capture

Capture contiguous RF data using USRP® embedded series radio hardware

Description

[data,mdata] = capture(rx,length) captures ceil(length) number of contiguous data samples using the USRP®[1] embedded series radio hardware associated with the rx receiver System object™. The function returns the captured data samples in the output data. The function also returns capture-time configuration settings for the radio device in the output structure mdata.

[___] = capture(rx,length,unit) also specifies the unit value for the capture length in addition to the previous syntax. The function converts length units into N samples, and returns ceil(N) number of samples.

example

[data,mdata,fn] = capture(___,'Filename',filename) also specifies a file name for the captured data, in addition to the input arguments in any of the previous syntaxes. The function saves the captured data and metadata to a baseband file reader file named fn.

[___] = capture(___,'Filename',filename,Name,Value) specifies options using one or more name-value pair arguments. For example, 'UserMetadata',moreInfo adds the moreInfo structure defined in the workspace to the mdata output. Specify name-value pair arguments after all other input arguments.

Examples

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Use the capture function to record RF signals for post-capture processing in MATLAB®. Save an FM broadcast signal to a file as baseband samples. Read the file containing the recorded signal and demodulate the baseband samples.

Configure SDR Hardware

If your radio hardware is already configured for host-radio communication, you can skip this section. To configure your radio hardware for host-radio communication, follow the steps in the Guided Host-Radio Hardware Setup. Attach an antenna suitable for the 88-108 MHz band to the first RX channel.

Configure Receiver System Object

Create a receiver System object with the specified properties. The specified center frequency corresponds to a local FM station.

stationCenterFrequency = 95e6;
deviceName = 'E3xx';
rx = sdrrx(deviceName,'BasebandSampleRate',528e3,...
    'CenterFrequency',stationCenterFrequency,'OutputDataType','Double');

Initiate Data Capture to File

Call the capture function, specifying the receiver object, capture duration, and file name. The function returns the captured data in a file named "FMRecording.bb". After capturing the FM signal, unlock the receiver object using the release function.

capture(rx,5,'Seconds','Filename','FMRecording.bb');
## Establishing connection to hardware. This process can take several seconds.
release(rx);

Demodulate FM Recording

Create a comm.BasebandFileReader System object to read the captured signal and extract frames of data from the file. Set baseband file reader to take 4400 samples per frame when reading the saved baseband signal.

bbr = comm.BasebandFileReader('FMRecording.bb');
bbr.SamplesPerFrame = 4400;

Use the BasebandSampleRate field of the baseband file reader object to set the SampleRate property of the demodulator. Find the BasebandSampleRate field in the MetaData structure. Create a comm.FMBroadcastDemodulator System object. Demodulate and play back each frame of the FM data. Use a while loop to read all frames of the captured data.

fmbDemod = comm.FMBroadcastDemodulator( ...
    'AudioSampleRate', 48e3, ...
    'SampleRate',bbr.Metadata.BasebandSampleRate,'PlaySound',true);

while ~isDone(bbr)
    fmbDemod(bbr());
end

Input Arguments

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Receiver, specified as a comm.SDRRxE3xx receiver System object. To create this object, use the sdrrx function.

Capture duration, specified as a scalar. The input argument unit specifies the unit of length. The default unit is samples of data. The maximum number of samples to capture is in the range [1, 16,777,216]. You can remove the limit on the maximum number of samples to capture but at the cost of guaranteed data contiguity. For more details, see the 'EnableOversizeCapture' name-value pair argument.

Data Types: double

Capture duration unit, specified as:

  • 'Samples' — The function returns ceil(length) number of samples.

  • 'Seconds' — The function converts length seconds into N samples, based on the BasebandSampleRate property of the receiver object rx, and returns ceil(N) number of samples.

File name for the captured data, specified as a character vector or string scalar. The function saves the captured data. To read this file, use the comm.BasebandFileReader System object. The saved file has the file name extension .bb.

Example: 'rxdata' or 'rxdata.bb' configures the function to save data to a file named rxdata.bb.

Data Types: char | string

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: [data,mdata,fn] = capture(rxsdr,5,'Filename','RFCapture','Timestamp',true)

Append a capture time stamp to the saved file name, specified as the comma separated pair consisting of 'Timestamp' and false or true.

  • When you specify false, no timestamp is appended to the saved file name.

  • When you specify true, the name of the created file takes the form <filename>_<Year>-<Month>-<Day>_<Hour>-<Minute>-<Second>-<Millisecond>.bb and indicates when the data was captured.

Data Types: logical

User data, specified as the comma separated pair consisting of 'UserMetadata' and a single-level structure of data that is added to the mdata structure output. By default there is no user data structure included in mdata.

Data Types: struct

Remove limit on capture duration, specified as the comma separated pair consisting of 'EnableOversizeCapture' and false or true. When you specify true, the function removes the limit on the number of samples and data contiguity is not guaranteed.

Data Types: logical

Output Arguments

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Output signal received from the radio hardware, returned as a complex matrix. The number of columns in the matrix depends on the number of channels in use, as specified by the ChannelMapping property of the receiver object rx. Each column corresponds to a channel of complex data received on one channel.

The output signal supports complex values with these data types:

  • 16-bit signed integers — Complex values are the raw 16-bit I and Q samples from the board. The 12-bit value from the ADC of the AD9361 RF chip is sign-extended to 16 bits.

  • Single-precision floating point — Complex values are scaled to the range of [–0.0625, 0.0625]. The object derives this value from the sign-extended 16 bits received from the board.

  • Double-precision floating point — Complex values are scaled to the range of [–0.0625, 0.0625]. The object derives this value from the sign-extended 16 bits received from the board.

To specify the base type, use the OutputDataType property of the receiver object rx.

Data Types: int16 | single | double
Complex Number Support: Yes

Capture-time configuration settings for the radio device, returned as a structure that contains these fields:

FieldDescription
Date

Date of the data capture.

BasebandSampleRate

Baseband sample rate, in Hz, configured in the SDR device.

CenterFrequency

Center frequency, in Hz, configured in the SDR device.

DeviceName

Name of the SDR device.

ChannelMapping

Channel mapping value configured in the SDR device. Channel mapping is always 1.

CaptureLength

Length of data capture, as specified by inputs length and unit.

Setup

Character vector list of property values and settings that describe the System object state.

GainSource

Gain source value configured in the SDR device.

DataContiguity

0 indicates the data is not contiguous.

1 indicates the data is contiguous.

UserMetaDataAdditional structure containing user-defined information included as an input to the capture function.

Data Types: struct

File name for saved captured data and metadata, returned as a character vector.

Data Types: char

Introduced in R2019b


[1] USRP, USRP2, UHD, and Ettus Research are trademarks of National Instruments Corp.