# User Controls

Devices typically have a number of user-settable controls such as brightness, saturation and so on, which would be presented to the user on a graphical user interface. But, different devices will have different controls available, and furthermore, the range of possible values, and the default value will vary from device to device. The control ioctls provide the information and a mechanism to create a nice user interface for these controls that will work correctly with any device.

All controls are accessed using an ID value. V4L2 defines several IDs for specific purposes. Drivers can also implement their own custom controls using `V4L2_CID_PRIVATE_BASE` and higher values. The pre-defined control IDs have the prefix `V4L2_CID_`, and are listed in control-id. The ID is used when querying the attributes of a control, and when getting or setting the current value.

Generally applications should present controls to the user without assumptions about their purpose. Each control comes with a name string the user is supposed to understand. When the purpose is non-intuitive the driver writer should provide a user manual, a user interface plug-in or a driver specific panel application. Predefined IDs were introduced to change a few controls programmatically, for example to mute a device during a channel switch.

Drivers may enumerate different controls after switching the current video input or output, tuner or modulator, or audio input or output. Different in the sense of other bounds, another default and current value, step size or other menu items. A control with a certain custom ID can also change name and type.

It will be more convenient for applications if drivers make use of the `V4L2_CTRL_FLAG_DISABLED` flag, but that was never required.

Control values are stored globally, they do not change when switching except to stay within the reported bounds. They also do not change ⪚ when the device is opened or closed, when the tuner radio frequency is changed or generally never without application request. Since V4L2 specifies no event mechanism, panel applications intended to cooperate with other panel applications (be they built into a larger application, as a TV viewer) may need to regularly poll control values to update their user interface.

Applications could call an ioctl to request events. After another process called &VIDIOC-S-CTRL; or another ioctl changing shared properties the &func-select; function would indicate readability until any ioctl (querying the properties) is called.

## Control IDs

&cs-def;
ID | Type | Description
`V4L2_CID_BASE` |  | First predefined ID, equal to `V4L2_CID_BRIGHTNESS`.
`V4L2_CID_USER_BASE` |  | Synonym of `V4L2_CID_BASE`.
`V4L2_CID_BRIGHTNESS` | integer | Picture brightness, or more precisely, the black level.
`V4L2_CID_CONTRAST` | integer | Picture contrast or luma gain.
`V4L2_CID_SATURATION` | integer | Picture color saturation or chroma gain.
`V4L2_CID_HUE` | integer | Hue or color balance.
`V4L2_CID_AUDIO_VOLUME` | integer | Overall audio volume. Note some drivers also provide an OSS or ALSA mixer interface.
`V4L2_CID_AUDIO_BALANCE` | integer | Audio stereo balance. Minimum corresponds to all the way left, maximum to right.
`V4L2_CID_AUDIO_BASS` | integer | Audio bass adjustment.
`V4L2_CID_AUDIO_TREBLE` | integer | Audio treble adjustment.
`V4L2_CID_AUDIO_MUTE` | boolean | Mute audio, &ie; set the volume to zero, however without affecting `V4L2_CID_AUDIO_VOLUME`. Like ALSA drivers, V4L2 drivers must mute at load time to avoid excessive noise. Actually the entire device should be reset to a low power consumption state.
`V4L2_CID_AUDIO_LOUDNESS` | boolean | Loudness mode (bass boost).
`V4L2_CID_BLACK_LEVEL` | integer | Another name for brightness (not a synonym of `V4L2_CID_BRIGHTNESS`). This control is deprecated and should not be used in new drivers and applications.
`V4L2_CID_AUTO_WHITE_BALANCE` | boolean | Automatic white balance (cameras).
`V4L2_CID_DO_WHITE_BALANCE` | button | This is an action control. When set (the value is ignored), the device will do a white balance and then hold the current setting. Contrast this with the boolean `V4L2_CID_AUTO_WHITE_BALANCE`, which, when activated, keeps adjusting the white balance.
`V4L2_CID_RED_BALANCE` | integer | Red chroma balance.
`V4L2_CID_BLUE_BALANCE` | integer | Blue chroma balance.
`V4L2_CID_GAMMA` | integer | Gamma adjust.
`V4L2_CID_WHITENESS` | integer | Whiteness for grey-scale devices. This is a synonym for `V4L2_CID_GAMMA`. This control is deprecated and should not be used in new drivers and applications.
`V4L2_CID_EXPOSURE` | integer | Exposure (cameras). [Unit?]
`V4L2_CID_AUTOGAIN` | boolean | Automatic gain/exposure control.
`V4L2_CID_GAIN` | integer | Gain control.
`V4L2_CID_HFLIP` | boolean | Mirror the picture horizontally.
`V4L2_CID_VFLIP` | boolean | Mirror the picture vertically.
`V4L2_CID_HCENTER_DEPRECATED` (formerly `V4L2_CID_HCENTER`) | integer | Horizontal image centering. This control is deprecated. New drivers and applications should use the Camera class controls `V4L2_CID_PAN_ABSOLUTE`, `V4L2_CID_PAN_RELATIVE` and `V4L2_CID_PAN_RESET` instead.
`V4L2_CID_VCENTER_DEPRECATED` (formerly `V4L2_CID_VCENTER`) | integer | Vertical image centering. Centering is intended to physically adjust cameras. For image cropping see crop, for clipping overlay. This control is deprecated. New drivers and applications should use the Camera class controls `V4L2_CID_TILT_ABSOLUTE`, `V4L2_CID_TILT_RELATIVE` and `V4L2_CID_TILT_RESET` instead.
`V4L2_CID_POWER_LINE_FREQUENCY` | enum | Enables a power line frequency filter to avoid flicker. Possible values for `enum v4l2_power_line_frequency` are: `V4L2_CID_POWER_LINE_FREQUENCY_DISABLED` (0), `V4L2_CID_POWER_LINE_FREQUENCY_50HZ` (1) and `V4L2_CID_POWER_LINE_FREQUENCY_60HZ` (2).
`V4L2_CID_HUE_AUTO` | boolean | Enables automatic hue control by the device. The effect of setting `V4L2_CID_HUE` while automatic hue control is enabled is undefined, drivers should ignore such request.
`V4L2_CID_WHITE_BALANCE_TEMPERATURE` | integer | This control specifies the white balance settings as a color temperature in Kelvin. A driver should have a minimum of 2800 (incandescent) to 6500 (daylight). For more information about color temperature see Wikipedia.
`V4L2_CID_SHARPNESS` | integer | Adjusts the sharpness filters in a camera. The minimum value disables the filters, higher values give a sharper picture.
`V4L2_CID_BACKLIGHT_COMPENSATION` | integer | Adjusts the backlight compensation in a camera. The minimum value disables backlight compensation.
`V4L2_CID_CHROMA_AGC` | boolean | Chroma automatic gain control.
`V4L2_CID_COLOR_KILLER` | boolean | Enable the color killer (&ie; force a black & white image in case of a weak video signal).
`V4L2_CID_COLORFX` | enum | Selects a color effect. Possible values for `enum v4l2_colorfx` are: `V4L2_COLORFX_NONE` (0), `V4L2_COLORFX_BW` (1) and `V4L2_COLORFX_SEPIA` (2).
`V4L2_CID_LASTP1` |  | End of the predefined control IDs (currently `V4L2_CID_COLORFX` + 1).
`V4L2_CID_PRIVATE_BASE` |  | ID of the first custom (driver specific) control. Applications depending on particular custom controls should check the driver name and version, see querycap.

Applications can enumerate the available controls with the &VIDIOC-QUERYCTRL; and &VIDIOC-QUERYMENU; ioctls, get and set a control value with the &VIDIOC-G-CTRL; and &VIDIOC-S-CTRL; ioctls. Drivers must implement `VIDIOC_QUERYCTRL`, `VIDIOC_G_CTRL` and `VIDIOC_S_CTRL` when the device has one or more controls, `VIDIOC_QUERYMENU` when it has one or more menu type controls.

## Enumerating all controls

    &v4l2-queryctrl; queryctrl;
    &v4l2-querymenu; querymenu;

    static void
    enumerate_menu (void)
    {
    	printf ("  Menu items:\n");

    	memset (&querymenu, 0, sizeof (querymenu));
    	querymenu.id = queryctrl.id;

    	for (querymenu.index = queryctrl.minimum;
    	     querymenu.index <= queryctrl.maximum;
    	      querymenu.index++) {
    		if (0 == ioctl (fd, &VIDIOC-QUERYMENU;, &querymenu)) {
    			printf ("  %s\n", querymenu.name);
    		} else {
    			perror ("VIDIOC_QUERYMENU");
    			exit (EXIT_FAILURE);
    		}
    	}
    }

    memset (&queryctrl, 0, sizeof (queryctrl));

    for (queryctrl.id = V4L2_CID_BASE;
         queryctrl.id < V4L2_CID_LASTP1;
         queryctrl.id++) {
    	if (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &queryctrl)) {
    		if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
    			continue;

    		printf ("Control %s\n", queryctrl.name);

    		if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
    			enumerate_menu ();
    	} else {
    		if (errno == EINVAL)
    			continue;

    		perror ("VIDIOC_QUERYCTRL");
    		exit (EXIT_FAILURE);
    	}
    }

    for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
         queryctrl.id++) {
    	if (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &queryctrl)) {
    		if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
    			continue;

    		printf ("Control %s\n", queryctrl.name);

    		if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
    			enumerate_menu ();
    	} else {
    		if (errno == EINVAL)
    			break;

    		perror ("VIDIOC_QUERYCTRL");
    		exit (EXIT_FAILURE);
    	}
    }

## Changing controls

    &v4l2-queryctrl; queryctrl;
    &v4l2-control; control;

    memset (&queryctrl, 0, sizeof (queryctrl));
    queryctrl.id = V4L2_CID_BRIGHTNESS;

    if (-1 == ioctl (fd, &VIDIOC-QUERYCTRL;, &queryctrl)) {
    	if (errno != EINVAL) {
    		perror ("VIDIOC_QUERYCTRL");
    		exit (EXIT_FAILURE);
    	} else {
    		printf ("V4L2_CID_BRIGHTNESS is not supported\n");
    	}
    } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
    	printf ("V4L2_CID_BRIGHTNESS is not supported\n");
    } else {
    	memset (&control, 0, sizeof (control));
    	control.id = V4L2_CID_BRIGHTNESS;
    	control.value = queryctrl.default_value;

    	if (-1 == ioctl (fd, &VIDIOC-S-CTRL;, &control)) {
    		perror ("VIDIOC_S_CTRL");
    		exit (EXIT_FAILURE);
    	}
    }

    memset (&control, 0, sizeof (control));
    control.id = V4L2_CID_CONTRAST;

    if (0 == ioctl (fd, &VIDIOC-G-CTRL;, &control)) {
    	control.value += 1;

    	/* The driver may clamp the value or return ERANGE, ignored here */

    	if (-1 == ioctl (fd, &VIDIOC-S-CTRL;, &control)
    	    && errno != ERANGE) {
    		perror ("VIDIOC_S_CTRL");
    		exit (EXIT_FAILURE);
    	}
    /* Ignore if V4L2_CID_CONTRAST is unsupported */
    } else if (errno != EINVAL) {
    	perror ("VIDIOC_G_CTRL");
    	exit (EXIT_FAILURE);
    }

    control.id = V4L2_CID_AUDIO_MUTE;
    control.value = TRUE; /* silence */

    /* Errors ignored */
    ioctl (fd, VIDIOC_S_CTRL, &control);

## Extended Controls

## Introduction

The control mechanism as originally designed was meant to be used for user settings (brightness, saturation, etc). However, it turned out to be a very useful model for implementing more complicated driver APIs where each driver implements only a subset of a larger API.

The MPEG encoding API was the driving force behind designing and implementing this extended control mechanism: the MPEG standard is quite large and the currently supported hardware MPEG encoders each only implement a subset of this standard. Further more, many parameters relating to how the video is encoded into an MPEG stream are specific to the MPEG encoding chip since the MPEG standard only defines the format of the resulting MPEG stream, not how the video is actually encoded into that format.

Unfortunately, the original control API lacked some features needed for these new uses and so it was extended into the (not terribly originally named) extended control API.

Even though the MPEG encoding API was the first effort to use the Extended Control API, nowadays there are also other classes of Extended Controls, such as Camera Controls and FM Transmitter Controls. The Extended Controls API as well as all Extended Controls classes are described in the following text.

## The Extended Control API

Three new ioctls are available: &VIDIOC-G-EXT-CTRLS;, &VIDIOC-S-EXT-CTRLS; and &VIDIOC-TRY-EXT-CTRLS;. These ioctls act on arrays of controls (as opposed to the &VIDIOC-G-CTRL; and &VIDIOC-S-CTRL; ioctls that act on a single control). This is needed since it is often required to atomically change several controls at once.

Each of the new ioctls expects a pointer to a &v4l2-ext-controls;. This structure contains a pointer to the control array, a count of the number of controls in that array and a control class. Control classes are used to group similar controls into a single class. For example, control class `V4L2_CTRL_CLASS_USER` contains all user controls (&ie; all controls that can also be set using the old `VIDIOC_S_CTRL` ioctl). Control class `V4L2_CTRL_CLASS_MPEG` contains all controls relating to MPEG encoding, etc.

All controls in the control array must belong to the specified control class. An error is returned if this is not the case.

It is also possible to use an empty control array (count == 0) to check whether the specified control class is supported.

The control array is a &v4l2-ext-control; array. The v4l2_ext_control structure is very similar to &v4l2-control;, except for the fact that it also allows for 64-bit values and pointers to be passed.

It is important to realize that due to the flexibility of controls it is necessary to check whether the control you want to set actually is supported in the driver and what the valid range of values is. So use the &VIDIOC-QUERYCTRL; and &VIDIOC-QUERYMENU; ioctls to check this. Also note that it is possible that some of the menu indices in a control of type `V4L2_CTRL_TYPE_MENU` may not be supported (`VIDIOC_QUERYMENU` will return an error). A good example is the list of supported MPEG audio bitrates. Some drivers only support one or two bitrates, others support a wider range.

## Enumerating Extended Controls

The recommended way to enumerate over the extended controls is by using &VIDIOC-QUERYCTRL; in combination with the `V4L2_CTRL_FLAG_NEXT_CTRL` flag:

    &v4l2-queryctrl; qctrl;

    qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
    while (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &qctrl)) {
    	/* ... */
    	qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
    }

The initial control ID is set to 0 ORed with the `V4L2_CTRL_FLAG_NEXT_CTRL` flag. The `VIDIOC_QUERYCTRL` ioctl will return the first control with a higher ID than the specified one. When no such controls are found an error is returned.

If you want to get all controls within a specific control class, then you can set the initial qctrl.id value to the control class and add an extra check to break out of the loop when a control of another control class is found:

    qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL;
    while (0 == ioctl (fd, &VIDIOC-QUERYCTRL;, &qctrl)) {
    	if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG)
    		break;
    		/* ... */
    		qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
    	}

The 32-bit qctrl.id value is subdivided into three bit ranges: the top 4 bits are reserved for flags (⪚ `V4L2_CTRL_FLAG_NEXT_CTRL`) and are not actually part of the ID. The remaining 28 bits form the control ID, of which the most significant 12 bits define the control class and the least significant 16 bits identify the control within the control class. It is guaranteed that these last 16 bits are always non-zero for controls. The range of 0x1000 and up are reserved for driver-specific controls. The macro `V4L2_CTRL_ID2CLASS(id)` returns the control class ID based on a control ID.

If the driver does not support extended controls, then `VIDIOC_QUERYCTRL` will fail when used in combination with `V4L2_CTRL_FLAG_NEXT_CTRL`. In that case the old method of enumerating control should be used (see 1.8). But if it is supported, then it is guaranteed to enumerate over all controls, including driver-private controls.

## Creating Control Panels

It is possible to create control panels for a graphical user interface where the user can select the various controls. Basically you will have to iterate over all controls using the method described above. Each control class starts with a control of type `V4L2_CTRL_TYPE_CTRL_CLASS`. `VIDIOC_QUERYCTRL` will return the name of this control class which can be used as the title of a tab page within a control panel.

The flags field of &v4l2-queryctrl; also contains hints on the behavior of the control. See the &VIDIOC-QUERYCTRL; documentation for more details.

## MPEG Control Reference

Below all controls within the MPEG control class are described. First the generic controls, then controls specific for certain hardware.

### Generic MPEG Controls

### MPEG Control IDs

ID | Type
Description
`V4L2_CID_MPEG_CLASS` | class
The MPEG class descriptor. Calling &VIDIOC-QUERYCTRL; for this control will return a description of this control class. This description can be used as the caption of a Tab page in a GUI, for example.
`V4L2_CID_MPEG_STREAM_TYPE` | enum v4l2_mpeg_stream_type
The MPEG-1, -2 or -4 output stream type. One cannot assume anything here. Each hardware MPEG encoder tends to support different subsets of the available MPEG stream types. The currently defined stream types are:

`V4L2_MPEG_STREAM_TYPE_MPEG2_PS` | MPEG-2 program stream
`V4L2_MPEG_STREAM_TYPE_MPEG2_TS` | MPEG-2 transport stream
`V4L2_MPEG_STREAM_TYPE_MPEG1_SS` | MPEG-1 system stream
`V4L2_MPEG_STREAM_TYPE_MPEG2_DVD` | MPEG-2 DVD-compatible stream
`V4L2_MPEG_STREAM_TYPE_MPEG1_VCD` | MPEG-1 VCD-compatible stream
`V4L2_MPEG_STREAM_TYPE_MPEG2_SVCD` | MPEG-2 SVCD-compatible stream

`V4L2_CID_MPEG_STREAM_PID_PMT` | integer
Program Map Table Packet ID for the MPEG transport stream (default 16)
`V4L2_CID_MPEG_STREAM_PID_AUDIO` | integer
Audio Packet ID for the MPEG transport stream (default 256)
`V4L2_CID_MPEG_STREAM_PID_VIDEO` | integer
Video Packet ID for the MPEG transport stream (default 260)
`V4L2_CID_MPEG_STREAM_PID_PCR` | integer
Packet ID for the MPEG transport stream carrying PCR fields (default 259)
`V4L2_CID_MPEG_STREAM_PES_ID_AUDIO` | integer
Audio ID for MPEG PES
`V4L2_CID_MPEG_STREAM_PES_ID_VIDEO` | integer
Video ID for MPEG PES
`V4L2_CID_MPEG_STREAM_VBI_FMT` | enum v4l2_mpeg_stream_vbi_fmt
Some cards can embed VBI data (⪚ Closed Caption, Teletext) into the MPEG stream. This control selects whether VBI data should be embedded, and if so, what embedding method should be used. The list of possible VBI formats depends on the driver. The currently defined VBI format types are:

`V4L2_MPEG_STREAM_VBI_FMT_NONE` | No VBI in the MPEG stream
`V4L2_MPEG_STREAM_VBI_FMT_IVTV` | VBI in private packets, IVTV format (documented in the kernel sources in the file `Documentation/video4linux/cx2341x/README.vbi`)

`V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ` | enum v4l2_mpeg_audio_sampling_freq
MPEG Audio sampling frequency. Possible values are:

`V4L2_MPEG_AUDIO_SAMPLING_FREQ_44100` | 44.1 kHz
`V4L2_MPEG_AUDIO_SAMPLING_FREQ_48000` | 48 kHz
`V4L2_MPEG_AUDIO_SAMPLING_FREQ_32000` | 32 kHz

`V4L2_CID_MPEG_AUDIO_ENCODING` | enum v4l2_mpeg_audio_encoding
MPEG Audio encoding. Possible values are:

`V4L2_MPEG_AUDIO_ENCODING_LAYER_1` | MPEG-1/2 Layer I encoding
`V4L2_MPEG_AUDIO_ENCODING_LAYER_2` | MPEG-1/2 Layer II encoding
`V4L2_MPEG_AUDIO_ENCODING_LAYER_3` | MPEG-1/2 Layer III encoding
`V4L2_MPEG_AUDIO_ENCODING_AAC` | MPEG-2/4 AAC (Advanced Audio Coding)
`V4L2_MPEG_AUDIO_ENCODING_AC3` | AC-3 aka ATSC A/52 encoding

`V4L2_CID_MPEG_AUDIO_L1_BITRATE` | enum v4l2_mpeg_audio_l1_bitrate
MPEG-1/2 Layer I bitrate. Possible values are:

`V4L2_MPEG_AUDIO_L1_BITRATE_32K` | 32 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_64K` | 64 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_96K` | 96 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_128K` | 128 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_160K` | 160 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_192K` | 192 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_224K` | 224 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_256K` | 256 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_288K` | 288 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_320K` | 320 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_352K` | 352 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_384K` | 384 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_416K` | 416 kbit/s
`V4L2_MPEG_AUDIO_L1_BITRATE_448K` | 448 kbit/s

`V4L2_CID_MPEG_AUDIO_L2_BITRATE` | enum v4l2_mpeg_audio_l2_bitrate
MPEG-1/2 Layer II bitrate. Possible values are:

`V4L2_MPEG_AUDIO_L2_BITRATE_32K` | 32 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_48K` | 48 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_56K` | 56 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_64K` | 64 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_80K` | 80 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_96K` | 96 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_112K` | 112 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_128K` | 128 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_160K` | 160 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_192K` | 192 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_224K` | 224 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_256K` | 256 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_320K` | 320 kbit/s
`V4L2_MPEG_AUDIO_L2_BITRATE_384K` | 384 kbit/s

`V4L2_CID_MPEG_AUDIO_L3_BITRATE` | enum v4l2_mpeg_audio_l3_bitrate
MPEG-1/2 Layer III bitrate. Possible values are:

`V4L2_MPEG_AUDIO_L3_BITRATE_32K` | 32 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_40K` | 40 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_48K` | 48 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_56K` | 56 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_64K` | 64 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_80K` | 80 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_96K` | 96 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_112K` | 112 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_128K` | 128 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_160K` | 160 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_192K` | 192 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_224K` | 224 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_256K` | 256 kbit/s
`V4L2_MPEG_AUDIO_L3_BITRATE_320K` | 320 kbit/s

`V4L2_CID_MPEG_AUDIO_AAC_BITRATE` | integer
AAC bitrate in bits per second.
`V4L2_CID_MPEG_AUDIO_AC3_BITRATE` | enum v4l2_mpeg_audio_ac3_bitrate
AC-3 bitrate. Possible values are:

`V4L2_MPEG_AUDIO_AC3_BITRATE_32K` | 32 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_40K` | 40 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_48K` | 48 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_56K` | 56 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_64K` | 64 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_80K` | 80 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_96K` | 96 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_112K` | 112 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_128K` | 128 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_160K` | 160 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_192K` | 192 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_224K` | 224 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_256K` | 256 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_320K` | 320 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_384K` | 384 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_448K` | 448 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_512K` | 512 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_576K` | 576 kbit/s
`V4L2_MPEG_AUDIO_AC3_BITRATE_640K` | 640 kbit/s

`V4L2_CID_MPEG_AUDIO_MODE` | enum v4l2_mpeg_audio_mode
MPEG Audio mode. Possible values are:

`V4L2_MPEG_AUDIO_MODE_STEREO` | Stereo
`V4L2_MPEG_AUDIO_MODE_JOINT_STEREO` | Joint Stereo
`V4L2_MPEG_AUDIO_MODE_DUAL` | Bilingual
`V4L2_MPEG_AUDIO_MODE_MONO` | Mono

`V4L2_CID_MPEG_AUDIO_MODE_EXTENSION` | enum v4l2_mpeg_audio_mode_extension
Joint Stereo audio mode extension. In Layer I and II they indicate which subbands are in intensity stereo. All other subbands are coded in stereo. Layer III is not (yet) supported. Possible values are:

`V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_4` | Subbands 4-31 in intensity stereo
`V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_8` | Subbands 8-31 in intensity stereo
`V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_12` | Subbands 12-31 in intensity stereo
`V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_16` | Subbands 16-31 in intensity stereo

`V4L2_CID_MPEG_AUDIO_EMPHASIS` | enum v4l2_mpeg_audio_emphasis
Audio Emphasis. Possible values are:

`V4L2_MPEG_AUDIO_EMPHASIS_NONE` | None
`V4L2_MPEG_AUDIO_EMPHASIS_50_DIV_15_uS` | 50/15 microsecond emphasis
`V4L2_MPEG_AUDIO_EMPHASIS_CCITT_J17` | CCITT J.17

`V4L2_CID_MPEG_AUDIO_CRC` | enum v4l2_mpeg_audio_crc
CRC method. Possible values are:

`V4L2_MPEG_AUDIO_CRC_NONE` | None
`V4L2_MPEG_AUDIO_CRC_CRC16` | 16 bit parity check

`V4L2_CID_MPEG_AUDIO_MUTE` | boolean
Mutes the audio when capturing. This is not done by muting audio hardware, which can still produce a slight hiss, but in the encoder itself, guaranteeing a fixed and reproducable audio bitstream. 0 = unmuted, 1 = muted.
`V4L2_CID_MPEG_VIDEO_ENCODING` | enum v4l2_mpeg_video_encoding
MPEG Video encoding method. Possible values are:

`V4L2_MPEG_VIDEO_ENCODING_MPEG_1` | MPEG-1 Video encoding
`V4L2_MPEG_VIDEO_ENCODING_MPEG_2` | MPEG-2 Video encoding
`V4L2_MPEG_VIDEO_ENCODING_MPEG_4_AVC` | MPEG-4 AVC (H.264) Video encoding

`V4L2_CID_MPEG_VIDEO_ASPECT` | enum v4l2_mpeg_video_aspect
Video aspect. Possible values are:

`V4L2_MPEG_VIDEO_ASPECT_1x1`
`V4L2_MPEG_VIDEO_ASPECT_4x3`
`V4L2_MPEG_VIDEO_ASPECT_16x9`
`V4L2_MPEG_VIDEO_ASPECT_221x100`

`V4L2_CID_MPEG_VIDEO_B_FRAMES` | integer
Number of B-Frames (default 2)
`V4L2_CID_MPEG_VIDEO_GOP_SIZE` | integer
GOP size (default 12)
`V4L2_CID_MPEG_VIDEO_GOP_CLOSURE` | boolean
GOP closure (default 1)
`V4L2_CID_MPEG_VIDEO_PULLDOWN` | boolean
Enable 3:2 pulldown (default 0)
`V4L2_CID_MPEG_VIDEO_BITRATE_MODE` | enum v4l2_mpeg_video_bitrate_mode
Video bitrate mode. Possible values are:

`V4L2_MPEG_VIDEO_BITRATE_MODE_VBR` | Variable bitrate
`V4L2_MPEG_VIDEO_BITRATE_MODE_CBR` | Constant bitrate

`V4L2_CID_MPEG_VIDEO_BITRATE` | integer
Video bitrate in bits per second.
`V4L2_CID_MPEG_VIDEO_BITRATE_PEAK` | integer
Peak video bitrate in bits per second. Must be larger or equal to the average video bitrate. It is ignored if the video bitrate mode is set to constant bitrate.
`V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION` | integer
For every captured frame, skip this many subsequent frames (default 0).
`V4L2_CID_MPEG_VIDEO_MUTE` | boolean
"Mutes" the video to a fixed color when capturing. This is useful for testing, to produce a fixed video bitstream. 0 = unmuted, 1 = muted.
`V4L2_CID_MPEG_VIDEO_MUTE_YUV` | integer
Sets the "mute" color of the video. The supplied 32-bit integer is interpreted as follows (bit 0 = least significant bit):

Bit 0:7 | V chrominance information
Bit 8:15 | U chrominance information
Bit 16:23 | Y luminance information
Bit 24:31 | Must be zero.

### CX2341x MPEG Controls

The following MPEG class controls deal with MPEG encoding settings that are specific to the Conexant CX23415 and CX23416 MPEG encoding chips.

### CX2341x Control IDs

ID | Type
Description
`V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE` | enum v4l2_mpeg_cx2341x_video_spatial_filter_mode
Sets the Spatial Filter mode (default `MANUAL`). Possible values are:

`V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_MANUAL` | Choose the filter manually
`V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_AUTO` | Choose the filter automatically

`V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER` | integer (0-15)
The setting for the Spatial Filter. 0 = off, 15 = maximum. (Default is 0.)
`V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE` | enum v4l2_mpeg_cx2341x_video_luma_spatial_filter_type
Select the algorithm to use for the Luma Spatial Filter (default `1D_HOR`). Possible values:

`V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_OFF` | No filter
`V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_HOR` | One-dimensional horizontal
`V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_VERT` | One-dimensional vertical
`V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_HV_SEPARABLE` | Two-dimensional separable
`V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_SYM_NON_SEPARABLE` | Two-dimensional symmetrical non-separable

`V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE` | enum v4l2_mpeg_cx2341x_video_chroma_spatial_filter_type
Select the algorithm for the Chroma Spatial Filter (default `1D_HOR`). Possible values are:

`V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_OFF` | No filter
`V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_1D_HOR` | One-dimensional horizontal

`V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE` | enum v4l2_mpeg_cx2341x_video_temporal_filter_mode
Sets the Temporal Filter mode (default `MANUAL`). Possible values are:

`V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_MANUAL` | Choose the filter manually
`V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_AUTO` | Choose the filter automatically

`V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER` | integer (0-31)
The setting for the Temporal Filter. 0 = off, 31 = maximum. (Default is 8 for full-scale capturing and 0 for scaled capturing.)
`V4L2_CID_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE` | enum v4l2_mpeg_cx2341x_video_median_filter_type
Median Filter Type (default `OFF`). Possible values are:

`V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_OFF` | No filter
`V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR` | Horizontal filter
`V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_VERT` | Vertical filter
`V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR_VERT` | Horizontal and vertical filter
`V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_DIAG` | Diagonal filter

`V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_BOTTOM` | integer (0-255)
Threshold above which the luminance median filter is enabled (default 0)
`V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_TOP` | integer (0-255)
Threshold below which the luminance median filter is enabled (default 255)
`V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_BOTTOM` | integer (0-255)
Threshold above which the chroma median filter is enabled (default 0)
`V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_TOP` | integer (0-255)
Threshold below which the chroma median filter is enabled (default 255)
`V4L2_CID_MPEG_CX2341X_STREAM_INSERT_NAV_PACKETS` | boolean
The CX2341X MPEG encoder can insert one empty MPEG-2 PES packet into the stream between every four video frames. The packet size is 2048 bytes, including the packet_start_code_prefix and stream_id fields. The stream_id is 0xBF (private stream 2). The payload consists of 0x00 bytes, to be filled in by the application. 0 = do not insert, 1 = insert packets.

## Camera Control Reference

The Camera class includes controls for mechanical (or equivalent digital) features of a device such as controllable lenses or sensors.

## Camera Control IDs

ID | Type
Description
`V4L2_CID_CAMERA_CLASS` | class
The Camera class descriptor. Calling &VIDIOC-QUERYCTRL; for this control will return a description of this control class.
`V4L2_CID_EXPOSURE_AUTO` | enum v4l2_exposure_auto_type
Enables automatic adjustments of the exposure time and/or iris aperture. The effect of manual changes of the exposure time or iris aperture while these features are enabled is undefined, drivers should ignore such requests. Possible values are:

`V4L2_EXPOSURE_AUTO` | Automatic exposure time, automatic iris aperture.
`V4L2_EXPOSURE_MANUAL` | Manual exposure time, manual iris.
`V4L2_EXPOSURE_SHUTTER_PRIORITY` | Manual exposure time, auto iris.
`V4L2_EXPOSURE_APERTURE_PRIORITY` | Auto exposure time, manual iris.

`V4L2_CID_EXPOSURE_ABSOLUTE` | integer
Determines the exposure time of the camera sensor. The exposure time is limited by the frame interval. Drivers should interpret the values as 100 µs units, where the value 1 stands for 1/10000th of a second, 10000 for 1 second and 100000 for 10 seconds.
`V4L2_CID_EXPOSURE_AUTO_PRIORITY` | boolean
When `V4L2_CID_EXPOSURE_AUTO` is set to `AUTO` or `APERTURE_PRIORITY`, this control determines if the device may dynamically vary the frame rate. By default this feature is disabled (0) and the frame rate must remain constant.
`V4L2_CID_PAN_RELATIVE` | integer
This control turns the camera horizontally by the specified amount. The unit is undefined. A positive value moves the camera to the right (clockwise when viewed from above), a negative value to the left. A value of zero does not cause motion. This is a write-only control.
`V4L2_CID_TILT_RELATIVE` | integer
This control turns the camera vertically by the specified amount. The unit is undefined. A positive value moves the camera up, a negative value down. A value of zero does not cause motion. This is a write-only control.
`V4L2_CID_PAN_RESET` | button
When this control is set, the camera moves horizontally to the default position.
`V4L2_CID_TILT_RESET` | button
When this control is set, the camera moves vertically to the default position.
`V4L2_CID_PAN_ABSOLUTE` | integer
This control turns the camera horizontally to the specified position. Positive values move the camera to the right (clockwise when viewed from above), negative values to the left. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive.
`V4L2_CID_TILT_ABSOLUTE` | integer
This control turns the camera vertically to the specified position. Positive values move the camera up, negative values down. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive.
`V4L2_CID_FOCUS_ABSOLUTE` | integer
This control sets the focal point of the camera to the specified position. The unit is undefined. Positive values set the focus closer to the camera, negative values towards infinity.
`V4L2_CID_FOCUS_RELATIVE` | integer
This control moves the focal point of the camera by the specified amount. The unit is undefined. Positive values move the focus closer to the camera, negative values towards infinity. This is a write-only control.
`V4L2_CID_FOCUS_AUTO` | boolean
Enables automatic focus adjustments. The effect of manual focus adjustments while this feature is enabled is undefined, drivers should ignore such requests.
`V4L2_CID_ZOOM_ABSOLUTE` | integer
Specify the objective lens focal length as an absolute value. The zoom unit is driver-specific and its value should be a positive integer.
`V4L2_CID_ZOOM_RELATIVE` | integer
Specify the objective lens focal length relatively to the current value. Positive values move the zoom lens group towards the telephoto direction, negative values towards the wide-angle direction. The zoom unit is driver-specific. This is a write-only control.
`V4L2_CID_ZOOM_CONTINUOUS` | integer
Move the objective lens group at the specified speed until it reaches physical device limits or until an explicit request to stop the movement. A positive value moves the zoom lens group towards the telephoto direction. A value of zero stops the zoom lens group movement. A negative value moves the zoom lens group towards the wide-angle direction. The zoom speed unit is driver-specific.
`V4L2_CID_PRIVACY` | boolean
Prevent video from being acquired by the camera. When this control is set to `TRUE` (1), no image can be captured by the camera. Common means to enforce privacy are mechanical obturation of the sensor and firmware image processing, but the device is not restricted to these methods. Devices that implement the privacy control must support read access and may support write access.
`V4L2_CID_BAND_STOP_FILTER` | integer
Switch the band-stop filter of a camera sensor on or off, or specify its strength. Such band-stop filters can be used, for example, to filter out the fluorescent light component.

## FM Transmitter Control Reference

The FM Transmitter (FM_TX) class includes controls for common features of FM transmissions capable devices. Currently this class includes parameters for audio compression, pilot tone generation, audio deviation limiter, RDS transmission and tuning power features.

## FM_TX Control IDs

ID | Type
Description
`V4L2_CID_FM_TX_CLASS` | class
The FM_TX class descriptor. Calling &VIDIOC-QUERYCTRL; for this control will return a description of this control class.
`V4L2_CID_RDS_TX_DEVIATION` | integer
Configures RDS signal frequency deviation level in Hz. The range and step are driver-specific.
`V4L2_CID_RDS_TX_PI` | integer
Sets the RDS Programme Identification field for transmission.
`V4L2_CID_RDS_TX_PTY` | integer
Sets the RDS Programme Type field for transmission. This encodes up to 31 pre-defined programme types.
`V4L2_CID_RDS_TX_PS_NAME` | string
Sets the Programme Service name (PS_NAME) for transmission. It is intended for static display on a receiver. It is the primary aid to listeners in programme service identification and selection. In Annex E of en50067, the RDS specification, there is a full description of the correct character encoding for Programme Service name strings. Also from RDS specification, PS is usually a single eight character text. However, it is also possible to find receivers which can scroll strings sized as 8 x N characters. So, this control must be configured with steps of 8 characters. The result is it must always contain a string with size multiple of 8.
`V4L2_CID_RDS_TX_RADIO_TEXT` | string
Sets the Radio Text info for transmission. It is a textual description of what is being broadcasted. RDS Radio Text can be applied when broadcaster wishes to transmit longer PS names, programme-related information or any other text. In these cases, RadioText should be used in addition to `V4L2_CID_RDS_TX_PS_NAME`. The encoding for Radio Text strings is also fully described in Annex E of en50067. The length of Radio Text strings depends on which RDS Block is being used to transmit it, either 32 (2A block) or 64 (2B block). However, it is also possible to find receivers which can scroll strings sized as 32 x N or 64 x N characters. So, this control must be configured with steps of 32 or 64 characters. The result is it must always contain a string with size multiple of 32 or 64.
`V4L2_CID_AUDIO_LIMITER_ENABLED` | boolean
Enables or disables the audio deviation limiter feature. The limiter is useful when trying to maximize the audio volume, minimize receiver-generated distortion and prevent overmodulation.
`V4L2_CID_AUDIO_LIMITER_RELEASE_TIME` | integer
Sets the audio deviation limiter feature release time. Unit is in useconds. Step and range are driver-specific.
`V4L2_CID_AUDIO_LIMITER_DEVIATION` | integer
Configures audio frequency deviation level in Hz. The range and step are driver-specific.
`V4L2_CID_AUDIO_COMPRESSION_ENABLED` | boolean
Enables or disables the audio compression feature. This feature amplifies signals below the threshold by a fixed gain and compresses audio signals above the threshold by the ratio of Threshold/(Gain + Threshold).
`V4L2_CID_AUDIO_COMPRESSION_GAIN` | integer
Sets the gain for audio compression feature. It is a dB value. The range and step are driver-specific.
`V4L2_CID_AUDIO_COMPRESSION_THRESHOLD` | integer
Sets the threshold level for audio compression freature. It is a dB value. The range and step are driver-specific.
`V4L2_CID_AUDIO_COMPRESSION_ATTACK_TIME` | integer
Sets the attack time for audio compression feature. It is a useconds value. The range and step are driver-specific.
`V4L2_CID_AUDIO_COMPRESSION_RELEASE_TIME` | integer
Sets the release time for audio compression feature. It is a useconds value. The range and step are driver-specific.
`V4L2_CID_PILOT_TONE_ENABLED` | boolean
Enables or disables the pilot tone generation feature.
`V4L2_CID_PILOT_TONE_DEVIATION` | integer
Configures pilot tone frequency deviation level. Unit is in Hz. The range and step are driver-specific.
`V4L2_CID_PILOT_TONE_FREQUENCY` | integer
Configures pilot tone frequency value. Unit is in Hz. The range and step are driver-specific.
`V4L2_CID_TUNE_PREEMPHASIS` | integer
Configures the pre-emphasis value for broadcasting. A pre-emphasis filter is applied to the broadcast to accentuate the high audio frequencies. Depending on the region, a time constant of either 50 or 75 useconds is used. The enum v4l2_preemphasis defines possible values for pre-emphasis. Here they are:

`V4L2_PREEMPHASIS_DISABLED` | No pre-emphasis is applied.
`V4L2_PREEMPHASIS_50_uS` | A pre-emphasis of 50 uS is used.
`V4L2_PREEMPHASIS_75_uS` | A pre-emphasis of 75 uS is used.

`V4L2_CID_TUNE_POWER_LEVEL` | integer
Sets the output power level for signal transmission. Unit is in dBuV. Range and step are driver-specific.
`V4L2_CID_TUNE_ANTENNA_CAPACITOR` | integer
This selects the value of antenna tuning capacitor manually or automatically if set to zero. Unit, range and step are driver-specific.

For more details about RDS specification, refer to en50067 document, from CENELEC.