4.10. Software Defined Radio Interface (SDR)¶

SDR is an abbreviation of Software Defined Radio, the radio device which uses application software for modulation or demodulation. This interface is intended for controlling and data streaming of such devices.

SDR devices are accessed through character device special files named /dev/swradio0 to /dev/swradio255 with major number 81 and dynamically allocated minor numbers 0 to 255.

4.10.1. Querying Capabilities¶

Devices supporting the SDR receiver interface set the V4L2_CAP_SDR_CAPTURE and V4L2_CAP_TUNER flag in the capabilities field of struct v4l2_capability returned by the ioctl VIDIOC_QUERYCAP ioctl. That flag means the device has an Analog to Digital Converter (ADC), which is a mandatory element for the SDR receiver.

Devices supporting the SDR transmitter interface set the V4L2_CAP_SDR_OUTPUT and V4L2_CAP_MODULATOR flag in the capabilities field of struct v4l2_capability returned by the ioctl VIDIOC_QUERYCAP ioctl. That flag means the device has an Digital to Analog Converter (DAC), which is a mandatory element for the SDR transmitter.

At least one of the read/write, streaming or asynchronous I/O methods must be supported.

4.10.2. Supplemental Functions¶

SDR devices can support controls, and must support the Tuners and Modulators ioctls. Tuner ioctls are used for setting the ADC/DAC sampling rate (sampling frequency) and the possible radio frequency (RF).

The V4L2_TUNER_SDR tuner type is used for setting SDR device ADC/DAC frequency, and the V4L2_TUNER_RF tuner type is used for setting radio frequency. The tuner index of the RF tuner (if any) must always follow the SDR tuner index. Normally the SDR tuner is #0 and the RF tuner is #1.

The ioctl VIDIOC_S_HW_FREQ_SEEK ioctl is not supported.

4.10.3. Data Format Negotiation¶

The SDR device uses the Data Formats ioctls to select the capture and output format. Both the sampling resolution and the data streaming format are bound to that selectable format. In addition to the basic Data Formats ioctls, the ioctl VIDIOC_ENUM_FMT ioctl must be supported as well.

To use the Data Formats ioctls applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_SDR_CAPTURE or V4L2_BUF_TYPE_SDR_OUTPUT and use the struct v4l2_sdr_format sdr member of the fmt union as needed per the desired operation. Currently there are two fields, pixelformat and buffersize, of struct v4l2_sdr_format which are used. Content of the pixelformat is V4L2 FourCC code of the data format. The buffersize field is maximum buffer size in bytes required for data transfer, set by the driver in order to inform application.

v4l2_sdr_format¶

struct v4l2_sdr_format¶
__u32	`pixelformat`	The data format or type of compression, set by the application. This is a little endian four character code. V4L2 defines SDR formats in SDR Formats.
__u32	`buffersize`	Maximum size in bytes required for data. Value is set by the driver.
__u8	`reserved[24]`	This array is reserved for future extensions. Drivers and applications must set it to zero.

An SDR device may support read/write and/or streaming (memory mapping or user pointer) I/O.