1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _radio: 4 5*************** 6Radio Interface 7*************** 8 9This interface is intended for AM and FM (analog) radio receivers and 10transmitters. 11 12Conventionally V4L2 radio devices are accessed through character device 13special files named ``/dev/radio`` and ``/dev/radio0`` to 14``/dev/radio63`` with major number 81 and minor numbers 64 to 127. 15 16 17Querying Capabilities 18===================== 19 20Devices supporting the radio interface set the ``V4L2_CAP_RADIO`` and 21``V4L2_CAP_TUNER`` or ``V4L2_CAP_MODULATOR`` flag in the 22``capabilities`` field of struct 23:c:type:`v4l2_capability` returned by the 24:ref:`VIDIOC_QUERYCAP` ioctl. Other combinations of 25capability flags are reserved for future extensions. 26 27 28Supplemental Functions 29====================== 30 31Radio devices can support :ref:`controls <control>`, and must support 32the :ref:`tuner or modulator <tuner>` ioctls. 33 34They do not support the video input or output, audio input or output, 35video standard, cropping and scaling, compression and streaming 36parameter, or overlay ioctls. All other ioctls and I/O methods are 37reserved for future extensions. 38 39 40Programming 41=========== 42 43Radio devices may have a couple audio controls (as discussed in 44:ref:`control`) such as a volume control, possibly custom controls. 45Further all radio devices have one tuner or modulator (these are 46discussed in :ref:`tuner`) with index number zero to select the radio 47frequency and to determine if a monaural or FM stereo program is 48received/emitted. Drivers switch automatically between AM and FM 49depending on the selected frequency. The 50:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` or 51:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl reports the 52supported frequency range. 53