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1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _standard:
5
6***************
7Video Standards
8***************
9
10Video devices typically support one or more different video standards or
11variations of standards. Each video input and output may support another
12set of standards. This set is reported by the ``std`` field of struct
13:c:type:`v4l2_input` and struct
14:c:type:`v4l2_output` returned by the
15:ref:`VIDIOC_ENUMINPUT` and
16:ref:`VIDIOC_ENUMOUTPUT` ioctls, respectively.
17
18V4L2 defines one bit for each analog video standard currently in use
19worldwide, and sets aside bits for driver defined standards, e. g.
20hybrid standards to watch NTSC video tapes on PAL TVs and vice versa.
21Applications can use the predefined bits to select a particular
22standard, although presenting the user a menu of supported standards is
23preferred. To enumerate and query the attributes of the supported
24standards applications use the :ref:`VIDIOC_ENUMSTD`
25ioctl.
26
27Many of the defined standards are actually just variations of a few
28major standards. The hardware may in fact not distinguish between them,
29or do so internal and switch automatically. Therefore enumerated
30standards also contain sets of one or more standard bits.
31
32Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL
33signals. The first enumerated standard is a set of B and G/PAL, switched
34automatically depending on the selected radio frequency in UHF or VHF
35band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a
36Composite input may collapse standards, enumerating "PAL-B/G/H/I",
37"NTSC-M" and "SECAM-D/K". [#f1]_
38
39To query and select the standard used by the current video input or
40output applications call the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>` and
41:ref:`VIDIOC_S_STD <VIDIOC_G_STD>` ioctl, respectively. The
42*received* standard can be sensed with the
43:ref:`VIDIOC_QUERYSTD` ioctl.
44
45.. note::
46
47 The parameter of all these ioctls is a pointer to a
48 :ref:`v4l2_std_id <v4l2-std-id>` type (a standard set), *not* an
49 index into the standard enumeration. Drivers must implement all video
50 standard ioctls when the device has one or more video inputs or outputs.
51
52Special rules apply to devices such as USB cameras where the notion of
53video standards makes little sense. More generally for any capture or
54output device which is:
55
56- incapable of capturing fields or frames at the nominal rate of the
57 video standard, or
58
59- that does not support the video standard formats at all.
60
61Here the driver shall set the ``std`` field of struct
62:c:type:`v4l2_input` and struct
63:c:type:`v4l2_output` to zero and the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>`,
64:ref:`VIDIOC_S_STD <VIDIOC_G_STD>`, :ref:`VIDIOC_QUERYSTD` and :ref:`VIDIOC_ENUMSTD` ioctls
65shall return the ``ENOTTY`` error code or the ``EINVAL`` error code.
66
67Applications can make use of the :ref:`input-capabilities` and
68:ref:`output-capabilities` flags to determine whether the video
69standard ioctls can be used with the given input or output.
70
71Example: Information about the current video standard
72=====================================================
73
74.. code-block:: c
75
76 v4l2_std_id std_id;
77 struct v4l2_standard standard;
78
79 if (-1 == ioctl(fd, VIDIOC_G_STD, &std_id)) {
80 /* Note when VIDIOC_ENUMSTD always returns ENOTTY this
81 is no video device or it falls under the USB exception,
82 and VIDIOC_G_STD returning ENOTTY is no error. */
83
84 perror("VIDIOC_G_STD");
85 exit(EXIT_FAILURE);
86 }
87
88 memset(&standard, 0, sizeof(standard));
89 standard.index = 0;
90
91 while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
92 if (standard.id & std_id) {
93 printf("Current video standard: %s\\n", standard.name);
94 exit(EXIT_SUCCESS);
95 }
96
97 standard.index++;
98 }
99
100 /* EINVAL indicates the end of the enumeration, which cannot be
101 empty unless this device falls under the USB exception. */
102
103 if (errno == EINVAL || standard.index == 0) {
104 perror("VIDIOC_ENUMSTD");
105 exit(EXIT_FAILURE);
106 }
107
108Example: Listing the video standards supported by the current input
109===================================================================
110
111.. code-block:: c
112
113 struct v4l2_input input;
114 struct v4l2_standard standard;
115
116 memset(&input, 0, sizeof(input));
117
118 if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
119 perror("VIDIOC_G_INPUT");
120 exit(EXIT_FAILURE);
121 }
122
123 if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
124 perror("VIDIOC_ENUM_INPUT");
125 exit(EXIT_FAILURE);
126 }
127
128 printf("Current input %s supports:\\n", input.name);
129
130 memset(&standard, 0, sizeof(standard));
131 standard.index = 0;
132
133 while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
134 if (standard.id & input.std)
135 printf("%s\\n", standard.name);
136
137 standard.index++;
138 }
139
140 /* EINVAL indicates the end of the enumeration, which cannot be
141 empty unless this device falls under the USB exception. */
142
143 if (errno != EINVAL || standard.index == 0) {
144 perror("VIDIOC_ENUMSTD");
145 exit(EXIT_FAILURE);
146 }
147
148Example: Selecting a new video standard
149=======================================
150
151.. code-block:: c
152
153 struct v4l2_input input;
154 v4l2_std_id std_id;
155
156 memset(&input, 0, sizeof(input));
157
158 if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
159 perror("VIDIOC_G_INPUT");
160 exit(EXIT_FAILURE);
161 }
162
163 if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
164 perror("VIDIOC_ENUM_INPUT");
165 exit(EXIT_FAILURE);
166 }
167
168 if (0 == (input.std & V4L2_STD_PAL_BG)) {
169 fprintf(stderr, "Oops. B/G PAL is not supported.\\n");
170 exit(EXIT_FAILURE);
171 }
172
173 /* Note this is also supposed to work when only B
174 or G/PAL is supported. */
175
176 std_id = V4L2_STD_PAL_BG;
177
178 if (-1 == ioctl(fd, VIDIOC_S_STD, &std_id)) {
179 perror("VIDIOC_S_STD");
180 exit(EXIT_FAILURE);
181 }
182
183.. [#f1]
184 Some users are already confused by technical terms PAL, NTSC and
185 SECAM. There is no point asking them to distinguish between B, G, D,
186 or K when the software or hardware can do that automatically.