1.. Permission is granted to copy, distribute and/or modify this 2.. document under the terms of the GNU Free Documentation License, 3.. Version 1.1 or any later version published by the Free Software 4.. Foundation, with no Invariant Sections, no Front-Cover Texts 5.. and no Back-Cover Texts. A copy of the license is included at 6.. Documentation/userspace-api/media/fdl-appendix.rst. 7.. 8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections 9 10.. _radio: 11 12*************** 13Radio Interface 14*************** 15 16This interface is intended for AM and FM (analog) radio receivers and 17transmitters. 18 19Conventionally V4L2 radio devices are accessed through character device 20special files named ``/dev/radio`` and ``/dev/radio0`` to 21``/dev/radio63`` with major number 81 and minor numbers 64 to 127. 22 23 24Querying Capabilities 25===================== 26 27Devices supporting the radio interface set the ``V4L2_CAP_RADIO`` and 28``V4L2_CAP_TUNER`` or ``V4L2_CAP_MODULATOR`` flag in the 29``capabilities`` field of struct 30:c:type:`v4l2_capability` returned by the 31:ref:`VIDIOC_QUERYCAP` ioctl. Other combinations of 32capability flags are reserved for future extensions. 33 34 35Supplemental Functions 36====================== 37 38Radio devices can support :ref:`controls <control>`, and must support 39the :ref:`tuner or modulator <tuner>` ioctls. 40 41They do not support the video input or output, audio input or output, 42video standard, cropping and scaling, compression and streaming 43parameter, or overlay ioctls. All other ioctls and I/O methods are 44reserved for future extensions. 45 46 47Programming 48=========== 49 50Radio devices may have a couple audio controls (as discussed in 51:ref:`control`) such as a volume control, possibly custom controls. 52Further all radio devices have one tuner or modulator (these are 53discussed in :ref:`tuner`) with index number zero to select the radio 54frequency and to determine if a monaural or FM stereo program is 55received/emitted. Drivers switch automatically between AM and FM 56depending on the selected frequency. The 57:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` or 58:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl reports the 59supported frequency range. 60