doc/spec/x7236.htm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Sliced VBI Data Interface</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REL="HOME"
TITLE="Video for Linux Two API Specification"
HREF="book1.htm"><LINK
REL="UP"
TITLE="Interfaces"
HREF="c6488.htm"><LINK
REL="PREVIOUS"
TITLE="Raw VBI Data Interface"
HREF="x7013.htm"><LINK
REL="NEXT"
TITLE="Teletext Interface"
HREF="x7561.htm"></HEAD
><BODY
CLASS="SECTION"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Video for Linux Two API Specification: Revision 0.24</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="x7013.htm"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 4. Interfaces</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="x7561.htm"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECTION"
><H1
CLASS="SECTION"
><A
NAME="SLICED"
>4.8. Sliced VBI Data Interface</A
></H1
><P
>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.</P
><P
>Sliced VBI devices use hardware to demodulate data transmitted
in the VBI. V4L2 drivers shall <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> do this by
software, see also the <A
HREF="x7013.htm"
>raw VBI
interface</A
>. The data is passed as short packets of fixed size,
covering one scan line each. The number of packets per video frame is
variable.</P
><P
>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
<TT
CLASS="FILENAME"
>/dev/vbi</TT
> device is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>raw</I
></SPAN
> VBI
capturing or output, and the sliced VBI function is only available
after calling the <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> ioctl as defined below. Likewise a
<TT
CLASS="FILENAME"
>/dev/video</TT
> 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.</P
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN7248"
>4.8.1. Querying Capabilities</A
></H2
><P
>Devices supporting the sliced VBI capturing or output API
set the <CODE
CLASS="CONSTANT"
>V4L2_CAP_SLICED_VBI_CAPTURE</CODE
> or
<CODE
CLASS="CONSTANT"
>V4L2_CAP_SLICED_VBI_OUTPUT</CODE
> flag respectively, in
the <CODE
CLASS="STRUCTFIELD"
>capabilities</CODE
> field of struct&nbsp;<A
HREF="r13105.htm#V4L2-CAPABILITY"
>v4l2_capability</A
>
returned by the <A
HREF="r13105.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_QUERYCAP</CODE
></A
> ioctl. At least one of the
read/write, streaming or asynchronous <A
HREF="c5742.htm"
>I/O
methods</A
> must be supported. Sliced VBI devices may have a tuner
or modulator.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN7258"
>4.8.2. Supplemental Functions</A
></H2
><P
>Sliced VBI devices shall support <A
HREF="x309.htm"
>video
input or output</A
> and <A
HREF="x394.htm"
>tuner or
modulator</A
> ioctls if they have these capabilities, and they may
support <A
HREF="x542.htm"
>control</A
> ioctls. The <A
HREF="x448.htm"
>video standard</A
> ioctls provide information
vital to program a sliced VBI device, therefore must be
supported.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN7265"
>4.8.3. Sliced VBI Format Negotiation</A
></H2
><P
>To find out which data services are supported by the
hardware applications can call the <A
HREF="r12051.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_G_SLICED_VBI_CAP</CODE
></A
> ioctl.
All drivers implementing the sliced VBI interface must support this
ioctl. The results may differ from those of the <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> 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.</P
><P
>To determine the currently selected services applications
set the <CODE
CLASS="STRUCTFIELD"
>type </CODE
> field of struct&nbsp;<A
HREF="r10944.htm#V4L2-FORMAT"
>v4l2_format</A
> to
<CODE
CLASS="CONSTANT"
> V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</CODE
> or <CODE
CLASS="CONSTANT"
>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</CODE
>, and the <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_G_FMT</CODE
></A
>
ioctl fills the <CODE
CLASS="STRUCTFIELD"
>fmt.sliced</CODE
> member, a
struct&nbsp;<A
HREF="x7236.htm#V4L2-SLICED-VBI-FORMAT"
>v4l2_sliced_vbi_format</A
>.</P
><P
>Applications can request different parameters by
initializing or modifying the <CODE
CLASS="STRUCTFIELD"
>fmt.sliced</CODE
>
member and calling the <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> ioctl with a pointer to the
<CODE
CLASS="STRUCTNAME"
>v4l2_format</CODE
> structure.</P
><P
>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
<CODE
CLASS="STRUCTFIELD"
>service_set</CODE
> field to the required services
and let the driver fill the <CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>
array according to hardware capabilities. Only if more precise control
is needed should the programmer set the
<CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
> array explicitly.</P
><P
>The <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> ioctl returns an <SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
> error code only when the
given parameters are ambiguous, otherwise it modifies the parameters
according to hardware capabilities. When the driver allocates
resources at this point, it may return an <SPAN
CLASS="ERRORCODE"
>EBUSY</SPAN
> error code if the required
resources are temporarily unavailable. Other resource allocation
points which may return <SPAN
CLASS="ERRORCODE"
>EBUSY</SPAN
> can be the
<A
HREF="r13817.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_STREAMON</CODE
></A
> ioctl and the first <A
HREF="r14264.htm"
><CODE
CLASS="FUNCTION"
>read()</CODE
></A
>, <A
HREF="r14496.htm"
><CODE
CLASS="FUNCTION"
>write()</CODE
></A
> and
<A
HREF="r14390.htm"
><CODE
CLASS="FUNCTION"
>select()</CODE
></A
> call.</P
><DIV
CLASS="TABLE"
><A
NAME="V4L2-SLICED-VBI-FORMAT"
></A
><P
><B
>Table 4-6. struct
<CODE
CLASS="STRUCTNAME"
>v4l2_sliced_vbi_format</CODE
></B
></P
><TABLE
BORDER="0"
FRAME="void"
WIDTH="100%"
CLASS="CALSTABLE"
><COL
WIDTH="25%"
TITLE="C1"><COL
WIDTH="25%"
TITLE="C2"><COL
WIDTH="17%"
TITLE="C3"><COL
WIDTH="17%"
TITLE="C4"><COL
WIDTH="17%"
TITLE="C5"><TBODY
VALIGN="TOP"
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>service_set</CODE
></TD
><TD
COLSPAN="3"
><P
>If
<CODE
CLASS="STRUCTFIELD"
>service_set</CODE
> is non-zero when passed with
<A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> or <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_TRY_FMT</CODE
></A
>, the
<CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
> array will be filled by the
driver according to the services specified in this field. For example,
if <CODE
CLASS="STRUCTFIELD"
>service_set</CODE
> is initialized with
<CODE
CLASS="CONSTANT"
>V4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625</CODE
>, a
driver for the cx25840 video decoder sets lines 7-22 of both
fields<SUP
>a</SUP
> to <CODE
CLASS="CONSTANT"
>V4L2_SLICED_TELETEXT_B</CODE
>
and line 23 of the first field to
<CODE
CLASS="CONSTANT"
>V4L2_SLICED_WSS_625</CODE
>. If
<CODE
CLASS="STRUCTFIELD"
>service_set</CODE
> is set to zero, then the values
of <CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
> will be used instead.</P
><P
>On return the driver sets this field to the union of all
elements of the returned <CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>
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.</P
></TD
></TR
><TR
><TD
>__u16</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[2][24]</TD
><TD
COLSPAN="3"
><P
>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.</P
><P
>Data
services are defined in <A
HREF="x7236.htm#VBI-SERVICES2"
>Table 4-7</A
>. Array indices
map to ITU-R line numbers (see also <A
HREF="x7013.htm#VBI-525"
>Figure 4-2</A
> and <A
HREF="x7013.htm#VBI-625"
>Figure 4-3</A
>) as follows: </P
></TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
><TD
>Element</TD
><TD
>525 line systems</TD
><TD
>625 line systems</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[0][1]</TD
><TD
ALIGN="CENTER"
>1</TD
><TD
ALIGN="CENTER"
>1</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[0][23]</TD
><TD
ALIGN="CENTER"
>23</TD
><TD
ALIGN="CENTER"
>23</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[1][1]</TD
><TD
ALIGN="CENTER"
>264</TD
><TD
ALIGN="CENTER"
>314</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[1][23]</TD
><TD
ALIGN="CENTER"
>286</TD
><TD
ALIGN="CENTER"
>336</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
><TD
COLSPAN="3"
>Drivers must set
<CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[0][0] and
<CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>[1][0] to zero.</TD
></TR
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>io_size</CODE
></TD
><TD
COLSPAN="3"
>Maximum number of bytes passed by
one <A
HREF="r14264.htm"
><CODE
CLASS="FUNCTION"
>read()</CODE
></A
> or <A
HREF="r14496.htm"
><CODE
CLASS="FUNCTION"
>write()</CODE
></A
> call, and the buffer size in bytes for
the <A
HREF="r12878.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_QBUF</CODE
></A
> and <A
HREF="r12878.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_DQBUF</CODE
></A
> ioctl. Drivers set this field to
the size of struct&nbsp;<A
HREF="x7236.htm#V4L2-SLICED-VBI-DATA"
>v4l2_sliced_vbi_data</A
> times the number of non-zero
elements in the returned <CODE
CLASS="STRUCTFIELD"
>service_lines</CODE
>
array (that is the number of lines potentially carrying data).</TD
></TR
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>reserved</CODE
>[2]</TD
><TD
COLSPAN="3"
>This array is reserved for future
extensions. Applications and drivers must set it to zero.</TD
></TR
></TBODY
><TR
><TD
COLSPAN="5"
>Notes:<BR><A
NAME="FTN.AEN7329"
>a. </A
>According to <A
HREF="b17127.htm#ETS300706"
>ETS&nbsp;300&nbsp;706</A
> lines 6-22 of the
first field and lines 5-22 of the second field may carry Teletext
data.<BR></TD
></TR
></TABLE
></DIV
><DIV
CLASS="TABLE"
><A
NAME="VBI-SERVICES2"
></A
><P
><B
>Table 4-7. Sliced VBI services</B
></P
><TABLE
BORDER="0"
FRAME="void"
WIDTH="100%"
CLASS="CALSTABLE"
><COL
WIDTH="25%"
TITLE="C1"><COL
WIDTH="12%"
TITLE="C2"><COL
WIDTH="12%"
TITLE="C3"><COL
WIDTH="25%"
TITLE="C4"><COL
WIDTH="25%"
TITLE="C5"><THEAD
><TR
><TH
>Symbol</TH
><TH
>Value</TH
><TH
>Reference</TH
><TH
>Lines, usually</TH
><TH
>Payload</TH
></TR
></THEAD
><TBODY
VALIGN="TOP"
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_SLICED_TELETEXT_B</CODE
>
(Teletext System B)</TD
><TD
>0x0001</TD
><TD
><A
HREF="b17127.htm#ETS300706"
><ABBR
CLASS="ABBREV"
>ETS&nbsp;300&nbsp;706</ABBR
></A
>, <A
HREF="b17127.htm#ITU653"
><ABBR
CLASS="ABBREV"
>ITU&nbsp;BT.653</ABBR
></A
></TD
><TD
>PAL/SECAM line 7-22, 320-335 (second field 7-22)</TD
><TD
>Last 42 of the 45 byte Teletext packet, that is
without clock run-in and framing code, lsb first transmitted.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_SLICED_VPS</CODE
></TD
><TD
>0x0400</TD
><TD
><A
HREF="b17127.htm#ETS300231"
><ABBR
CLASS="ABBREV"
>ETS&nbsp;300&nbsp;231</ABBR
></A
></TD
><TD
>PAL line 16</TD
><TD
>Byte number 3 to 15 according to Figure 9 of
ETS&nbsp;300&nbsp;231, lsb first transmitted.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_SLICED_CAPTION_525</CODE
></TD
><TD
>0x1000</TD
><TD
><A
HREF="b17127.htm#EIA608"
><ABBR
CLASS="ABBREV"
>EIA&nbsp;608-B</ABBR
></A
></TD
><TD
>NTSC line 21, 284 (second field 21)</TD
><TD
>Two bytes in transmission order, including parity
bit, lsb first transmitted.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_SLICED_WSS_625</CODE
></TD
><TD
>0x4000</TD
><TD
><A
HREF="b17127.htm#ITU1119"
><ABBR
CLASS="ABBREV"
>ITU&nbsp;BT.1119</ABBR
></A
>, <A
HREF="b17127.htm#EN300294"
><ABBR
CLASS="ABBREV"
>EN&nbsp;300&nbsp;294</ABBR
></A
></TD
><TD
>PAL/SECAM line 23</TD
><TD
><PRE
CLASS="SCREEN"
>Byte         0                 1
      msb         lsb  msb           lsb
 Bit  7 6 5 4 3 2 1 0  x x 13 12 11 10 9</PRE
></TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_SLICED_VBI_525</CODE
></TD
><TD
>0x1000</TD
><TD
COLSPAN="3"
>Set of services applicable to 525
line systems.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_SLICED_VBI_625</CODE
></TD
><TD
>0x4401</TD
><TD
COLSPAN="3"
>Set of services applicable to 625
line systems.</TD
></TR
></TBODY
></TABLE
></DIV
><P
>Drivers may return an <SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
> error code when applications attempt to
read or write data without prior format negotiation, after switching
the video standard (which may invalidate the negotiated VBI
parameters) and after switching the video input (which may change the
video standard as a side effect). The <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> ioctl may return
an <SPAN
CLASS="ERRORCODE"
>EBUSY</SPAN
> error code when applications attempt to change the format while i/o is
in progress (between a <A
HREF="r13817.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_STREAMON</CODE
></A
> and <A
HREF="r13817.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_STREAMOFF</CODE
></A
> call,
and after the first <A
HREF="r14264.htm"
><CODE
CLASS="FUNCTION"
>read()</CODE
></A
> or <A
HREF="r14496.htm"
><CODE
CLASS="FUNCTION"
>write()</CODE
></A
> call).</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN7483"
>4.8.4. Reading and writing sliced VBI data</A
></H2
><P
>A single <A
HREF="r14264.htm"
><CODE
CLASS="FUNCTION"
>read()</CODE
></A
> or <A
HREF="r14496.htm"
><CODE
CLASS="FUNCTION"
>write()</CODE
></A
> call must pass all data
belonging to one video frame. That is an array of
<CODE
CLASS="STRUCTNAME"
>v4l2_sliced_vbi_data</CODE
> structures with one or
more elements and a total size not exceeding
<CODE
CLASS="STRUCTFIELD"
>io_size</CODE
> bytes. Likewise in streaming I/O
mode one buffer of <CODE
CLASS="STRUCTFIELD"
>io_size</CODE
> bytes must
contain data of one video frame. The <CODE
CLASS="STRUCTFIELD"
>id</CODE
> of
unused <CODE
CLASS="STRUCTNAME"
>v4l2_sliced_vbi_data</CODE
> elements must be
zero.</P
><DIV
CLASS="TABLE"
><A
NAME="V4L2-SLICED-VBI-DATA"
></A
><P
><B
>Table 4-8. struct
<CODE
CLASS="STRUCTNAME"
>v4l2_sliced_vbi_data</CODE
></B
></P
><TABLE
BORDER="0"
FRAME="void"
WIDTH="100%"
CLASS="CALSTABLE"
><COL
WIDTH="38%"
TITLE="C1"><COL
WIDTH="12%"
TITLE="C2"><COL
WIDTH="50%"
TITLE="C3"><TBODY
VALIGN="TOP"
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>id</CODE
></TD
><TD
>A flag from <A
HREF="r12051.htm#VBI-SERVICES"
>Table 2</A
>
identifying the type of data in this packet. Only a single bit must be
set. When the <CODE
CLASS="STRUCTFIELD"
>id</CODE
> of a captured packet is
zero, the packet is empty and the contents of other fields are
undefined. Applications shall ignore empty packets. When the
<CODE
CLASS="STRUCTFIELD"
>id</CODE
> of a packet for output is zero the
contents of the <CODE
CLASS="STRUCTFIELD"
>data</CODE
> field are undefined
and the driver must no longer insert data on the requested
<CODE
CLASS="STRUCTFIELD"
>field</CODE
> and
<CODE
CLASS="STRUCTFIELD"
>line</CODE
>.</TD
></TR
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>field</CODE
></TD
><TD
>The video field number this data has been captured
from, or shall be inserted at. <CODE
CLASS="CONSTANT"
>0</CODE
> for the first
field, <CODE
CLASS="CONSTANT"
>1</CODE
> for the second field.</TD
></TR
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>line</CODE
></TD
><TD
>The field (as opposed to frame) line number this
data has been captured from, or shall be inserted at. See <A
HREF="x7013.htm#VBI-525"
>Figure 4-2</A
> and <A
HREF="x7013.htm#VBI-625"
>Figure 4-3</A
> for valid
values. Sliced VBI capture devices can set the line number of all
packets to <CODE
CLASS="CONSTANT"
>0</CODE
> if the hardware cannot reliably
identify scan lines. The field number must always be valid.</TD
></TR
><TR
><TD
>__u32</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>reserved</CODE
></TD
><TD
>This field is reserved for future extensions.
Applications and drivers must set it to zero.</TD
></TR
><TR
><TD
>__u8</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>data</CODE
>[48]</TD
><TD
>The packet payload. See <A
HREF="r12051.htm#VBI-SERVICES"
>Table 2</A
> for the contents and number of
bytes passed for each data type. The contents of padding bytes at the
end of this array are undefined, drivers and applications shall ignore
them.</TD
></TR
></TBODY
></TABLE
></DIV
><P
>Packets are always passed in ascending line number order,
without duplicate line numbers. The <A
HREF="r14496.htm"
><CODE
CLASS="FUNCTION"
>write()</CODE
></A
> function and the
<A
HREF="r12878.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_QBUF</CODE
></A
> ioctl must return an <SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
> error code when applications violate
this rule. They must also return an <SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
> error code when applications pass an
incorrect field or line number, or a combination of
<CODE
CLASS="STRUCTFIELD"
>field</CODE
>, <CODE
CLASS="STRUCTFIELD"
>line</CODE
> and
<CODE
CLASS="STRUCTFIELD"
>id</CODE
> which has not been negotiated with the
<A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_G_FMT</CODE
></A
> or <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> ioctl. When the line numbers are
unknown the driver must pass the packets in transmitted order. The
driver can insert empty packets with <CODE
CLASS="STRUCTFIELD"
>id</CODE
> set
to zero anywhere in the packet array.</P
><P
>To assure synchronization and to distinguish from frame
dropping, when a captured frame does not carry any of the requested
data services drivers must pass one or more empty packets. When an
application fails to pass VBI data in time for output, the driver
must output the last VPS and WSS packet again, and disable the output
of Closed Caption and Teletext data, or output data which is ignored
by Closed Caption and Teletext decoders.</P
><P
>A sliced VBI device may support <A
HREF="c5742.htm#RW"
>read/write</A
> and/or streaming (<A
HREF="x5791.htm"
>memory mapping</A
> and/or <A
HREF="x5884.htm"
>user
pointer</A
>) I/O. The latter bears the possibility of synchronizing
video and VBI data by using buffer timestamps.</P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="x7013.htm"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="book1.htm"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="x7561.htm"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Raw VBI Data Interface</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="c6488.htm"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Teletext Interface</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>