Video4Linux provides the following sets of device files. These live on the character device formerly known as "/dev/bttv". /dev/bttv should be a symlink to /dev/video0 for most people.

Device NameMinor RangeFunction
/dev/video0-63Video Capture Interface
/dev/radio64-127AM/FM Radio Devices
/dev/vtx192-223Teletext Interface Chips
/dev/vbi224-239Raw VBI Data (Intercast/teletext)

Video4Linux programs open and scan the devices to find what they are looking for. Capability queries define what each interface supports. The described API is only defined for video capture cards. The relevant subset applies to radio cards. Teletext interfaces talk the existing VTX API.

Capability Query Ioctl

The VIDIOCGCAP ioctl call is used to obtain the capability information for a video device. The struct video_capability object passed to the ioctl is completed and returned. It contains the following information

name[32]Cannonical name for this interface
typeType of interface
channelsNumber of radio/tv channels if appropriate
audiosNumber of audio devices if appropriate
maxwidthMaximum capture width in pixels
maxheightMaximum capture height in pixels
minwidthMinimum capture width in pixels
minheightMinimum capture height in pixels

The type field lists the capability flags for the device. These are as follows

VID_TYPE_CAPTURECan capture to memory
VID_TYPE_TUNERHas a tuner of some form
VID_TYPE_TELETEXTHas teletext capability
VID_TYPE_OVERLAYCan overlay its image onto the frame buffer
VID_TYPE_CHROMAKEYOverlay is Chromakeyed
VID_TYPE_CLIPPINGOverlay clipping is supported
VID_TYPE_FRAMERAMOverlay overwrites frame buffer memory
VID_TYPE_SCALESThe hardware supports image scaling
VID_TYPE_MONOCHROMEImage capture is grey scale only
VID_TYPE_SUBCAPTURECapture can be of only part of the image

The minimum and maximum sizes listed for a capture device do not imply all that all height/width ratios or sizes within the range are possible. A request to set a size will be honoured by the largest available capture size whose capture is no large than the requested rectangle in either direction. For example the quickcam has 3 fixed settings.

Frame Buffer

Capture cards that drop data directly onto the frame buffer must be told the base address of the frame buffer, its size and organisation. This is a privileged ioctl and one that eventually X itself should set.

The VIDIOCSFBUF ioctl sets the frame buffer parameters for a capture card. If the card does not do direct writes to the frame buffer then this ioctl will be unsupported. The VIDIOCGFBUF ioctl returns the currently used parameters. The structure used in both cases is a struct video_buffer.

void *baseBase physical address of the buffer
int heightHeight of the frame buffer
int widthWidth of the frame buffer
int depthDepth of the frame buffer
int bytesperlineNumber of bytes of memory between the start of two adjacent lines

Note that these values reflect the physical layout of the frame buffer. The visible area may be smaller. In fact under XFree86 this is commonly the case. XFree86 DGA can provide the parameters required to set up this ioctl. Setting the base address to NULL indicates there is no physical frame buffer access.

Capture Windows

The capture area is described by a struct video_window. This defines a capture area and the clipping information if relevant. The VIDIOCGWIN ioctl recovers the current settings and the VIDIOCSWIN sets new values. A successful call to VIDIOCSWIN indicates that a suitable set of parameters have been chosen. They do not indicate that exactly what was requested was granted. The program should call VIDIOCGWIN to check if the nearest match was suitable. The struct video_window contains the following fields.

xThe X co-ordinate specified in X windows format.
yThe Y co-ordinate specified in X windows format.
widthThe width of the image capture.
heightThe height of the image capture.
chromakeyA host order RGB32 value for the chroma key.
flagsAdditional capture flags.
clipsA list of clipping rectangles. (Set only)
clipcountThe number of clipping rectangles. (Set only)

Clipping rectangles are passed as an array. Each clip consists of the following fields available to the user.

xX co-ordinate of rectangle to skip
yY co-ordinate of rectangle to skip
widthWidth of rectangle to skip
heightHeight of rectangle to skip

Merely setting the window does not enable capturing. Overlay capturing is activated by passing the VIDIOCCAPTURE ioctl a value of 1, and disabled by passing it a value of 0.

Some capture devices can capture a subfield of the image they actually see. This is indicated when VIDEO_TYPE_SUBCAPTURE is defined. The video_capture describes the time and spacial subfields to capture. The video_capture structure contains the following fields.

xX co-ordinate of source rectangle to grab
yY co-ordinate of source rectangle to grab
widthWidth of source rectangle to grab
heightHeight of source rectangle to grab
decimationDecimation to apply
flagsFlag settings for grabbing
The available flags are

VIDEO_CAPTURE_ODDCapture only odd frames
VIDEO_CAPTURE_EVENCapture only even frames

Video Sources

Each video4linux video or audio device captures from one or more source channels. Each channel can be queries with the VDIOCGCHAN ioctl call. Before invoking this function the caller must set the channel field to the channel that is being queried. On return the struct video_channel is filled in with information about the nature of the channel itself.

The VIDIOCSCHAN ioctl takes an integer argument and switches the capture to this input. It is not defined whether parameters such as colour settings or tuning are maintained across a channel switch. The caller should maintain settings as desired for each channel. (This is reasonable as different video inputs may have different properties).

The struct video_channel consists of the following

channelThe channel number
nameThe input name - preferably reflecting the label on the card input itself
tunersNumber of tuners for this input
flagsProperties the tuner has
typeInput type (if known)
normThe norm for this channel

The flags defined are

VIDEO_VC_TUNERChannel has tuners.
VIDEO_VC_AUDIOChannel has audio.
VIDEO_VC_NORMChannel has norm setting.

The types defined are

VIDEO_TYPE_TVThe input is a TV input.
VIDEO_TYPE_CAMERAThe input is a camera.

Image Properties

The image properties of the picture can be queried with the VIDIOCGPICT ioctl which fills in a struct video_picture. The VIDIOCSPICT ioctl allows values to be changed. All values except for the palette type are scaled between 0-65535.

The struct video_picture consists of the following fields

brightnessPicture brightness
huePicture hue (colour only)
colourPicture colour (colour only)
contrastPicture contrast
whitenessThe whiteness (greyscale only)
depthThe capture depth (may need to match the frame buffer depth)
paletteReports the palette that should be used for this image

The following palettes are defined

VIDEO_PALETTE_GREYLinear intensity grey scale (255 is brightest).
VIDEO_PALETTE_HI240The BT848 8bit colour cube.
VIDEO_PALETTE_RGB565RGB565 packed into 16 bit words.
VIDEO_PALETTE_RGB555RGV555 packed into 16 bit words, top bit undefined.
VIDEO_PALETTE_RGB24RGB888 packed into 24bit words.
VIDEO_PALETTE_RGB32RGB888 packed into the low 3 bytes of 32bit words. The top 8bits are undefined.
VIDEO_PALETTE_YUV422Video style YUV422 - 8bits packed 4bits Y 2bits U 2bits V


Each video input channel can have one or more tuners associated with it. Many devices will not have tuners. TV cards and radio cards will have one or more tuners attached.

Tuners are described by a struct video_tuner which can be obtained by the VIDIOCGTUNER ioctl. Fill in the tuner number in the structure then pass the structure to the ioctl to have the data filled in. The tuner can be switched using VIDIOCSTUNER which takes an integer argument giving the tuner to use. A struct tuner has the following fields

tunerNumber of the tuner
nameCannonical name for this tuner (eg FM/AM/TV)
rangelowLowest tunable frequency
rangehighHighest tunable frequency
flagsFlags describing the tuner
modeThe video signal mode if relevant
signalSignal strength if known - between 0-65535

The following flags exist

VIDEO_TUNER_PALPAL tuning is supported
VIDEO_TUNER_NTSCNTSC tuning is supported
VIDEO_TUNER_SECAMSECAM tuning is supported
VIDEO_TUNER_LOWFrequency is in a lower range
VIDEO_TUNER_NORMThe norm for this tuner is settable
VIDEO_TUNER_STEREO_ONThe tuner is seeing stereo audio
VIDEO_TUNER_RDS_ONThe tuner is seeing a RDS datastream
VIDEO_TUNER_MBS_ONThe tuner is seeing a MBS datastream

The following modes are defined

VIDEO_MODE_PALThe tuner is in PAL mode
VIDEO_MODE_NTSCThe tuner is in NTSC mode
VIDEO_MODE_SECAMThe tuner is in SECAM mode
VIDEO_MODE_AUTOThe tuner auto switches, or mode does not apply

Tuning frequencies are an unsigned 32bit value in 1/16th MHz or if the VIDEO_TUNER_LOW flag is set they are in 1/16th KHz. The current frequency is obtained as an unsigned long via the VIDIOCGFREQ ioctl and set by the VIDIOCSFREQ ioctl.


TV and Radio devices have one or more audio inputs that may be selected. The audio properties are queried by passing a struct video_audio to VIDIOCGAUDIO ioctl. The VIDIOCSAUDIO ioctl sets audio properties.

The structure contains the following fields

audioThe channel number
volumeThe voume level
bassThe bass level
trebleThe treble level
flagsFlags describing the audio channel
nameCanonical name for the audio input
modeThe mode the audio input is in
balanceThe left/right balance
stepActual step used by the hardware

The following flags are defined

VIDEO_AUDIO_MUTEThe audio is muted
VIDEO_AUDIO_MUTABLEAudio muting is supported
VIDEO_AUDIO_VOLUMEThe volume is controllable
VIDEO_AUDIO_BASSThe bass is controllable
VIDEO_AUDIO_TREBLEThe treble is controllable
VIDEO_AUDIO_BALANCEThe balance is controllable

The following decoding modes are defined

VIDEO_SOUND_LANG1European TV alternate language 1
VIDEO_SOUND_LANG2European TV alternate language 2

Reading Images

Each call to the read syscall returns the next available image from the device. It is up to the caller to set the format and then to pass a suitable size buffer and length to the function. Not all devices will support read operations.

A second way to handle image capture is via the mmap interface if supported. To use the mmap interface a user first sets the desired image size and depth properties. Next the VIDIOCGMBUF ioctl is issued. This reports the size of buffer to mmap and the offset within the buffer for each frame. The number of frames supported is device dependent and may only be one.

The video_mbuf structure contains the following fields

sizeThe number of bytes to map
framesThe number of frames
offsetsThe offset of each frame

Once the mmap has been made the VIDIOCMCAPTURE ioctl sets the image size you wish to use (which should match or be below the initial query size). Having done so it will begin capturing to the memory mapped buffer. Whenever a buffer is "used" by the program it should called VIDIOCSYNC to free this frame up and continue. to add:VIDIOCSYNC takes the frame number you are freeing as its argument. When the buffer is unmapped or all the buffers are full capture ceases. While capturing to memory the driver will make a "best effort" attempt to capture to screen as well if requested. This normally means all frames that "miss" memory mapped capture will go to the display.

A final ioctl exists to allow a device to obtain related devices if a driver has multiple components (for example video0 may not be associated with vbi0 which would cause an intercast display program to make a bad mistake). The VIDIOCGUNIT ioctl reports the unit numbers of the associated devices if any exist. The video_unit structure has the following fields.

videoVideo capture device
vbiVBI capture device
radioRadio device
audioAudio mixer
teletextTeletext device

RDS Datastreams

For radio devices that support it, it is possible to receive Radio Data System (RDS) data by means of a read() on the device. The data is packed in groups of three, as follows:
First OctetLeast Siginificant Byte of RDS Block
Second OctetMost Siginificant Byte of RDS Block
Third OctetBit 7:Error bit. Indicates that an uncorrectable error occured during reception of this block.
 Bit 6:Corrected bit. Indicates that an error was corrected for this data block.
 Bits 5-3:Reeived Offset. Indicates the offset received by the sync system.
 Bits 2-0:Offset Name. Indicates the offset applied to this data.