dev-raw-vbi.xml

  <title>Raw VBI Data Interface</title>

  <para>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<footnote><para>ASK: Amplitude-Shift Keying. A high signal
level represents a '1' bit, a low level a '0' bit.</para></footnote>
onto the video signal. These are transmissions of services such as
Teletext or Closed Caption.</para>

  <para>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.</para>

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

  <para>To address the problems of finding related video and VBI
devices VBI capturing and output is also available as device function
under <filename>/dev/video</filename>. To capture or output raw VBI
data with these devices applications must call the &VIDIOC-S-FMT;
ioctl. Accessed as <filename>/dev/vbi</filename>, raw VBI capturing
or output is the default device function.</para>

    <section>
      <title>Querying Capabilities</title>

      <para>Devices supporting the raw VBI capturing or output API set
the <constant>V4L2_CAP_VBI_CAPTURE</constant> or
<constant>V4L2_CAP_VBI_OUTPUT</constant> flags, respectively, in the
<structfield>capabilities</structfield> field of &v4l2-capability;
returned by the &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.</para>
    </section>

    <section>
      <title>Supplemental Functions</title>

      <para>VBI devices shall support <link linkend="video">video
input or output</link>, <link linkend="tuner">tuner or
modulator</link>, and <link linkend="control">controls</link> ioctls
as needed. The <link linkend="standard">video standard</link> ioctls provide
information vital to program a VBI device, therefore must be
supported.</para>
    </section>

    <section>
      <title>Raw VBI Format Negotiation</title>

      <para>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.</para>

      <para>As usual these parameters are <emphasis>not</emphasis>
reset at &func-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.</para>

      <para>To query the current raw VBI capture parameters
applications set the <structfield>type</structfield> field of a
&v4l2-format; to <constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant> or
<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>, and call the
&VIDIOC-G-FMT; ioctl with a pointer to this structure. Drivers fill
the &v4l2-vbi-format; <structfield>vbi</structfield> member of the
<structfield>fmt</structfield> union.</para>

      <para>To request different parameters applications set the
<structfield>type</structfield> field of a &v4l2-format; as above and
initialize all fields of the &v4l2-vbi-format;
<structfield>vbi</structfield> member of the
<structfield>fmt</structfield> union, or better just modify the
results of <constant>VIDIOC_G_FMT</constant>, and call the
&VIDIOC-S-FMT; ioctl with a pointer to this structure. Drivers return
an &EINVAL; only when the given parameters are ambiguous, otherwise
they modify the parameters according to the hardware capabilites and
return the actual parameters. When the driver allocates resources at
this point, it may return an &EBUSY; 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 <errorcode>EBUSY</errorcode>, at the &VIDIOC-STREAMON; ioctl
and the first read(), write() and select() call.</para>

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

      <table pgwide="1" frame="none" id="v4l2-vbi-format">
	<title>struct <structname>v4l2_vbi_format</structname></title>
	<tgroup cols="3">
	  &cs-str;
	  <tbody valign="top">
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>sampling_rate</structfield></entry>
	      <entry>Samples per second, i.&nbsp;e. unit 1 Hz.</entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>offset</structfield></entry>
	      <entry><para>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
<structfield>offset</structfield> /
<structfield>sampling_rate</structfield> seconds following the leading
edge. See also <xref linkend="vbi-hsync" />.</para></entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>samples_per_line</structfield></entry>
	      <entry></entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>sample_format</structfield></entry>
	      <entry><para>Defines the sample format as in <xref
linkend="pixfmt" />, a four-character-code.<footnote>
		    <para>A few devices may be unable to
sample VBI data at all but can extend the video capture window to the
VBI region.</para>
		  </footnote> Usually this is
<constant>V4L2_PIX_FMT_GREY</constant>, i.&nbsp;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.</para></entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>start</structfield>[2]</entry>
	      <entry>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 <xref linkend="vbi-525" /> and
<xref linkend="vbi-625" /> for valid values. VBI input drivers can
return start values 0 if the hardware cannot reliable identify
scanning lines, VBI acquisition may not require this
information.</entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>count</structfield>[2]</entry>
	      <entry>The number of lines in the first and second
field image, respectively.</entry>
	  </row>
	  <row>
	    <entry spanname="hspan"><para>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.</para><para>An application can set the first or second
<structfield>count</structfield> value to zero if no data is required
from the respective field; <structfield>count</structfield>[1] if the
scanning system is progressive, &ie; 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.</para><para>Both
<structfield>count</structfield> values set to zero, or line numbers
outside the bounds depicted in <xref linkend="vbi-525" /> and <xref
		    linkend="vbi-625" />, or a field image covering
lines of two fields, are invalid and shall not be returned by the
driver.</para><para>To initialize the <structfield>start</structfield>
and <structfield>count</structfield> fields, applications must first
determine the current video standard selection. The &v4l2-std-id; or
the <structfield>framelines</structfield> field of &v4l2-standard; can
be evaluated for this purpose.</para></entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>flags</structfield></entry>
	      <entry>See <xref linkend="vbifmt-flags" /> below. Currently
only drivers set flags, applications must set this field to
zero.</entry>
	    </row>
	    <row>
	      <entry>__u32</entry>
	      <entry><structfield>reserved</structfield>[2]</entry>
	      <entry>This array is reserved for future extensions.
Drivers and applications must set it to zero.</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <table pgwide="1" frame="none" id="vbifmt-flags">
	<title>Raw VBI Format Flags</title>
	<tgroup cols="3">
	  &cs-def;
	  <tbody valign="top">
	    <row>
	      <entry><constant>V4L2_VBI_UNSYNC</constant></entry>
	      <entry>0x0001</entry>
	      <entry><para>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.<footnote>
		  <para>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
<constant>V4L2_VBI_UNSYNC</constant> is set.</para>
		</footnote></para></entry>
	    </row>
	    <row>
	      <entry><constant>V4L2_VBI_INTERLACED</constant></entry>
	      <entry>0x0002</entry>
	      <entry>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 <xref linkend="field-order" />
<constant>V4L2_FIELD_SEQ_TB</constant> and
<constant>V4L2_FIELD_SEQ_BT</constant>, 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.
<constant>V4L2_FIELD_INTERLACED</constant>). 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
<structfield>count</structfield> values are equal and non-zero.</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <figure id="vbi-hsync">
	<title>Line synchronization</title>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="vbi_hsync.pdf" format="PS" />
	  </imageobject>
	  <imageobject>
	    <imagedata fileref="vbi_hsync.gif" format="GIF" />
	  </imageobject>
	  <textobject>
	    <phrase>Line synchronization diagram</phrase>
	  </textobject>
	</mediaobject>
      </figure>

      <figure id="vbi-525">
	<title>ITU-R 525 line numbering (M/NTSC and M/PAL)</title>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="vbi_525.pdf" format="PS" />
	  </imageobject>
	  <imageobject>
	    <imagedata fileref="vbi_525.gif" format="GIF" />
	  </imageobject>
	  <textobject>
	    <phrase>NTSC field synchronization diagram</phrase>
	  </textobject>
	  <caption>
	    <para>(1) For the purpose of this specification field 2
starts in line 264 and not 263.5 because half line capturing is not
supported.</para>
	  </caption>
	</mediaobject>
      </figure>

      <figure id="vbi-625">
	<title>ITU-R 625 line numbering</title>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="vbi_625.pdf" format="PS" />
	  </imageobject>
	  <imageobject>
	    <imagedata fileref="vbi_625.gif" format="GIF" />
	  </imageobject>
	  <textobject>
	    <phrase>PAL/SECAM field synchronization diagram</phrase>
	  </textobject>
	  <caption>
	    <para>(1) For the purpose of this specification field 2
starts in line 314 and not 313.5 because half line capturing is not
supported.</para>
	  </caption>
	</mediaobject>
      </figure>

      <para>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.</para>
    </section>

    <section>
      <title>Reading and writing VBI images</title>

      <para>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.</para>

      <para>The total size of a frame computes as follows:</para>

      <programlisting>
(<structfield>count</structfield>[0] + <structfield>count</structfield>[1]) *
<structfield>samples_per_line</structfield> * sample size in bytes</programlisting>

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

      <para>A VBI device may support <link
      linkend="rw">read/write</link> and/or streaming (<link
      linkend="mmap">memory mapping</link> or <link
      linkend="userp">user pointer</link>) I/O. The latter bears the
possibility of synchronizing video and
VBI data by using buffer timestamps.</para>

      <para>Remember the &VIDIOC-STREAMON; ioctl and the first read(),
write() and select() call can be resource allocation points returning
an &EBUSY; if the required hardware resources are temporarily
unavailable, for example the device is already in use by another
process.</para>
  </section>

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