1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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 :c:type:`v4l2_input` and struct
13 :c:type:`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". [#f1]_
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.
43
44 .. note::
45
46 The parameter of all these ioctls is a pointer to a
47 :ref:`v4l2_std_id <v4l2-std-id>` type (a standard set), *not* an
48 index into the standard enumeration. Drivers must implement all video
49 standard ioctls when the device has one or more video inputs or outputs.
50
51 Special rules apply to devices such as USB cameras where the notion of
52 video standards makes little sense. More generally for any capture or
53 output device which is:
54
55 - incapable of capturing fields or frames at the nominal rate of the
56 video standard, or
57
58 - that does not support the video standard formats at all.
59
60 Here the driver shall set the ``std`` field of struct
61 :c:type:`v4l2_input` and struct
62 :c:type:`v4l2_output` to zero and the :ref:`VIDIOC_G_STD <VIDIOC_G_STD>`,
63 :ref:`VIDIOC_S_STD <VIDIOC_G_STD>`, :ref:`VIDIOC_QUERYSTD` and :ref:`VIDIOC_ENUMSTD` ioctls
64 shall return the ``ENOTTY`` error code or the ``EINVAL`` error code.
65
66 Applications can make use of the :ref:`input-capabilities` and
67 :ref:`output-capabilities` flags to determine whether the video
68 standard ioctls can be used with the given input or output.
69
70 Example: Information about the current video standard
71 =====================================================
72
73 .. code-block:: c
74
75 v4l2_std_id std_id;
76 struct v4l2_standard standard;
77
78 if (-1 == ioctl(fd, VIDIOC_G_STD, &std_id)) {
79 /* Note when VIDIOC_ENUMSTD always returns ENOTTY this
80 is no video device or it falls under the USB exception,
81 and VIDIOC_G_STD returning ENOTTY is no error. */
82
83 perror("VIDIOC_G_STD");
84 exit(EXIT_FAILURE);
85 }
86
87 memset(&standard, 0, sizeof(standard));
88 standard.index = 0;
89
90 while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
91 if (standard.id & std_id) {
92 printf("Current video standard: %s\\n", standard.name);
93 exit(EXIT_SUCCESS);
94 }
95
96 standard.index++;
97 }
98
99 /* EINVAL indicates the end of the enumeration, which cannot be
100 empty unless this device falls under the USB exception. */
101
102 if (errno == EINVAL || standard.index == 0) {
103 perror("VIDIOC_ENUMSTD");
104 exit(EXIT_FAILURE);
105 }
106
107 Example: Listing the video standards supported by the current input
108 ===================================================================
109
110 .. code-block:: c
111
112 struct v4l2_input input;
113 struct v4l2_standard standard;
114
115 memset(&input, 0, sizeof(input));
116
117 if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
118 perror("VIDIOC_G_INPUT");
119 exit(EXIT_FAILURE);
120 }
121
122 if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
123 perror("VIDIOC_ENUM_INPUT");
124 exit(EXIT_FAILURE);
125 }
126
127 printf("Current input %s supports:\\n", input.name);
128
129 memset(&standard, 0, sizeof(standard));
130 standard.index = 0;
131
132 while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
133 if (standard.id & input.std)
134 printf("%s\\n", standard.name);
135
136 standard.index++;
137 }
138
139 /* EINVAL indicates the end of the enumeration, which cannot be
140 empty unless this device falls under the USB exception. */
141
142 if (errno != EINVAL || standard.index == 0) {
143 perror("VIDIOC_ENUMSTD");
144 exit(EXIT_FAILURE);
145 }
146
147 Example: Selecting a new video standard
148 =======================================
149
150 .. code-block:: c
151
152 struct v4l2_input input;
153 v4l2_std_id std_id;
154
155 memset(&input, 0, sizeof(input));
156
157 if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
158 perror("VIDIOC_G_INPUT");
159 exit(EXIT_FAILURE);
160 }
161
162 if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
163 perror("VIDIOC_ENUM_INPUT");
164 exit(EXIT_FAILURE);
165 }
166
167 if (0 == (input.std & V4L2_STD_PAL_BG)) {
168 fprintf(stderr, "Oops. B/G PAL is not supported.\\n");
169 exit(EXIT_FAILURE);
170 }
171
172 /* Note this is also supposed to work when only B
173 or G/PAL is supported. */
174
175 std_id = V4L2_STD_PAL_BG;
176
177 if (-1 == ioctl(fd, VIDIOC_S_STD, &std_id)) {
178 perror("VIDIOC_S_STD");
179 exit(EXIT_FAILURE);
180 }
181
182 .. [#f1]
183 Some users are already confused by technical terms PAL, NTSC and
184 SECAM. There is no point asking them to distinguish between B, G, D,
185 or K when the software or hardware can do that automatically.
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