1.9. Extended Controls

1.9.1. 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.

1.9.2. 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 struct 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 (i. e. 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 struct v4l2_ext_control array. The v4l2_ext_control structure is very similar to struct 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.

1.9.3. 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:

struct 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:

while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
	if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG)
		/* ... */
		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 (e. g. 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.

1.9.4. 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 struct v4l2_queryctrl also contains hints on the behavior of the control. See the VIDIOC_QUERYCTRL documentation for more details.

1.9.5. 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

Table 1.2. MPEG Control IDs

 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_MPEG2_DVD MPEG-2 DVD-compatible stream
V4L2_MPEG_STREAM_TYPE_MPEG1_VCD MPEG-1 VCD-compatible stream
 Program Map Table Packet ID for the MPEG transport stream (default 16)
 Audio Packet ID for the MPEG transport stream (default 256)
 Video Packet ID for the MPEG transport stream (default 260)
 Packet ID for the MPEG transport stream carrying PCR fields (default 259)
 Audio ID for MPEG PES
 Video ID for MPEG PES
V4L2_CID_MPEG_STREAM_VBI_FMT enum v4l2_mpeg_stream_vbi_fmt 
 Some cards can embed VBI data (e. g. 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_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_CID_MPEG_AUDIO_ENCODING enum v4l2_mpeg_audio_encoding 
 MPEG Audio encoding. Possible values are:
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
 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_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_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_50_DIV_15_uS 50/15 microsecond emphasis
V4L2_CID_MPEG_AUDIO_CRC enum v4l2_mpeg_audio_crc 
 CRC method. Possible values are:
V4L2_MPEG_AUDIO_CRC_CRC16 16 bit parity check
 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_CID_MPEG_VIDEO_ASPECT enum v4l2_mpeg_video_aspect 
 Video aspect. Possible values are:
 Number of B-Frames (default 2)
 GOP size (default 12)
 GOP closure (default 1)
 Enable 3:2 pulldown (default 0)
V4L2_CID_MPEG_VIDEO_BITRATE_MODE enum v4l2_mpeg_video_bitrate_mode 
 Video bitrate mode. Possible values are:
 Video bitrate in bits per second.
 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.
 For every captured frame, skip this many subsequent frames (default 0).
 "Mutes" the video to a fixed color when capturing. This is useful for testing, to produce a fixed video bitstream. 0 = unmuted, 1 = muted.
 Sets the "mute" color of the video. The supplied 32-bit integer is interpreted as follows (bit 0 = least significant bit):
Bit 0:7V chrominance information
Bit 8:15U chrominance information
Bit 16:23Y luminance information
Bit 24:31Must 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.

Table 1.3. CX2341x Control IDs

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
 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_1D_HOR One-dimensional horizontal
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_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
 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_HOR_VERT Horizontal and vertical filter
 Threshold above which the luminance median filter is enabled (default 0)
 Threshold below which the luminance median filter is enabled (default 255)
 Threshold above which the chroma median filter is enabled (default 0)
 Threshold below which the chroma median filter is enabled (default 255)
 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.

1.9.6. Camera Control Reference

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

Table 1.4. Camera Control IDs

 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.
 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.
 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.
 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.
 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.
 When this control is set, the camera moves vertically to the default position.
 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.
 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.
 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.
 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.
 Specify the objective lens focal length as an absolute value. The zoom unit is driver-specific and its value should be a positive 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.
 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.
 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.

1.9.7. 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.

Table 1.5. FM_TX Control IDs

 The FM_TX class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class.
 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.
 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 [EN 50067], 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.
 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 [EN 50067]. 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.
 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.
 Sets the audio deviation limiter feature release time. Unit is in useconds. Step and range are driver-specific.
 Configures audio frequency deviation level in Hz. The range and step are driver-specific.
 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).
 Sets the gain for audio compression feature. It is a dB value. The range and step are driver-specific.
 Sets the threshold level for audio compression freature. It is a dB value. The range and step are driver-specific.
 Sets the attack time for audio compression feature. It is a useconds value. The range and step are driver-specific.
 Sets the release time for audio compression feature. It is a useconds value. The range and step are driver-specific.
 Enables or disables the pilot tone generation feature.
 Configures pilot tone frequency deviation level. Unit is in Hz. The range and step are driver-specific.
 Configures pilot tone frequency value. Unit is in Hz. The range and step are driver-specific.
 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.
 Sets the output power level for signal transmission. Unit is in dBuV. Range and step are driver-specific.
 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 [EN 50067] document, from CENELEC.