standard.rst

  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.