1 .. -*- coding: utf-8; mode: rst -*-
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.
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`
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.
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]_
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.
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:
52 - incapable of capturing fields or frames at the nominal rate of the
55 - that does not support the video standard formats at all.
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.
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.
69 :caption: Example 1.5. Information about the current video standard
72 struct v4l2_standard standard;
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. */
79 perror("VIDIOC_G_STD");
83 memset(&standard, 0, sizeof(standard));
86 while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
87 if (standard.id & std_id) {
88 printf("Current video standard: %s\\n", standard.name);
95 /* EINVAL indicates the end of the enumeration, which cannot be
96 empty unless this device falls under the USB exception. */
98 if (errno == EINVAL || standard.index == 0) {
99 perror("VIDIOC_ENUMSTD");
105 :caption: Example 1.6. Listing the video standards supported by the current input
107 struct v4l2_input input;
108 struct v4l2_standard standard;
110 memset(&input, 0, sizeof(input));
112 if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
113 perror("VIDIOC_G_INPUT");
117 if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
118 perror("VIDIOC_ENUM_INPUT");
122 printf("Current input %s supports:\\n", input.name);
124 memset(&standard, 0, sizeof(standard));
127 while (0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
128 if (standard.id & input.std)
129 printf("%s\\n", standard.name);
134 /* EINVAL indicates the end of the enumeration, which cannot be
135 empty unless this device falls under the USB exception. */
137 if (errno != EINVAL || standard.index == 0) {
138 perror("VIDIOC_ENUMSTD");
144 :caption: Example 1.7. Selecting a new video standard
146 struct v4l2_input input;
149 memset(&input, 0, sizeof(input));
151 if (-1 == ioctl(fd, VIDIOC_G_INPUT, &input.index)) {
152 perror("VIDIOC_G_INPUT");
156 if (-1 == ioctl(fd, VIDIOC_ENUMINPUT, &input)) {
157 perror("VIDIOC_ENUM_INPUT");
161 if (0 == (input.std & V4L2_STD_PAL_BG)) {
162 fprintf(stderr, "Oops. B/G PAL is not supported.\\n");
166 /* Note this is also supposed to work when only B
167 or G/PAL is supported. */
169 std_id = V4L2_STD_PAL_BG;
171 if (-1 == ioctl(fd, VIDIOC_S_STD, &std_id)) {
172 perror("VIDIOC_S_STD");
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.