| 1 | .. -*- coding: utf-8; mode: rst -*- |
| 2 | |
| 3 | .. _standard: |
| 4 | |
| 5 | *************** |
| 6 | Video Standards |
| 7 | *************** |
| 8 | |
| 9 | Video devices typically support one or more different video standards or |
| 10 | variations of standards. Each video input and output may support another |
| 11 | set of standards. This set is reported by the ``std`` field of struct |
| 12 | :ref:`v4l2_input <v4l2-input>` and struct |
| 13 | :ref:`v4l2_output <v4l2-output>` returned by the |
| 14 | :ref:`VIDIOC_ENUMINPUT` and |
| 15 | :ref:`VIDIOC_ENUMOUTPUT` ioctls, respectively. |
| 16 | |
| 17 | V4L2 defines one bit for each analog video standard currently in use |
| 18 | worldwide, and sets aside bits for driver defined standards, e. g. |
| 19 | hybrid standards to watch NTSC video tapes on PAL TVs and vice versa. |
| 20 | Applications can use the predefined bits to select a particular |
| 21 | standard, although presenting the user a menu of supported standards is |
| 22 | preferred. To enumerate and query the attributes of the supported |
| 23 | standards applications use the :ref:`VIDIOC_ENUMSTD` |
| 24 | ioctl. |
| 25 | |
| 26 | Many of the defined standards are actually just variations of a few |
| 27 | major standards. The hardware may in fact not distinguish between them, |
| 28 | or do so internal and switch automatically. Therefore enumerated |
| 29 | standards also contain sets of one or more standard bits. |
| 30 | |
| 31 | Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL |
| 32 | signals. The first enumerated standard is a set of B and G/PAL, switched |
| 33 | automatically depending on the selected radio frequency in UHF or VHF |
| 34 | band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a |
| 35 | Composite input may collapse standards, enumerating "PAL-B/G/H/I", |
| 36 | "NTSC-M" and "SECAM-D/K". [1]_ |
| 37 | |
| 38 | To query and select the standard used by the current video input or |
| 39 | output applications call the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>` and |
| 40 | :ref:`VIDIOC_S_STD <VIDIOC_G_STD>` ioctl, respectively. The |
| 41 | *received* standard can be sensed with the |
| 42 | :ref:`VIDIOC_QUERYSTD` ioctl. Note that the |
| 43 | parameter of all these ioctls is a pointer to a |
| 44 | :ref:`v4l2_std_id <v4l2-std-id>` type (a standard set), *not* an |
| 45 | index into the standard enumeration. Drivers must implement all video |
| 46 | standard ioctls when the device has one or more video inputs or outputs. |
| 47 | |
| 48 | Special rules apply to devices such as USB cameras where the notion of |
| 49 | video standards makes little sense. More generally for any capture or |
| 50 | output device which is: |
| 51 | |
| 52 | - incapable of capturing fields or frames at the nominal rate of the |
| 53 | video standard, or |
| 54 | |
| 55 | - that does not support the video standard formats at all. |
| 56 | |
| 57 | Here the driver shall set the ``std`` field of struct |
| 58 | :ref:`v4l2_input <v4l2-input>` and struct |
| 59 | :ref:`v4l2_output <v4l2-output>` to zero and the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>`, |
| 60 | :ref:`VIDIOC_S_STD <VIDIOC_G_STD>`, :ref:`VIDIOC_QUERYSTD` and :ref:`VIDIOC_ENUMSTD` ioctls |
| 61 | shall return the ENOTTY error code or the EINVAL error code. |
| 62 | |
| 63 | Applications can make use of the :ref:`input-capabilities` and |
| 64 | :ref:`output-capabilities` flags to determine whether the video |
| 65 | standard ioctls can be used with the given input or output. |
| 66 | |
| 67 | |
| 68 | .. code-block:: c |
| 69 | :caption: Example 5: Information about the current video standard |
| 70 | |
| 71 | v4l2_std_id std_id; |
| 72 | struct v4l2_standard standard; |
| 73 | |
| 74 | if (-1 == ioctl(fd, VIDIOC_G_STD, &std_id)) { |
| 75 | /* Note when VIDIOC_ENUMSTD always returns ENOTTY this |
| 76 | is no video device or it falls under the USB exception, |
| 77 | and VIDIOC_G_STD returning ENOTTY is no error. */ |
| 78 | |
| 79 | perror("VIDIOC_G_STD"); |
| 80 | exit(EXIT_FAILURE); |
| 81 | } |
| 82 | |
| 83 | memset(&standard, 0, sizeof(standard)); |
| 84 | standard.index = 0; |
| 85 | |
| 86 | while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) { |
| 87 | if (standard.id & std_id) { |
| 88 | printf("Current video standard: %s\\n", standard.name); |
| 89 | exit(EXIT_SUCCESS); |
| 90 | } |
| 91 | |
| 92 | standard.index++; |
| 93 | } |
| 94 | |
| 95 | /* EINVAL indicates the end of the enumeration, which cannot be |
| 96 | empty unless this device falls under the USB exception. */ |
| 97 | |
| 98 | if (errno == EINVAL || standard.index == 0) { |
| 99 | perror("VIDIOC_ENUMSTD"); |
| 100 | exit(EXIT_FAILURE); |
| 101 | } |
| 102 | |
| 103 | |
| 104 | .. code-block:: c |
| 105 | :caption: Example 6: Listing the video standards supported by the current input |
| 106 | |
| 107 | struct v4l2_input input; |
| 108 | struct v4l2_standard standard; |
| 109 | |
| 110 | memset(&input, 0, sizeof(input)); |
| 111 | |
| 112 | if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) { |
| 113 | perror("VIDIOC_G_INPUT"); |
| 114 | exit(EXIT_FAILURE); |
| 115 | } |
| 116 | |
| 117 | if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) { |
| 118 | perror("VIDIOC_ENUM_INPUT"); |
| 119 | exit(EXIT_FAILURE); |
| 120 | } |
| 121 | |
| 122 | printf("Current input %s supports:\\n", input.name); |
| 123 | |
| 124 | memset(&standard, 0, sizeof(standard)); |
| 125 | standard.index = 0; |
| 126 | |
| 127 | while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) { |
| 128 | if (standard.id & input.std) |
| 129 | printf("%s\\n", standard.name); |
| 130 | |
| 131 | standard.index++; |
| 132 | } |
| 133 | |
| 134 | /* EINVAL indicates the end of the enumeration, which cannot be |
| 135 | empty unless this device falls under the USB exception. */ |
| 136 | |
| 137 | if (errno != EINVAL || standard.index == 0) { |
| 138 | perror("VIDIOC_ENUMSTD"); |
| 139 | exit(EXIT_FAILURE); |
| 140 | } |
| 141 | |
| 142 | |
| 143 | .. code-block:: c |
| 144 | :caption: Example 7: Selecting a new video standard |
| 145 | |
| 146 | struct v4l2_input input; |
| 147 | v4l2_std_id std_id; |
| 148 | |
| 149 | memset(&input, 0, sizeof(input)); |
| 150 | |
| 151 | if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) { |
| 152 | perror("VIDIOC_G_INPUT"); |
| 153 | exit(EXIT_FAILURE); |
| 154 | } |
| 155 | |
| 156 | if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) { |
| 157 | perror("VIDIOC_ENUM_INPUT"); |
| 158 | exit(EXIT_FAILURE); |
| 159 | } |
| 160 | |
| 161 | if (0 == (input.std & V4L2_STD_PAL_BG)) { |
| 162 | fprintf(stderr, "Oops. B/G PAL is not supported.\\n"); |
| 163 | exit(EXIT_FAILURE); |
| 164 | } |
| 165 | |
| 166 | /* Note this is also supposed to work when only B |
| 167 | or G/PAL is supported. */ |
| 168 | |
| 169 | std_id = V4L2_STD_PAL_BG; |
| 170 | |
| 171 | if (-1 == ioctl(fd, VIDIOC_S_STD, &std_id)) { |
| 172 | perror("VIDIOC_S_STD"); |
| 173 | exit(EXIT_FAILURE); |
| 174 | } |
| 175 | |
| 176 | .. [1] |
| 177 | Some users are already confused by technical terms PAL, NTSC and |
| 178 | SECAM. There is no point asking them to distinguish between B, G, D, |
| 179 | or K when the software or hardware can do that automatically. |
| 180 | |
| 181 | |
| 182 | .. ------------------------------------------------------------------------------ |
| 183 | .. This file was automatically converted from DocBook-XML with the dbxml |
| 184 | .. library (https://github.com/return42/sphkerneldoc). The origin XML comes |
| 185 | .. from the linux kernel, refer to: |
| 186 | .. |
| 187 | .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook |
| 188 | .. ------------------------------------------------------------------------------ |