Software Defined Radio Interface (SDR)

Querying Capabilities
Supplemental Functions
Data Format Negotiation


This is an experimental interface and may change in the future.

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.

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 VIDIOC_QUERYCAP ioctl. That flag means the device has an Analog to Digital Converter (ADC), which is a mandatory element for the SDR receiver. At least one of the read/write, streaming or asynchronous I/O methods must be supported.

Supplemental Functions

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

The V4L2_TUNER_ADC tuner type is used for ADC tuners, and the V4L2_TUNER_RF tuner type is used for RF tuners. The tuner index of the RF tuner (if any) must always follow the ADC tuner index. Normally the ADC tuner is #0 and the RF tuner is #1.

The VIDIOC_S_HW_FREQ_SEEK ioctl is not supported.

Data Format Negotiation

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

To use the format ioctls applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_SDR_CAPTURE and use the struct v4l2_sdr_format sdr member of the fmt union as needed per the desired operation. Currently there is two fields, pixelformat and buffersize, of struct 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.

Table 4.18. struct v4l2_sdr_format

__u32pixelformat The data format or type of compression, set by the application. This is a little endian four character code. V4L2 defines SDR formats in the section called “SDR Formats”.
__u32buffersize Maximum size in bytes required for data. Value is set by the driver.
__u8reserved[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.