testcases/kernel/device-drivers/v4l/user_space/doc/spec/x448.htm - platform/external/ltp - Gitiles

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 ><DIV
 CLASS="SECTION"
 ><H1
 CLASS="SECTION"
 ><A
 NAME="STANDARD"
 >1.7. Video Standards</A
 ></H1
 ><P
 >Video devices typically support one or more different video
 standards or variations of standards. Each video input and output may
 support another set of standards. This set is reported by the
 <CODE
 CLASS="STRUCTFIELD"
 >std</CODE
 > field of struct&nbsp;<A
 HREF="r8936.htm#V4L2-INPUT"
 >v4l2_input</A
 > and
 struct&nbsp;<A
 HREF="r9149.htm#V4L2-OUTPUT"
 >v4l2_output</A
 > returned by the <A
 HREF="r8936.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMINPUT</CODE
 ></A
 > and
 <A
 HREF="r9149.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMOUTPUT</CODE
 ></A
 > ioctl, respectively.</P
 ><P
 >V4L2 defines one bit for each analog video standard
 currently in use worldwide, and sets aside bits for driver defined
 standards, e.&nbsp;g. hybrid standards to watch NTSC video tapes on PAL TVs
 and vice versa. Applications can use the predefined bits to select a
 particular standard, although presenting the user a menu of supported
 standards is preferred. To enumerate and query the attributes of the
 supported standards applications use the <A
 HREF="r9288.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMSTD</CODE
 ></A
 > ioctl.</P
 ><P
 >Many of the defined standards are actually just variations
 of a few major standards. The hardware may in fact not distinguish
 between them, or do so internal and switch automatically. Therefore
 enumerated standards also contain sets of one or more standard
 bits.</P
 ><P
 >Assume a hypothetic tuner capable of demodulating B/PAL,
 G/PAL and I/PAL signals. The first enumerated standard is a set of B
 and G/PAL, switched automatically depending on the selected radio
 frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I"
 choice. Similar a Composite input may collapse standards, enumerating
 "PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".<A
 NAME="AEN463"
 HREF="x448.htm#FTN.AEN463"
 ><SPAN
 CLASS="footnote"
 >[1]</SPAN
 ></A
 ></P
 ><P
 >To query and select the standard used by the current video
 input or output applications call the <A
 HREF="r12265.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_G_STD</CODE
 ></A
 > and
 <A
 HREF="r12265.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_S_STD</CODE
 ></A
 > ioctl, respectively. The <SPAN
 CLASS="emphasis"
 ><I
 CLASS="EMPHASIS"
 >received</I
 ></SPAN
 >
 standard can be sensed with the <A
 HREF="r13641.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_QUERYSTD</CODE
 ></A
 > ioctl. Note parameter of all these ioctls is a pointer to a <A
 HREF="r9288.htm#V4L2-STD-ID"
 >v4l2_std_id</A
 > type (a standard set), <SPAN
 CLASS="emphasis"
 ><I
 CLASS="EMPHASIS"
 >not</I
 ></SPAN
 > an index into the standard enumeration.<A
 NAME="AEN475"
 HREF="x448.htm#FTN.AEN475"
 ><SPAN
 CLASS="footnote"
 >[2]</SPAN
 ></A
 > Drivers must implement all video standard ioctls
 when the device has one or more video inputs or outputs.</P
 ><P
 >Special rules apply to USB cameras where the notion of video
 standards makes little sense. More generally any capture device,
 output devices accordingly, which is <P
 ></P
 ><UL
 ><LI
 ><P
 >incapable of capturing fields or frames at the nominal
 rate of the video standard, or</P
 ></LI
 ><LI
 ><P
 >where <A
 HREF="x5953.htm"
 >timestamps</A
 > refer
 to the instant the field or frame was received by the driver, not the
 capture time, or</P
 ></LI
 ><LI
 ><P
 >where <A
 HREF="x5953.htm"
 >sequence numbers</A
 >
 refer to the frames received by the driver, not the captured
 frames.</P
 ></LI
 ></UL
 > Here the driver shall set the
 <CODE
 CLASS="STRUCTFIELD"
 >std</CODE
 > field of struct&nbsp;<A
 HREF="r8936.htm#V4L2-INPUT"
 >v4l2_input</A
 > and struct&nbsp;<A
 HREF="r9149.htm#V4L2-OUTPUT"
 >v4l2_output</A
 >
 to zero, the <CODE
 CLASS="CONSTANT"
 >VIDIOC_G_STD</CODE
 >,
 <CODE
 CLASS="CONSTANT"
 >VIDIOC_S_STD</CODE
 >,
 <CODE
 CLASS="CONSTANT"
 >VIDIOC_QUERYSTD</CODE
 > and
 <CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMSTD</CODE
 > ioctls shall return the
 <SPAN
 CLASS="ERRORCODE"
 >EINVAL</SPAN
 > error code.<A
 NAME="AEN507"
 HREF="x448.htm#FTN.AEN507"
 ><SPAN
 CLASS="footnote"
 >[3]</SPAN
 ></A
 ></P
 ><DIV
 CLASS="EXAMPLE"
 ><A
 NAME="AEN510"
 ></A
 ><P
 ><B
 >Example 1-5. Information about the current video standard</B
 ></P
 ><PRE
 CLASS="PROGRAMLISTING"
 ><A
 HREF="r9288.htm#V4L2-STD-ID"
 >v4l2_std_id</A
 > std_id;
 struct&nbsp;<A
 HREF="r9288.htm#V4L2-STANDARD"
 >v4l2_standard</A
 > standard;

 if (-1 == ioctl (fd, <A
 HREF="r12265.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_G_STD</CODE
 ></A
 >, &amp;std_id)) {
         /* Note when VIDIOC_ENUMSTD always returns EINVAL this
            is no video device or it falls under the USB exception,
            and VIDIOC_G_STD returning EINVAL is no error. */

         perror ("VIDIOC_G_STD");
         exit (EXIT_FAILURE);
 }

 memset (&amp;standard, 0, sizeof (standard));
 standard.index = 0;

 while (0 == ioctl (fd, <A
 HREF="r9288.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMSTD</CODE
 ></A
 >, &amp;standard)) {
         if (standard.id &amp; std_id) {
                printf ("Current video standard: %s\n", standard.name);
                exit (EXIT_SUCCESS);
         }

         standard.index++;
 }

 /* EINVAL indicates the end of the enumeration, which cannot be
    empty unless this device falls under the USB exception. */

 if (errno == EINVAL || standard.index == 0) {
         perror ("VIDIOC_ENUMSTD");
         exit (EXIT_FAILURE);
 }
       </PRE
 ></DIV
 ><DIV
 CLASS="EXAMPLE"
 ><A
 NAME="AEN519"
 ></A
 ><P
 ><B
 >Example 1-6. Listing the video standards supported by the current
 input</B
 ></P
 ><PRE
 CLASS="PROGRAMLISTING"
 >struct&nbsp;<A
 HREF="r8936.htm#V4L2-INPUT"
 >v4l2_input</A
 > input;
 struct&nbsp;<A
 HREF="r9288.htm#V4L2-STANDARD"
 >v4l2_standard</A
 > standard;

 memset (&amp;input, 0, sizeof (input));

 if (-1 == ioctl (fd, <A
 HREF="r11217.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_G_INPUT</CODE
 ></A
 >, &amp;input.index)) {
         perror ("VIDIOC_G_INPUT");
         exit (EXIT_FAILURE);
 }

 if (-1 == ioctl (fd, <A
 HREF="r8936.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMINPUT</CODE
 ></A
 >, &amp;input)) {
         perror ("VIDIOC_ENUM_INPUT");
         exit (EXIT_FAILURE);
 }

 printf ("Current input %s supports:\n", input.name);

 memset (&amp;standard, 0, sizeof (standard));
 standard.index = 0;

 while (0 == ioctl (fd, <A
 HREF="r9288.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMSTD</CODE
 ></A
 >, &amp;standard)) {
         if (standard.id &amp; input.std)
                 printf ("%s\n", standard.name);

         standard.index++;
 }

 /* EINVAL indicates the end of the enumeration, which cannot be
    empty unless this device falls under the USB exception. */

 if (errno != EINVAL || standard.index == 0) {
         perror ("VIDIOC_ENUMSTD");
         exit (EXIT_FAILURE);
 }
       </PRE
 ></DIV
 ><DIV
 CLASS="EXAMPLE"
 ><A
 NAME="AEN530"
 ></A
 ><P
 ><B
 >Example 1-7. Selecting a new video standard</B
 ></P
 ><PRE
 CLASS="PROGRAMLISTING"
 >struct&nbsp;<A
 HREF="r8936.htm#V4L2-INPUT"
 >v4l2_input</A
 > input;
 <A
 HREF="r9288.htm#V4L2-STD-ID"
 >v4l2_std_id</A
 > std_id;

 memset (&amp;input, 0, sizeof (input));

 if (-1 == ioctl (fd, <A
 HREF="r11217.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_G_INPUT</CODE
 ></A
 >, &amp;input.index)) {
         perror ("VIDIOC_G_INPUT");
         exit (EXIT_FAILURE);
 }

 if (-1 == ioctl (fd, <A
 HREF="r8936.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_ENUMINPUT</CODE
 ></A
 >, &amp;input)) {
         perror ("VIDIOC_ENUM_INPUT");
         exit (EXIT_FAILURE);
 }

 if (0 == (input.std &amp; V4L2_STD_PAL_BG)) {
         fprintf (stderr, "Oops. B/G PAL is not supported.\n");
         exit (EXIT_FAILURE);
 }

 /* Note this is also supposed to work when only B
    <SPAN
 CLASS="emphasis"
 ><I
 CLASS="EMPHASIS"
 >or</I
 ></SPAN
 > G/PAL is supported. */

 std_id = V4L2_STD_PAL_BG;

 if (-1 == ioctl (fd, <A
 HREF="r12265.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_S_STD</CODE
 ></A
 >, &amp;std_id)) {
         perror ("VIDIOC_S_STD");
         exit (EXIT_FAILURE);
 }
       </PRE
 ></DIV
 ></DIV
 ><H3
 CLASS="FOOTNOTES"
 >Notes</H3
 ><TABLE
 BORDER="0"
 CLASS="FOOTNOTES"
 WIDTH="100%"
 ><TR
 ><TD
 ALIGN="LEFT"
 VALIGN="TOP"
 WIDTH="5%"
 ><A
 NAME="FTN.AEN463"
 HREF="x448.htm#AEN463"
 ><SPAN
 CLASS="footnote"
 >[1]</SPAN
 ></A
 ></TD
 ><TD
 ALIGN="LEFT"
 VALIGN="TOP"
 WIDTH="95%"
 ><P
 >Some users are already confused by technical terms PAL,
 NTSC and SECAM. There is no point asking them to distinguish between
 B, G, D, or K when the software or hardware can do that
 automatically.</P
 ></TD
 ></TR
 ><TR
 ><TD
 ALIGN="LEFT"
 VALIGN="TOP"
 WIDTH="5%"
 ><A
 NAME="FTN.AEN475"
 HREF="x448.htm#AEN475"
 ><SPAN
 CLASS="footnote"
 >[2]</SPAN
 ></A
 ></TD
 ><TD
 ALIGN="LEFT"
 VALIGN="TOP"
 WIDTH="95%"
 ><P
 >An alternative to the current scheme is to use pointers
 to indices as arguments of <CODE
 CLASS="CONSTANT"
 >VIDIOC_G_STD</CODE
 > and
 <CODE
 CLASS="CONSTANT"
 >VIDIOC_S_STD</CODE
 >, the struct&nbsp;<A
 HREF="r8936.htm#V4L2-INPUT"
 >v4l2_input</A
 > and
 struct&nbsp;<A
 HREF="r9149.htm#V4L2-OUTPUT"
 >v4l2_output</A
 > <CODE
 CLASS="STRUCTFIELD"
 >std</CODE
 > field would be a set of
 indices like <CODE
 CLASS="STRUCTFIELD"
 >audioset</CODE
 >.</P
 ><P
 >Indices are consistent with the rest of the API
 and identify the standard unambiguously. In the present scheme of
 things an enumerated standard is looked up by <A
 HREF="r9288.htm#V4L2-STD-ID"
 >v4l2_std_id</A
 >. Now the
 standards supported by the inputs of a device can overlap. Just
 assume the tuner and composite input in the example above both
 exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests
 a choice which does not exist. We cannot merge or omit sets, because
 applications would be unable to find the standards reported by
 <CODE
 CLASS="CONSTANT"
 >VIDIOC_G_STD</CODE
 >. That leaves separate enumerations
 for each input. Also selecting a standard by <A
 HREF="r9288.htm#V4L2-STD-ID"
 >v4l2_std_id</A
 > can be
 ambiguous. Advantage of this method is that applications need not
 identify the standard indirectly, after enumerating.</P
 ><P
 >So in
 summary, the lookup itself is unavoidable. The difference is only
 whether the lookup is necessary to find an enumerated standard or to
 switch to a standard by <A
 HREF="r9288.htm#V4L2-STD-ID"
 >v4l2_std_id</A
 >.</P
 ></TD
 ></TR
 ><TR
 ><TD
 ALIGN="LEFT"
 VALIGN="TOP"
 WIDTH="5%"
 ><A
 NAME="FTN.AEN507"
 HREF="x448.htm#AEN507"
 ><SPAN
 CLASS="footnote"
 >[3]</SPAN
 ></A
 ></TD
 ><TD
 ALIGN="LEFT"
 VALIGN="TOP"
 WIDTH="95%"
 ><P
 >See <A
 HREF="x5953.htm"
 >Section 3.5</A
 > for a rationale. Probably
 even USB cameras follow some well known video standard. It might have
 been better to explicitly indicate elsewhere if a device cannot live
 up to normal expectations, instead of this exception.</P
 ></TD
 ></TR
 ></TABLE
 ><DIV
 CLASS="NAVFOOTER"
 ><HR
 ALIGN="LEFT"
 WIDTH="100%"><TABLE
 SUMMARY="Footer navigation table"
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 ><A
 HREF="x394.htm"
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 ALIGN="right"
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	>Video for Linux Two API Specification: Revision 0.24</TH
	></TR
	><TR
	><TD
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	ALIGN="left"
	VALIGN="bottom"
	><A
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	ACCESSKEY="P"
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	><DIV
	CLASS="SECTION"
	><H1
	CLASS="SECTION"
	><A
	NAME="STANDARD"
	>1.7. Video Standards</A
	></H1
	><P
	>Video devices typically support one or more different video
	standards or variations of standards. Each video input and output may
	support another set of standards. This set is reported by the
	<CODE
	CLASS="STRUCTFIELD"
	>std</CODE
	> field of struct <A
	HREF="r8936.htm#V4L2-INPUT"
	>v4l2_input</A
	> and
	struct <A
	HREF="r9149.htm#V4L2-OUTPUT"
	>v4l2_output</A
	> returned by the <A
	HREF="r8936.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMINPUT</CODE
	></A
	> and
	<A
	HREF="r9149.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMOUTPUT</CODE
	></A
	> ioctl, respectively.</P
	><P
	>V4L2 defines one bit for each analog video standard
	currently in use worldwide, and sets aside bits for driver defined
	standards, e. g. hybrid standards to watch NTSC video tapes on PAL TVs
	and vice versa. Applications can use the predefined bits to select a
	particular standard, although presenting the user a menu of supported
	standards is preferred. To enumerate and query the attributes of the
	supported standards applications use the <A
	HREF="r9288.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMSTD</CODE
	></A
	> ioctl.</P
	><P
	>Many of the defined standards are actually just variations
	of a few major standards. The hardware may in fact not distinguish
	between them, or do so internal and switch automatically. Therefore
	enumerated standards also contain sets of one or more standard
	bits.</P
	><P
	>Assume a hypothetic tuner capable of demodulating B/PAL,
	G/PAL and I/PAL signals. The first enumerated standard is a set of B
	and G/PAL, switched automatically depending on the selected radio
	frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I"
	choice. Similar a Composite input may collapse standards, enumerating
	"PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".<A
	NAME="AEN463"
	HREF="x448.htm#FTN.AEN463"
	><SPAN
	CLASS="footnote"
	>[1]</SPAN
	></A
	></P
	><P
	>To query and select the standard used by the current video
	input or output applications call the <A
	HREF="r12265.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_G_STD</CODE
	></A
	> and
	<A
	HREF="r12265.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_S_STD</CODE
	></A
	> ioctl, respectively. The <SPAN
	CLASS="emphasis"
	><I
	CLASS="EMPHASIS"
	>received</I
	></SPAN
	>
	standard can be sensed with the <A
	HREF="r13641.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_QUERYSTD</CODE
	></A
	> ioctl. Note parameter of all these ioctls is a pointer to a <A
	HREF="r9288.htm#V4L2-STD-ID"
	>v4l2_std_id</A
	> type (a standard set), <SPAN
	CLASS="emphasis"
	><I
	CLASS="EMPHASIS"
	>not</I
	></SPAN
	> an index into the standard enumeration.<A
	NAME="AEN475"
	HREF="x448.htm#FTN.AEN475"
	><SPAN
	CLASS="footnote"
	>[2]</SPAN
	></A
	> Drivers must implement all video standard ioctls
	when the device has one or more video inputs or outputs.</P
	><P
	>Special rules apply to USB cameras where the notion of video
	standards makes little sense. More generally any capture device,
	output devices accordingly, which is <P
	></P
	><UL
	><LI
	><P
	>incapable of capturing fields or frames at the nominal
	rate of the video standard, or</P
	></LI
	><LI
	><P
	>where <A
	HREF="x5953.htm"
	>timestamps</A
	> refer
	to the instant the field or frame was received by the driver, not the
	capture time, or</P
	></LI
	><LI
	><P
	>where <A
	HREF="x5953.htm"
	>sequence numbers</A
	>
	refer to the frames received by the driver, not the captured
	frames.</P
	></LI
	></UL
	> Here the driver shall set the
	<CODE
	CLASS="STRUCTFIELD"
	>std</CODE
	> field of struct <A
	HREF="r8936.htm#V4L2-INPUT"
	>v4l2_input</A
	> and struct <A
	HREF="r9149.htm#V4L2-OUTPUT"
	>v4l2_output</A
	>
	to zero, the <CODE
	CLASS="CONSTANT"
	>VIDIOC_G_STD</CODE
	>,
	<CODE
	CLASS="CONSTANT"
	>VIDIOC_S_STD</CODE
	>,
	<CODE
	CLASS="CONSTANT"
	>VIDIOC_QUERYSTD</CODE
	> and
	<CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMSTD</CODE
	> ioctls shall return the
	<SPAN
	CLASS="ERRORCODE"
	>EINVAL</SPAN
	> error code.<A
	NAME="AEN507"
	HREF="x448.htm#FTN.AEN507"
	><SPAN
	CLASS="footnote"
	>[3]</SPAN
	></A
	></P
	><DIV
	CLASS="EXAMPLE"
	><A
	NAME="AEN510"
	></A
	><P
	><B
	>Example 1-5. Information about the current video standard</B
	></P
	><PRE
	CLASS="PROGRAMLISTING"
	><A
	HREF="r9288.htm#V4L2-STD-ID"
	>v4l2_std_id</A
	> std_id;
	struct <A
	HREF="r9288.htm#V4L2-STANDARD"
	>v4l2_standard</A
	> standard;

	if (-1 == ioctl (fd, <A
	HREF="r12265.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_G_STD</CODE
	></A
	>, &std_id)) {
	/* Note when VIDIOC_ENUMSTD always returns EINVAL this
	is no video device or it falls under the USB exception,
	and VIDIOC_G_STD returning EINVAL is no error. */

	perror ("VIDIOC_G_STD");
	exit (EXIT_FAILURE);
	}

	memset (&standard, 0, sizeof (standard));
	standard.index = 0;

	while (0 == ioctl (fd, <A
	HREF="r9288.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMSTD</CODE
	></A
	>, &standard)) {
	if (standard.id & std_id) {
	printf ("Current video standard: %s\n", standard.name);
	exit (EXIT_SUCCESS);
	}

	standard.index++;
	}

	/* EINVAL indicates the end of the enumeration, which cannot be
	empty unless this device falls under the USB exception. */

	if (errno == EINVAL \|\| standard.index == 0) {
	perror ("VIDIOC_ENUMSTD");
	exit (EXIT_FAILURE);
	}
	</PRE
	></DIV
	><DIV
	CLASS="EXAMPLE"
	><A
	NAME="AEN519"
	></A
	><P
	><B
	>Example 1-6. Listing the video standards supported by the current
	input</B
	></P
	><PRE
	CLASS="PROGRAMLISTING"
	>struct <A
	HREF="r8936.htm#V4L2-INPUT"
	>v4l2_input</A
	> input;
	struct <A
	HREF="r9288.htm#V4L2-STANDARD"
	>v4l2_standard</A
	> standard;

	memset (&input, 0, sizeof (input));

	if (-1 == ioctl (fd, <A
	HREF="r11217.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_G_INPUT</CODE
	></A
	>, &input.index)) {
	perror ("VIDIOC_G_INPUT");
	exit (EXIT_FAILURE);
	}

	if (-1 == ioctl (fd, <A
	HREF="r8936.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMINPUT</CODE
	></A
	>, &input)) {
	perror ("VIDIOC_ENUM_INPUT");
	exit (EXIT_FAILURE);
	}

	printf ("Current input %s supports:\n", input.name);

	memset (&standard, 0, sizeof (standard));
	standard.index = 0;

	while (0 == ioctl (fd, <A
	HREF="r9288.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMSTD</CODE
	></A
	>, &standard)) {
	if (standard.id & input.std)
	printf ("%s\n", standard.name);

	standard.index++;
	}

	/* EINVAL indicates the end of the enumeration, which cannot be
	empty unless this device falls under the USB exception. */

	if (errno != EINVAL \|\| standard.index == 0) {
	perror ("VIDIOC_ENUMSTD");
	exit (EXIT_FAILURE);
	}
	</PRE
	></DIV
	><DIV
	CLASS="EXAMPLE"
	><A
	NAME="AEN530"
	></A
	><P
	><B
	>Example 1-7. Selecting a new video standard</B
	></P
	><PRE
	CLASS="PROGRAMLISTING"
	>struct <A
	HREF="r8936.htm#V4L2-INPUT"
	>v4l2_input</A
	> input;
	<A
	HREF="r9288.htm#V4L2-STD-ID"
	>v4l2_std_id</A
	> std_id;

	memset (&input, 0, sizeof (input));

	if (-1 == ioctl (fd, <A
	HREF="r11217.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_G_INPUT</CODE
	></A
	>, &input.index)) {
	perror ("VIDIOC_G_INPUT");
	exit (EXIT_FAILURE);
	}

	if (-1 == ioctl (fd, <A
	HREF="r8936.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_ENUMINPUT</CODE
	></A
	>, &input)) {
	perror ("VIDIOC_ENUM_INPUT");
	exit (EXIT_FAILURE);
	}

	if (0 == (input.std & V4L2_STD_PAL_BG)) {
	fprintf (stderr, "Oops. B/G PAL is not supported.\n");
	exit (EXIT_FAILURE);
	}

	/* Note this is also supposed to work when only B
	<SPAN
	CLASS="emphasis"
	><I
	CLASS="EMPHASIS"
	>or</I
	></SPAN
	> G/PAL is supported. */

	std_id = V4L2_STD_PAL_BG;

	if (-1 == ioctl (fd, <A
	HREF="r12265.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_S_STD</CODE
	></A
	>, &std_id)) {
	perror ("VIDIOC_S_STD");
	exit (EXIT_FAILURE);
	}
	</PRE
	></DIV
	></DIV
	><H3
	CLASS="FOOTNOTES"
	>Notes</H3
	><TABLE
	BORDER="0"
	CLASS="FOOTNOTES"
	WIDTH="100%"
	><TR
	><TD
	ALIGN="LEFT"
	VALIGN="TOP"
	WIDTH="5%"
	><A
	NAME="FTN.AEN463"
	HREF="x448.htm#AEN463"
	><SPAN
	CLASS="footnote"
	>[1]</SPAN
	></A
	></TD
	><TD
	ALIGN="LEFT"
	VALIGN="TOP"
	WIDTH="95%"
	><P
	>Some users are already confused by technical terms PAL,
	NTSC and SECAM. There is no point asking them to distinguish between
	B, G, D, or K when the software or hardware can do that
	automatically.</P
	></TD
	></TR
	><TR
	><TD
	ALIGN="LEFT"
	VALIGN="TOP"
	WIDTH="5%"
	><A
	NAME="FTN.AEN475"
	HREF="x448.htm#AEN475"
	><SPAN
	CLASS="footnote"
	>[2]</SPAN
	></A
	></TD
	><TD
	ALIGN="LEFT"
	VALIGN="TOP"
	WIDTH="95%"
	><P
	>An alternative to the current scheme is to use pointers
	to indices as arguments of <CODE
	CLASS="CONSTANT"
	>VIDIOC_G_STD</CODE
	> and
	<CODE
	CLASS="CONSTANT"
	>VIDIOC_S_STD</CODE
	>, the struct <A
	HREF="r8936.htm#V4L2-INPUT"
	>v4l2_input</A
	> and
	struct <A
	HREF="r9149.htm#V4L2-OUTPUT"
	>v4l2_output</A
	> <CODE
	CLASS="STRUCTFIELD"
	>std</CODE
	> field would be a set of
	indices like <CODE
	CLASS="STRUCTFIELD"
	>audioset</CODE
	>.</P
	><P
	>Indices are consistent with the rest of the API
	and identify the standard unambiguously. In the present scheme of
	things an enumerated standard is looked up by <A
	HREF="r9288.htm#V4L2-STD-ID"
	>v4l2_std_id</A
	>. Now the
	standards supported by the inputs of a device can overlap. Just
	assume the tuner and composite input in the example above both
	exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests
	a choice which does not exist. We cannot merge or omit sets, because
	applications would be unable to find the standards reported by
	<CODE
	CLASS="CONSTANT"
	>VIDIOC_G_STD</CODE
	>. That leaves separate enumerations
	for each input. Also selecting a standard by <A
	HREF="r9288.htm#V4L2-STD-ID"
	>v4l2_std_id</A
	> can be
	ambiguous. Advantage of this method is that applications need not
	identify the standard indirectly, after enumerating.</P
	><P
	>So in
	summary, the lookup itself is unavoidable. The difference is only
	whether the lookup is necessary to find an enumerated standard or to
	switch to a standard by <A
	HREF="r9288.htm#V4L2-STD-ID"
	>v4l2_std_id</A
	>.</P
	></TD
	></TR
	><TR
	><TD
	ALIGN="LEFT"
	VALIGN="TOP"
	WIDTH="5%"
	><A
	NAME="FTN.AEN507"
	HREF="x448.htm#AEN507"
	><SPAN
	CLASS="footnote"
	>[3]</SPAN
	></A
	></TD
	><TD
	ALIGN="LEFT"
	VALIGN="TOP"
	WIDTH="95%"
	><P
	>See <A
	HREF="x5953.htm"
	>Section 3.5</A
	> for a rationale. Probably
	even USB cameras follow some well known video standard. It might have
	been better to explicitly indicate elsewhere if a device cannot live
	up to normal expectations, instead of this exception.</P
	></TD
	></TR
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