Video for Linux Two API Specification: Revision 0.24
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# 4.10. Radio Interface

This interface is intended for AM and FM (analog) radio receivers.

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.

## 4.10.1. Querying Capabilities

Devices supporting the radio interface set the `V4L2_CAP_RADIO` and `V4L2_CAP_TUNER` 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.

## 4.10.2. Supplemental Functions

Radio devices can support controls, and must support the tuner 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.

## 4.10.3. Programming

Radio devices may have a couple audio controls (as discussed in Section 1.8) such as a volume control, possibly custom controls. Further all radio devices have one tuner (these are discussed in Section 1.6) with index number zero to select the radio frequency and to determine if a monaural or FM stereo program is received. Drivers switch automatically between AM and FM depending on the selected frequency. The `VIDIOC_G_TUNER` ioctl reports the supported frequency range.

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