path: root/Documentation/DocBook/v4l/dev-sliced-vbi.xml

  <title>Sliced VBI Data Interface</title>

  <para>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.</para>

  <para>Sliced VBI devices use hardware to demodulate data transmitted
in the VBI. V4L2 drivers shall <emphasis>not</emphasis> do this by
software, see also the <link linkend="raw-vbi">raw VBI
interface</link>. The data is passed as short packets of fixed size,
covering one scan line each. The number of packets per video frame is
variable.</para>

  <para>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
<filename>/dev/vbi</filename> device is <emphasis>raw</emphasis> VBI
capturing or output, and the sliced VBI function is only available
after calling the &VIDIOC-S-FMT; ioctl as defined below. Likewise a
<filename>/dev/video</filename> 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.</para>

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

    <para>Devices supporting the sliced VBI capturing or output API
set the <constant>V4L2_CAP_SLICED_VBI_CAPTURE</constant> or
<constant>V4L2_CAP_SLICED_VBI_OUTPUT</constant> flag 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 <link linkend="io">I/O
methods</link> must be supported. Sliced VBI devices may have a tuner
or modulator.</para>
  </section>

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

    <para>Sliced VBI devices shall support <link linkend="video">video
input or output</link> and <link linkend="tuner">tuner or
modulator</link> ioctls if they have these capabilities, and they may
support <link linkend="control">control</link> ioctls. The <link
linkend="standard">video standard</link> ioctls provide information
vital to program a sliced VBI device, therefore must be
supported.</para>
  </section>

  <section id="sliced-vbi-format-negotitation">
    <title>Sliced VBI Format Negotiation</title>

    <para>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.</para>

    <para>To determine the currently selected services applications
set the <structfield>type </structfield> field of &v4l2-format; to
<constant> V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant> or <constant>
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant>, and the &VIDIOC-G-FMT;
ioctl fills the <structfield>fmt.sliced</structfield> member, a
&v4l2-sliced-vbi-format;.</para>

    <para>Applications can request different parameters by
initializing or modifying the <structfield>fmt.sliced</structfield>
member and calling the &VIDIOC-S-FMT; ioctl with a pointer to the
<structname>v4l2_format</structname> structure.</para>

    <para>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
<structfield>service_set</structfield> field to the required services
and let the driver fill the <structfield>service_lines</structfield>
array according to hardware capabilities. Only if more precise control
is needed should the programmer set the
<structfield>service_lines</structfield> array explicitly.</para>

    <para>The &VIDIOC-S-FMT; ioctl modifies the parameters
according to hardware capabilities. When the driver allocates
resources at this point, it may return an &EBUSY; if the required
resources are temporarily unavailable. Other resource allocation
points which may return <errorcode>EBUSY</errorcode> can be the
&VIDIOC-STREAMON; ioctl and the first &func-read;, &func-write; and
&func-select; call.</para>

    <table frame="none" pgwide="1" id="v4l2-sliced-vbi-format">
      <title>struct
<structname>v4l2_sliced_vbi_format</structname></title>
      <tgroup cols="5">
	<colspec colname="c1" colwidth="3*" />
	<colspec colname="c2" colwidth="3*" />
	<colspec colname="c3" colwidth="2*" />
	<colspec colname="c4" colwidth="2*" />
	<colspec colname="c5" colwidth="2*" />
	<spanspec namest="c3" nameend="c5" spanname="hspan" />
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>service_set</structfield></entry>
	    <entry spanname="hspan"><para>If
<structfield>service_set</structfield> is non-zero when passed with
&VIDIOC-S-FMT; or &VIDIOC-TRY-FMT;, the
<structfield>service_lines</structfield> array will be filled by the
driver according to the services specified in this field. For example,
if <structfield>service_set</structfield> is initialized with
<constant>V4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625</constant>, a
driver for the cx25840 video decoder sets lines 7-22 of both
fields<footnote><para>According to <link
linkend="ets300706">ETS&nbsp;300&nbsp;706</link> lines 6-22 of the
first field and lines 5-22 of the second field may carry Teletext
data.</para></footnote> to <constant>V4L2_SLICED_TELETEXT_B</constant>
and line 23 of the first field to
<constant>V4L2_SLICED_WSS_625</constant>. If
<structfield>service_set</structfield> is set to zero, then the values
of <structfield>service_lines</structfield> will be used instead.
</para><para>On return the driver sets this field to the union of all
elements of the returned <structfield>service_lines</structfield>
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.</para></entry>
	  </row>
	  <row>
	    <entry>__u16</entry>
	    <entry><structfield>service_lines</structfield>[2][24]</entry>
	    <entry spanname="hspan"><para>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.</para><para>Data
services are defined in <xref linkend="vbi-services2" />. Array indices
map to ITU-R line numbers (see also <xref linkend="vbi-525" /> and <xref
		  linkend="vbi-625"