulses and
14 short data packages ASK modulated [#f1]_ onto the video signal. These are
15 transmissions of services such as Teletext or Closed Caption.
17 Subject of this interface type is raw VBI data, as sampled off a video
18 signal, or to be added to a signal for output. The data format is
19 similar to uncompressed video images, a number of lines times a number
20 of samples per line, we call this a VBI image.
22 Conventionally V4L2 VBI devices are accessed through character device
23 special files named ``/dev/vbi`` and ``/dev/vbi0`` to ``/dev/vbi31``
24 with major number 81 and minor numbers 224 to 255. ``/dev/vbi`` is
25 typically a symbolic link to the preferred VBI device. This convention
26 applies to both input and output devices.
28 To address the problems of finding related video and VBI devices VBI
29 capturing and output is also available as device function under
30 ``/dev/video``. To capture or output raw VBI data with these devices
31 applications must call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
32 Accessed as ``/dev/vbi``, raw VBI capturing or output is the default
39 Devices supporting the raw VBI capturing or output API set the
40 ``V4L2_CAP_VBI_CAPTURE`` or ``V4L2_CAP_VBI_OUTPUT`` flags, respectively,
41 in the ``capabilities`` field of struct
42 :ref:`v4l2_capability <v4l2-capability>` returned by the
43 :ref:`VIDIOC_QUERYCAP` ioctl. At least one of the
44 read/write, streaming or asynchronous I/O methods must be supported. VBI
45 devices may or may not have a tuner or modulator.
48 Supplemental Functions
49 ======================
51 VBI devices shall support :ref:`video input or output <video>`,
52 :ref:`tuner or modulator <tuner>`, and :ref:`controls <control>`
53 ioctls as needed. The :ref:`video standard <standard>` ioctls provide
54 information vital to program a VBI device, therefore must be supported.
57 Raw VBI Format Negotiation
58 ==========================
60 Raw VBI sampling abilities can vary, in particular the sampling
61 frequency. To properly interpret the data V4L2 specifies an ioctl to
62 query the sampling parameters. Moreover, to allow for some flexibility
63 applications can also suggest different parameters.
65 As usual these parameters are *not* reset at :ref:`open() <func-open>`
66 time to permit Unix tool chains, programming a device and then reading
67 from it as if it was a plain file. Well written V4L2 applications should
68 always ensure they really get what they want, requesting reasonable
69 parameters and then checking if the actual parameters are suitable.
71 To query the current raw VBI capture parameters applications set the
72 ``type`` field of a struct :ref:`v4l2_format <v4l2-format>` to
73 ``V4L2_BUF_TYPE_VBI_CAPTURE`` or ``V4L2_BUF_TYPE_VBI_OUTPUT``, and call
74 the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this
75 structure. Drivers fill the struct
76 :ref:`v4l2_vbi_format <v4l2-vbi-format>` ``vbi`` member of the
79 To request different parameters applications set the ``type`` field of a
80 struct :ref:`v4l2_format <v4l2-format>` as above and initialize all
81 fields of the struct :ref:`v4l2_vbi_format <v4l2-vbi-format>`
82 ``vbi`` member of the ``fmt`` union, or better just modify the results
83 of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
84 ioctl with a pointer to this structure. Drivers return an ``EINVAL`` error
85 code only when the given parameters are ambiguous, otherwise they modify
86 the parameters according to the hardware capabilities and return the
87 actual parameters. When the driver allocates resources at this point, it
88 may return an ``EBUSY`` error code to indicate the returned parameters are
89 valid but the required resources are currently not available. That may
90 happen for instance when the video and VBI areas to capture would
91 overlap, or when the driver supports multiple opens and another process
92 already requested VBI capturing or output. Anyway, applications must
93 expect other resource allocation points which may return ``EBUSY``, at the
94 :ref:`VIDIOC_STREAMON` ioctl and the first :ref:`read() <func-read>`
95 , :ref:`write() <func-write>` and :ref:`select() <func-select>` calls.
97 VBI devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and
98 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all requests
99 and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
100 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
105 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
107 .. flat-table:: struct v4l2_vbi_format
119 - Samples per second, i. e. unit 1 Hz.
127 - Horizontal offset of the VBI image, relative to the leading edge
128 of the line synchronization pulse and counted in samples: The
129 first sample in the VBI image will be located ``offset`` /
130 ``sampling_rate`` seconds following the leading edge. See also
137 - ``samples_per_line``
147 - Defines the sample format as in :ref:`pixfmt`, a
148 four-character-code. [#f2]_ Usually this is ``V4L2_PIX_FMT_GREY``,
149 i. e. each sample consists of 8 bits with lower values oriented
150 towards the black level. Do not assume any other correlation of
151 values with the signal level. For example, the MSB does not
152 necessarily indicate if the signal is 'high' or 'low' because 128
153 may not be the mean value of the signal. Drivers shall not convert
154 the sample format by software.
162 - This is the scanning system line number associated with the first
163 line of the VBI image, of the first and the second field
164 respectively. See :ref:`vbi-525` and :ref:`vbi-625` for valid
165 values. The ``V4L2_VBI_ITU_525_F1_START``,
166 ``V4L2_VBI_ITU_525_F2_START``, ``V4L2_VBI_ITU_625_F1_START`` and
167 ``V4L2_VBI_ITU_625_F2_START`` defines give the start line numbers
168 for each field for each 525 or 625 line format as a convenience.
169 Don't forget that ITU line numbering starts at 1, not 0. VBI input
170 drivers can return start values 0 if the hardware cannot reliable
171 identify scanning lines, VBI acquisition may not require this
180 - The number of lines in the first and second field image,
187 Drivers should be as flexibility as possible. For example, it may
188 be possible to extend or move the VBI capture window down to the
189 picture area, implementing a 'full field mode' to capture data
190 service transmissions embedded in the picture.
192 An application can set the first or second ``count`` value to zero
193 if no data is required from the respective field; ``count``\ [1]
194 if the scanning system is progressive, i. e. not interlaced. The
195 corresponding start value shall be ignored by the application and
196 driver. Anyway, drivers may not support single field capturing and
197 return both count values non-zero.
199 Both ``count`` values set to zero, or line numbers are outside the
200 bounds depicted\ [#f4]_, or a field image covering lines of two
201 fields, are invalid and shall not be returned by the driver.
203 To initialize the ``start`` and ``count`` fields, applications
204 must first determine the current video standard selection. The
205 :ref:`v4l2_std_id <v4l2-std-id>` or the ``framelines`` field
206 of struct :ref:`v4l2_standard <v4l2-standard>` can be evaluated
215 - See :ref:`vbifmt-flags` below. Currently only drivers set flags,
216 applications must set this field to zero.
222 - ``reserved``\ [#f2]_
224 - This array is reserved for future extensions. Drivers and
225 applications must set it to zero.
231 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
233 .. flat-table:: Raw VBI Format Flags
241 - ``V4L2_VBI_UNSYNC``
245 - This flag indicates hardware which does not properly distinguish
246 between fields. Normally the VBI image stores the first field
247 (lower scanning line numbers) first in memory. This may be a top
248 or bottom field depending on the video standard. When this flag is
249 set the first or second field may be stored first, however the
250 fields are still in correct temporal order with the older field
251 first in memory. [#f3]_
255 - ``V4L2_VBI_INTERLACED``
259 - By default the two field images will be passed sequentially; all
260 lines of the first field followed by all lines of the second field
261 (compare :ref:`field-order` ``V4L2_FIELD_SEQ_TB`` and
262 ``V4L2_FIELD_SEQ_BT``, whether the top or bottom field is first in
263 memory depends on the video standard). When this flag is set, the
264 two fields are interlaced (cf. ``V4L2_FIELD_INTERLACED``). The
265 first line of the first field followed by the first line of the
266 second field, then the two second lines, and so on. Such a layout
267 may be necessary when the hardware has been programmed to capture
268 or output interlaced video images and is unable to separate the
269 fields for VBI capturing at the same time. For simplicity setting
270 this flag implies that both ``count`` values are equal and
277 .. figure:: dev-raw-vbi_files/vbi_hsync.*
278 :alt: vbi_hsync.pdf / vbi_hsync.gif
281 **Figure 4.1. Line synchronization**
286 .. figure:: dev-raw-vbi_files/vbi_525.*
287 :alt: vbi_525.pdf / vbi_525.gif
290 **Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL)**
296 .. figure:: dev-raw-vbi_files/vbi_625.*
297 :alt: vbi_625.pdf / vbi_625.gif
300 **Figure 4.3. ITU-R 625 line numbering**
304 Remember the VBI image format depends on the selected video standard,
305 therefore the application must choose a new standard or query the
306 current standard first. Attempts to read or write data ahead of format
307 negotiation, or after switching the video standard which may invalidate
308 the negotiated VBI parameters, should be refused by the driver. A format
309 change during active I/O is not permitted.
312 Reading and writing VBI images
313 ==============================
315 To assure synchronization with the field number and easier
316 implementation, the smallest unit of data passed at a time is one frame,
317 consisting of two fields of VBI images immediately following in memory.
319 The total size of a frame computes as follows:
324 (count[0] + count[1]) * samples_per_line * sample size in bytes
326 The sample size is most likely always one byte, applications must check
327 the ``sample_format`` field though, to function properly with other
330 A VBI device may support :ref:`read/write <rw>` and/or streaming
331 (:ref:`memory mapping <mmap>` or :ref:`user pointer <userp>`) I/O.
332 The latter bears the possibility of synchronizing video and VBI data by
333 using buffer timestamps.
335 Remember the :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` ioctl and the
336 first :ref:`read() <func-read>`, :ref:`write() <func-write>` and
337 :ref:`select() <func-select>` call can be resource allocation
338 points returning an ``EBUSY`` error code if the required hardware resources
339 are temporarily unavailable, for example the device is already in use by
343 ASK: Amplitude-Shift Keying. A high signal level represents a '1'
344 bit, a low level a '0' bit.
347 A few devices may be unable to sample VBI data at all but can extend
348 the video capture window to the VBI region.
351 Most VBI services transmit on both fields, but some have different
352 semantics depending on the field number. These cannot be reliable
353 decoded or encoded when ``V4L2_VBI_UNSYNC`` is set.
356 The valid values ar shown at :ref:`vbi-525` and :ref:`vbi-625`.
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