4.10. Radio Interface¶
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/radio63 with major number 81 and minor numbers 64 to 127.
4.10.1. Querying Capabilities¶
Devices supporting the radio interface set the
V4L2_CAP_MODULATOR flag in the
capabilities field of struct
v4l2_capability returned by the
ioctl VIDIOC_QUERYCAP ioctl. Other combinations of
capability flags are reserved for future extensions.
4.10.2. Supplemental Functions¶
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 User Controls) such as a volume control, possibly custom controls. Further all radio devices have one tuner or modulator (these are discussed in 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.