348 lines
15 KiB
XML
348 lines
15 KiB
XML
<title>Raw VBI Data Interface</title>
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<para>VBI is an abbreviation of Vertical Blanking Interval, a gap
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in the sequence of lines of an analog video signal. During VBI
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no picture information is transmitted, allowing some time while the
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electron beam of a cathode ray tube TV returns to the top of the
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screen. Using an oscilloscope you will find here the vertical
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synchronization pulses and short data packages ASK
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modulated<footnote><para>ASK: Amplitude-Shift Keying. A high signal
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level represents a '1' bit, a low level a '0' bit.</para></footnote>
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onto the video signal. These are transmissions of services such as
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Teletext or Closed Caption.</para>
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<para>Subject of this interface type is raw VBI data, as sampled off
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a video signal, or to be added to a signal for output.
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The data format is similar to uncompressed video images, a number of
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lines times a number of samples per line, we call this a VBI image.</para>
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<para>Conventionally V4L2 VBI devices are accessed through character
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device special files named <filename>/dev/vbi</filename> and
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<filename>/dev/vbi0</filename> to <filename>/dev/vbi31</filename> with
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major number 81 and minor numbers 224 to 255.
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<filename>/dev/vbi</filename> is typically a symbolic link to the
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preferred VBI device. This convention applies to both input and output
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devices.</para>
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<para>To address the problems of finding related video and VBI
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devices VBI capturing and output is also available as device function
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under <filename>/dev/video</filename>. To capture or output raw VBI
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data with these devices applications must call the &VIDIOC-S-FMT;
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ioctl. Accessed as <filename>/dev/vbi</filename>, raw VBI capturing
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or output is the default device function.</para>
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<section>
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<title>Querying Capabilities</title>
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<para>Devices supporting the raw VBI capturing or output API set
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the <constant>V4L2_CAP_VBI_CAPTURE</constant> or
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<constant>V4L2_CAP_VBI_OUTPUT</constant> flags, respectively, in the
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<structfield>capabilities</structfield> field of &v4l2-capability;
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returned by the &VIDIOC-QUERYCAP; ioctl. At least one of the
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read/write, streaming or asynchronous I/O methods must be
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supported. VBI devices may or may not have a tuner or modulator.</para>
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</section>
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<section>
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<title>Supplemental Functions</title>
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<para>VBI devices shall support <link linkend="video">video
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input or output</link>, <link linkend="tuner">tuner or
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modulator</link>, and <link linkend="control">controls</link> ioctls
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as needed. The <link linkend="standard">video standard</link> ioctls provide
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information vital to program a VBI device, therefore must be
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supported.</para>
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</section>
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<section>
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<title>Raw VBI Format Negotiation</title>
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<para>Raw VBI sampling abilities can vary, in particular the
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sampling frequency. To properly interpret the data V4L2 specifies an
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ioctl to query the sampling parameters. Moreover, to allow for some
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flexibility applications can also suggest different parameters.</para>
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<para>As usual these parameters are <emphasis>not</emphasis>
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reset at &func-open; time to permit Unix tool chains, programming a
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device and then reading from it as if it was a plain file. Well
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written V4L2 applications should always ensure they really get what
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they want, requesting reasonable parameters and then checking if the
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actual parameters are suitable.</para>
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<para>To query the current raw VBI capture parameters
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applications set the <structfield>type</structfield> field of a
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&v4l2-format; to <constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant> or
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<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>, and call the
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&VIDIOC-G-FMT; ioctl with a pointer to this structure. Drivers fill
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the &v4l2-vbi-format; <structfield>vbi</structfield> member of the
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<structfield>fmt</structfield> union.</para>
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<para>To request different parameters applications set the
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<structfield>type</structfield> field of a &v4l2-format; as above and
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initialize all fields of the &v4l2-vbi-format;
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<structfield>vbi</structfield> member of the
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<structfield>fmt</structfield> union, or better just modify the
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results of <constant>VIDIOC_G_FMT</constant>, and call the
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&VIDIOC-S-FMT; ioctl with a pointer to this structure. Drivers return
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an &EINVAL; only when the given parameters are ambiguous, otherwise
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they modify the parameters according to the hardware capabilites and
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return the actual parameters. When the driver allocates resources at
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this point, it may return an &EBUSY; to indicate the returned
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parameters are valid but the required resources are currently not
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available. That may happen for instance when the video and VBI areas
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to capture would overlap, or when the driver supports multiple opens
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and another process already requested VBI capturing or output. Anyway,
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applications must expect other resource allocation points which may
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return <errorcode>EBUSY</errorcode>, at the &VIDIOC-STREAMON; ioctl
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and the first read(), write() and select() call.</para>
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<para>VBI devices must implement both the
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<constant>VIDIOC_G_FMT</constant> and
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<constant>VIDIOC_S_FMT</constant> ioctl, even if
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<constant>VIDIOC_S_FMT</constant> ignores all requests and always
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returns default parameters as <constant>VIDIOC_G_FMT</constant> does.
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<constant>VIDIOC_TRY_FMT</constant> is optional.</para>
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<table pgwide="1" frame="none" id="v4l2-vbi-format">
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<title>struct <structname>v4l2_vbi_format</structname></title>
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<tgroup cols="3">
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&cs-str;
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<tbody valign="top">
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<row>
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<entry>__u32</entry>
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<entry><structfield>sampling_rate</structfield></entry>
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<entry>Samples per second, i. e. unit 1 Hz.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>offset</structfield></entry>
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<entry><para>Horizontal offset of the VBI image,
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relative to the leading edge of the line synchronization pulse and
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counted in samples: The first sample in the VBI image will be located
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<structfield>offset</structfield> /
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<structfield>sampling_rate</structfield> seconds following the leading
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edge. See also <xref linkend="vbi-hsync" />.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>samples_per_line</structfield></entry>
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<entry></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>sample_format</structfield></entry>
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<entry><para>Defines the sample format as in <xref
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linkend="pixfmt" />, a four-character-code.<footnote>
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<para>A few devices may be unable to
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sample VBI data at all but can extend the video capture window to the
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VBI region.</para>
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</footnote> Usually this is
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<constant>V4L2_PIX_FMT_GREY</constant>, i. e. each sample
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consists of 8 bits with lower values oriented towards the black level.
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Do not assume any other correlation of values with the signal level.
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For example, the MSB does not necessarily indicate if the signal is
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'high' or 'low' because 128 may not be the mean value of the
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signal. Drivers shall not convert the sample format by software.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>start</structfield>[2]</entry>
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<entry>This is the scanning system line number
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associated with the first line of the VBI image, of the first and the
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second field respectively. See <xref linkend="vbi-525" /> and
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<xref linkend="vbi-625" /> for valid values. VBI input drivers can
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return start values 0 if the hardware cannot reliable identify
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scanning lines, VBI acquisition may not require this
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information.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>count</structfield>[2]</entry>
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<entry>The number of lines in the first and second
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field image, respectively.</entry>
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</row>
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<row>
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<entry spanname="hspan"><para>Drivers should be as
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flexibility as possible. For example, it may be possible to extend or
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move the VBI capture window down to the picture area, implementing a
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'full field mode' to capture data service transmissions embedded in
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the picture.</para><para>An application can set the first or second
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<structfield>count</structfield> value to zero if no data is required
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from the respective field; <structfield>count</structfield>[1] if the
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scanning system is progressive, &ie; not interlaced. The
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corresponding start value shall be ignored by the application and
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driver. Anyway, drivers may not support single field capturing and
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return both count values non-zero.</para><para>Both
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<structfield>count</structfield> values set to zero, or line numbers
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outside the bounds depicted in <xref linkend="vbi-525" /> and <xref
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linkend="vbi-625" />, or a field image covering
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lines of two fields, are invalid and shall not be returned by the
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driver.</para><para>To initialize the <structfield>start</structfield>
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and <structfield>count</structfield> fields, applications must first
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determine the current video standard selection. The &v4l2-std-id; or
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the <structfield>framelines</structfield> field of &v4l2-standard; can
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be evaluated for this purpose.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>flags</structfield></entry>
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<entry>See <xref linkend="vbifmt-flags" /> below. Currently
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only drivers set flags, applications must set this field to
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zero.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>reserved</structfield>[2]</entry>
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<entry>This array is reserved for future extensions.
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Drivers and applications must set it to zero.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table pgwide="1" frame="none" id="vbifmt-flags">
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<title>Raw VBI Format Flags</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_VBI_UNSYNC</constant></entry>
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<entry>0x0001</entry>
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<entry><para>This flag indicates hardware which does not
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properly distinguish between fields. Normally the VBI image stores the
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first field (lower scanning line numbers) first in memory. This may be
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a top or bottom field depending on the video standard. When this flag
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is set the first or second field may be stored first, however the
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fields are still in correct temporal order with the older field first
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in memory.<footnote>
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<para>Most VBI services transmit on both fields, but
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some have different semantics depending on the field number. These
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cannot be reliable decoded or encoded when
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<constant>V4L2_VBI_UNSYNC</constant> is set.</para>
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</footnote></para></entry>
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</row>
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<row>
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<entry><constant>V4L2_VBI_INTERLACED</constant></entry>
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<entry>0x0002</entry>
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<entry>By default the two field images will be passed
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sequentially; all lines of the first field followed by all lines of
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the second field (compare <xref linkend="field-order" />
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<constant>V4L2_FIELD_SEQ_TB</constant> and
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<constant>V4L2_FIELD_SEQ_BT</constant>, whether the top or bottom
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field is first in memory depends on the video standard). When this
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flag is set, the two fields are interlaced (cf.
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<constant>V4L2_FIELD_INTERLACED</constant>). The first line of the
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first field followed by the first line of the second field, then the
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two second lines, and so on. Such a layout may be necessary when the
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hardware has been programmed to capture or output interlaced video
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images and is unable to separate the fields for VBI capturing at
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the same time. For simplicity setting this flag implies that both
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<structfield>count</structfield> values are equal and non-zero.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<figure id="vbi-hsync">
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<title>Line synchronization</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="vbi_hsync.pdf" format="PS" />
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</imageobject>
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<imageobject>
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<imagedata fileref="vbi_hsync.gif" format="GIF" />
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</imageobject>
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<textobject>
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<phrase>Line synchronization diagram</phrase>
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</textobject>
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</mediaobject>
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</figure>
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<figure id="vbi-525">
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<title>ITU-R 525 line numbering (M/NTSC and M/PAL)</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="vbi_525.pdf" format="PS" />
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</imageobject>
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<imageobject>
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<imagedata fileref="vbi_525.gif" format="GIF" />
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</imageobject>
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<textobject>
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<phrase>NTSC field synchronization diagram</phrase>
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</textobject>
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<caption>
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<para>(1) For the purpose of this specification field 2
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starts in line 264 and not 263.5 because half line capturing is not
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supported.</para>
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</caption>
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</mediaobject>
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</figure>
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<figure id="vbi-625">
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<title>ITU-R 625 line numbering</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="vbi_625.pdf" format="PS" />
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</imageobject>
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<imageobject>
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<imagedata fileref="vbi_625.gif" format="GIF" />
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</imageobject>
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<textobject>
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<phrase>PAL/SECAM field synchronization diagram</phrase>
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</textobject>
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<caption>
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<para>(1) For the purpose of this specification field 2
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starts in line 314 and not 313.5 because half line capturing is not
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supported.</para>
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</caption>
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</mediaobject>
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</figure>
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<para>Remember the VBI image format depends on the selected
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video standard, therefore the application must choose a new standard or
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query the current standard first. Attempts to read or write data ahead
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of format negotiation, or after switching the video standard which may
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invalidate the negotiated VBI parameters, should be refused by the
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driver. A format change during active I/O is not permitted.</para>
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</section>
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<section>
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<title>Reading and writing VBI images</title>
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<para>To assure synchronization with the field number and easier
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implementation, the smallest unit of data passed at a time is one
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frame, consisting of two fields of VBI images immediately following in
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memory.</para>
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<para>The total size of a frame computes as follows:</para>
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<programlisting>
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(<structfield>count</structfield>[0] + <structfield>count</structfield>[1]) *
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<structfield>samples_per_line</structfield> * sample size in bytes</programlisting>
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<para>The sample size is most likely always one byte,
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applications must check the <structfield>sample_format</structfield>
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field though, to function properly with other drivers.</para>
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<para>A VBI device may support <link
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linkend="rw">read/write</link> and/or streaming (<link
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linkend="mmap">memory mapping</link> or <link
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linkend="userp">user pointer</link>) I/O. The latter bears the
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possibility of synchronizing video and
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VBI data by using buffer timestamps.</para>
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<para>Remember the &VIDIOC-STREAMON; ioctl and the first read(),
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write() and select() call can be resource allocation points returning
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an &EBUSY; if the required hardware resources are temporarily
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unavailable, for example the device is already in use by another
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process.</para>
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</section>
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