This interface is intended for AM and FM (analog) radio receivers and transmitters.
Conventionally V4L2 radio devices are accessed through
character device special files named /dev/radio
and /dev/radio0 to
/dev/radio63 with major number 81 and minor
numbers 64 to 127.
Devices supporting the radio interface set the
V4L2_CAP_RADIO and
V4L2_CAP_TUNER or
V4L2_CAP_MODULATOR flag in the
capabilities field of struct v4l2_capability
returned by the VIDIOC_QUERYCAP ioctl. Other combinations of
capability flags are reserved for future extensions.
Radio devices can support controls, and must support the tuner or modulator ioctls.
They do not support the video input or output, audio input or output, video standard, cropping and scaling, compression and streaming parameter, or overlay ioctls. All other ioctls and I/O methods are reserved for future extensions.
Radio devices may have a couple audio controls (as discussed
in Section 1.8, “User Controls”) such as a volume control, possibly custom
controls. Further all radio devices have one tuner or modulator (these are
discussed in Section 1.6, “Tuners and Modulators”) with index number zero to select
the radio frequency and to determine if a monaural or FM stereo
program is received/emitted. Drivers switch automatically between AM and FM
depending on the selected frequency. The VIDIOC_G_TUNER or
VIDIOC_G_MODULATOR ioctl
reports the supported frequency range.