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185 lines
5.8 KiB
185 lines
5.8 KiB
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later |
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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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