1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _standard:
4
5***************
6Video Standards
7***************
8
9Video devices typically support one or more different video standards or
10variations of standards. Each video input and output may support another
11set 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
17V4L2 defines one bit for each analog video standard currently in use
18worldwide, and sets aside bits for driver defined standards, e. g.
19hybrid standards to watch NTSC video tapes on PAL TVs and vice versa.
20Applications can use the predefined bits to select a particular
21standard, although presenting the user a menu of supported standards is
22preferred. To enumerate and query the attributes of the supported
23standards applications use the :ref:`VIDIOC_ENUMSTD`
24ioctl.
25
26Many of the defined standards are actually just variations of a few
27major standards. The hardware may in fact not distinguish between them,
28or do so internal and switch automatically. Therefore enumerated
29standards also contain sets of one or more standard bits.
30
31Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL
32signals. The first enumerated standard is a set of B and G/PAL, switched
33automatically depending on the selected radio frequency in UHF or VHF
34band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a
35Composite input may collapse standards, enumerating "PAL-B/G/H/I",
36"NTSC-M" and "SECAM-D/K". [#f1]_
37
38To query and select the standard used by the current video input or
39output 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
51Special rules apply to devices such as USB cameras where the notion of
52video standards makes little sense. More generally for any capture or
53output 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
60Here 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
64shall return the ``ENOTTY`` error code or the ``EINVAL`` error code.
65
66Applications can make use of the :ref:`input-capabilities` and
67:ref:`output-capabilities` flags to determine whether the video
68standard ioctls can be used with the given input or output.
69
70Example: 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
107Example: 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
147Example: 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.
186