1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _radio: 5 6*************** 7Radio Interface 8*************** 9 10This interface is intended for AM and FM (analog) radio receivers and 11transmitters. 12 13Conventionally V4L2 radio devices are accessed through character device 14special files named ``/dev/radio`` and ``/dev/radio0`` to 15``/dev/radio63`` with major number 81 and minor numbers 64 to 127. 16 17 18Querying Capabilities 19===================== 20 21Devices supporting the radio interface set the ``V4L2_CAP_RADIO`` and 22``V4L2_CAP_TUNER`` or ``V4L2_CAP_MODULATOR`` flag in the 23``capabilities`` field of struct 24:c:type:`v4l2_capability` returned by the 25:ref:`VIDIOC_QUERYCAP` ioctl. Other combinations of 26capability flags are reserved for future extensions. 27 28 29Supplemental Functions 30====================== 31 32Radio devices can support :ref:`controls <control>`, and must support 33the :ref:`tuner or modulator <tuner>` ioctls. 34 35They do not support the video input or output, audio input or output, 36video standard, cropping and scaling, compression and streaming 37parameter, or overlay ioctls. All other ioctls and I/O methods are 38reserved for future extensions. 39 40 41Programming 42=========== 43 44Radio devices may have a couple audio controls (as discussed in 45:ref:`control`) such as a volume control, possibly custom controls. 46Further all radio devices have one tuner or modulator (these are 47discussed in :ref:`tuner`) with index number zero to select the radio 48frequency and to determine if a monaural or FM stereo program is 49received/emitted. Drivers switch automatically between AM and FM 50depending on the selected frequency. The 51:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` or 52:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl reports the 53supported frequency range. 54