193 lines
6.2 KiB
ReStructuredText
193 lines
6.2 KiB
ReStructuredText
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.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/media/uapi/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _standard:
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***************
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Video Standards
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***************
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Video devices typically support one or more different video standards or
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variations of standards. Each video input and output may support another
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set of standards. This set is reported by the ``std`` field of struct
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:c:type:`v4l2_input` and struct
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:c:type:`v4l2_output` returned by the
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:ref:`VIDIOC_ENUMINPUT` and
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:ref:`VIDIOC_ENUMOUTPUT` ioctls, respectively.
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V4L2 defines one bit for each analog video standard currently in use
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worldwide, and sets aside bits for driver defined standards, e. g.
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hybrid standards to watch NTSC video tapes on PAL TVs and vice versa.
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Applications can use the predefined bits to select a particular
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standard, although presenting the user a menu of supported standards is
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preferred. To enumerate and query the attributes of the supported
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standards applications use the :ref:`VIDIOC_ENUMSTD`
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ioctl.
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Many of the defined standards are actually just variations of a few
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major standards. The hardware may in fact not distinguish between them,
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or do so internal and switch automatically. Therefore enumerated
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standards also contain sets of one or more standard bits.
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Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL
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signals. The first enumerated standard is a set of B and G/PAL, switched
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automatically depending on the selected radio frequency in UHF or VHF
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band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a
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Composite input may collapse standards, enumerating "PAL-B/G/H/I",
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"NTSC-M" and "SECAM-D/K". [#f1]_
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To query and select the standard used by the current video input or
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output applications call the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>` and
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:ref:`VIDIOC_S_STD <VIDIOC_G_STD>` ioctl, respectively. The
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*received* standard can be sensed with the
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:ref:`VIDIOC_QUERYSTD` ioctl.
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.. note::
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The parameter of all these ioctls is a pointer to a
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:ref:`v4l2_std_id <v4l2-std-id>` type (a standard set), *not* an
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index into the standard enumeration. Drivers must implement all video
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standard ioctls when the device has one or more video inputs or outputs.
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Special rules apply to devices such as USB cameras where the notion of
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video standards makes little sense. More generally for any capture or
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output device which is:
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- incapable of capturing fields or frames at the nominal rate of the
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video standard, or
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- that does not support the video standard formats at all.
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Here the driver shall set the ``std`` field of struct
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:c:type:`v4l2_input` and struct
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:c:type:`v4l2_output` to zero and the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>`,
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:ref:`VIDIOC_S_STD <VIDIOC_G_STD>`, :ref:`VIDIOC_QUERYSTD` and :ref:`VIDIOC_ENUMSTD` ioctls
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shall return the ``ENOTTY`` error code or the ``EINVAL`` error code.
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Applications can make use of the :ref:`input-capabilities` and
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:ref:`output-capabilities` flags to determine whether the video
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standard ioctls can be used with the given input or output.
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Example: Information about the current video standard
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=====================================================
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.. code-block:: c
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v4l2_std_id std_id;
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struct v4l2_standard standard;
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if (-1 == ioctl(fd, VIDIOC_G_STD, &std_id)) {
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/* Note when VIDIOC_ENUMSTD always returns ENOTTY this
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is no video device or it falls under the USB exception,
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and VIDIOC_G_STD returning ENOTTY is no error. */
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perror("VIDIOC_G_STD");
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exit(EXIT_FAILURE);
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}
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memset(&standard, 0, sizeof(standard));
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standard.index = 0;
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while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
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if (standard.id & std_id) {
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printf("Current video standard: %s\\n", standard.name);
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exit(EXIT_SUCCESS);
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}
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standard.index++;
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}
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/* EINVAL indicates the end of the enumeration, which cannot be
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empty unless this device falls under the USB exception. */
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if (errno == EINVAL || standard.index == 0) {
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perror("VIDIOC_ENUMSTD");
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exit(EXIT_FAILURE);
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}
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Example: Listing the video standards supported by the current input
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===================================================================
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.. code-block:: c
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struct v4l2_input input;
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struct v4l2_standard standard;
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memset(&input, 0, sizeof(input));
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if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
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perror("VIDIOC_G_INPUT");
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exit(EXIT_FAILURE);
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}
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if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
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perror("VIDIOC_ENUM_INPUT");
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exit(EXIT_FAILURE);
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}
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printf("Current input %s supports:\\n", input.name);
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memset(&standard, 0, sizeof(standard));
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standard.index = 0;
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while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
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if (standard.id & input.std)
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printf("%s\\n", standard.name);
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standard.index++;
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}
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/* EINVAL indicates the end of the enumeration, which cannot be
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empty unless this device falls under the USB exception. */
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if (errno != EINVAL || standard.index == 0) {
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perror("VIDIOC_ENUMSTD");
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exit(EXIT_FAILURE);
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}
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Example: Selecting a new video standard
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=======================================
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.. code-block:: c
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struct v4l2_input input;
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v4l2_std_id std_id;
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memset(&input, 0, sizeof(input));
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if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
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perror("VIDIOC_G_INPUT");
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exit(EXIT_FAILURE);
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}
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if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
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perror("VIDIOC_ENUM_INPUT");
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exit(EXIT_FAILURE);
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}
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if (0 == (input.std & V4L2_STD_PAL_BG)) {
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fprintf(stderr, "Oops. B/G PAL is not supported.\\n");
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exit(EXIT_FAILURE);
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}
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/* Note this is also supposed to work when only B
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or G/PAL is supported. */
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std_id = V4L2_STD_PAL_BG;
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if (-1 == ioctl(fd, VIDIOC_S_STD, &std_id)) {
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perror("VIDIOC_S_STD");
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exit(EXIT_FAILURE);
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}
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.. [#f1]
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Some users are already confused by technical terms PAL, NTSC and
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SECAM. There is no point asking them to distinguish between B, G, D,
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or K when the software or hardware can do that automatically.
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