mirror of https://gitee.com/openkylin/linux.git
348 lines
15 KiB
XML
348 lines
15 KiB
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. 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. 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>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-parent-document: "v4l2.sgml"
|
|
indent-tabs-mode: nil
|
|
End:
|
|
-->
|