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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _standard:
+
+***************
+Video Standards
+***************
+
+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 ``std`` field of struct
+:ref:`v4l2_input <v4l2-input>` and struct
+:ref:`v4l2_output <v4l2-output>` returned by the
+:ref:`VIDIOC_ENUMINPUT <vidioc-enuminput>` and
+:ref:`VIDIOC_ENUMOUTPUT <vidioc-enumoutput>` ioctls, respectively.
+
+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 :ref:`VIDIOC_ENUMSTD <vidioc-enumstd>`
+ioctl.
+
+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.
+
+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". [1]_
+
+To query and select the standard used by the current video input or
+output applications call the :ref:`VIDIOC_G_STD <vidioc-g-std>` and
+:ref:`VIDIOC_S_STD <vidioc-g-std>` ioctl, respectively. The
+*received* standard can be sensed with the
+:ref:`VIDIOC_QUERYSTD <vidioc-querystd>` ioctl. Note that the
+parameter of all these ioctls is a pointer to a
+:ref:`v4l2_std_id <v4l2-std-id>` type (a standard set), *not* an
+index into the standard enumeration. Drivers must implement all video
+standard ioctls when the device has one or more video inputs or outputs.
+
+Special rules apply to devices such as USB cameras where the notion of
+video standards makes little sense. More generally for any capture or
+output device which is:
+
+-  incapable of capturing fields or frames at the nominal rate of the
+   video standard, or
+
+-  that does not support the video standard formats at all.
+
+Here the driver shall set the ``std`` field of struct
+:ref:`v4l2_input <v4l2-input>` and struct
+:ref:`v4l2_output <v4l2-output>` to zero and the ``VIDIOC_G_STD``,
+``VIDIOC_S_STD``, ``VIDIOC_QUERYSTD`` and ``VIDIOC_ENUMSTD`` ioctls
+shall return the ENOTTY error code or the EINVAL error code.
+
+Applications can make use of the :ref:`input-capabilities` and
+:ref:`output-capabilities` flags to determine whether the video
+standard ioctls can be used with the given input or output.
+
+
+.. code-block:: c
+
+    v4l2_std_id std_id;
+    struct v4l2_standard standard;
+
+    if (-1 == ioctl(fd, VIDIOC_G_STD, &std_id)) {
+        /* Note when VIDIOC_ENUMSTD always returns ENOTTY this
+           is no video device or it falls under the USB exception,
+           and VIDIOC_G_STD returning ENOTTY is no error. */
+
+        perror("VIDIOC_G_STD");
+        exit(EXIT_FAILURE);
+    }
+
+    memset(&standard, 0, sizeof(standard));
+    standard.index = 0;
+
+    while (0 == ioctl(fd, VIDIOC_ENUMSTD, &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);
+    }
+
+
+.. code-block:: c
+
+    struct v4l2_input input;
+    struct v4l2_standard standard;
+
+    memset(&input, 0, sizeof(input));
+
+    if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
+        perror("VIDIOC_G_INPUT");
+        exit(EXIT_FAILURE);
+    }
+
+    if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &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, VIDIOC_ENUMSTD, &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);
+    }
+
+
+.. code-block:: c
+
+    struct v4l2_input input;
+    v4l2_std_id std_id;
+
+    memset(&input, 0, sizeof(input));
+
+    if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
+        perror("VIDIOC_G_INPUT");
+        exit(EXIT_FAILURE);
+    }
+
+    if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &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
+       or G/PAL is supported. */
+
+    std_id = V4L2_STD_PAL_BG;
+
+    if (-1 == ioctl(fd, VIDIOC_S_STD, &std_id)) {
+        perror("VIDIOC_S_STD");
+        exit(EXIT_FAILURE);
+    }
+
+.. [1]
+   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.
+
+
+.. ------------------------------------------------------------------------------
+.. This file was automatically converted from DocBook-XML with the dbxml
+.. library (https://github.com/return42/sphkerneldoc). The origin XML comes
+.. from the linux kernel, refer to:
+..
+.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
+.. ------------------------------------------------------------------------------