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1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later2 3.. _radio:4 5***************6Radio Interface7***************8 9This interface is intended for AM and FM (analog) radio receivers and10transmitters.11 12Conventionally V4L2 radio devices are accessed through character device13special files named ``/dev/radio`` and ``/dev/radio0`` to14``/dev/radio63`` with major number 81 and minor numbers 64 to 127.15 16 17Querying Capabilities18=====================19 20Devices supporting the radio interface set the ``V4L2_CAP_RADIO`` and21``V4L2_CAP_TUNER`` or ``V4L2_CAP_MODULATOR`` flag in the22``capabilities`` field of struct23:c:type:`v4l2_capability` returned by the24:ref:`VIDIOC_QUERYCAP` ioctl. Other combinations of25capability flags are reserved for future extensions.26 27 28Supplemental Functions29======================30 31Radio devices can support :ref:`controls <control>`, and must support32the :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 streaming36parameter, or overlay ioctls. All other ioctls and I/O methods are37reserved for future extensions.38 39 40Programming41===========42 43Radio devices may have a couple audio controls (as discussed in44:ref:`control`) such as a volume control, possibly custom controls.45Further all radio devices have one tuner or modulator (these are46discussed in :ref:`tuner`) with index number zero to select the radio47frequency and to determine if a monaural or FM stereo program is48received/emitted. Drivers switch automatically between AM and FM49depending on the selected frequency. The50:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` or51:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl reports the52supported frequency range.53