MATLAB Examples

FM Stereo Receiver Using Analog Devices AD9361/AD9364

This example shows how to use the Xilinx® Zynq-Based Radio Support Package with MATLAB® to build an FM stereo receiver. The receiver receives and demodulates the FM signal transmitted by the stereo FM broadcast radio.

Refer to the Getting Started documentation for details on configuring your host computer to work with the Support Package for Xilinx® Zynq-Based Radio.

Contents

Introduction

The example configures the SDR hardware to receive an FM stereo signal over the air. The example plays both the left and right channels. The example is designed to run on a Xilinx Zynq-Based SDR hardware Using Analog Devices™ AD9361/AD9364.

Setup

Before running the example, ensure you have performed the following steps:

Configure your host computer to work with the Support Package for Xilinx® Zynq-Based Radio. See Getting Started for help.

FM Stereo Receiver Design: System Architecture

Initialization

The getParamsFMExamplesAD9361AD9364.m script initialize some simulation parameters and generates a structure, stereoFMRxParams. The fields of this structure are the parameters of the FM receiver system.

stereoFMRxParams = getParamsFMExamplesAD9361AD9364 %#ok<NOPTS>

By default, the example is configured to run with ZC706 and ADI FMCOMMS2/3/4 hardware. You can uncomment one of the following lines as applicable to set the RadioDeviceName field in structure variable stereoFMRxParams.

% % stereoFM.RadioDeviceName='ZedBoard and FMCOMMS2/3/4';
% % stereoFM.RadioDeviceName='ADI RF SOM';

stereoFMRxParams.RadioDeviceName = 'ZC706 and FMCOMMS2/3/4';

SDR Receiver

The SDR receiver SDR Receiver System object™ used with the named radio 'ZC706 and FMCOMMS2/3/4' receives the radio baseband data from the SDR Hardware. The baseband sampling rate of the SDR Hardware is set to 960 kHz using the BasebandSampleRate parameter of the SDR receiver System object. The center frequency is set as 102.5 MHz.

Set the baseband sampling rate to 960 kHz so that the example uses round numbers to convert the sampling rate to 152 kHz and then 48 kHz using small resampling filters. Frame length controls the number of samples at the output of the SDR Receiver System object, which is the input to the rate converter filter. The frame length is set to 4800 samples. The output data type is set as single to reduce the required memory and speed up execution.

sdrReceiver = sdrrx(stereoFMRxParams.RadioDeviceName, ...
    'IPAddress', '192.168.3.2', ...
    'CenterFrequency', stereoFMRxParams.CenterFrequency, ...
    'GainSource', stereoFMRxParams.RadioGainControlMode, ...
    'SamplesPerFrame', stereoFMRxParams.RadioFrameLength, ...
    'BasebandSampleRate', stereoFMRxParams.RadioSampleRate, ...
    'OutputDataType', stereoFMRxParams.RadioOutputDataType);

FM Demodulator

This example uses the FM Broadcast Demodulator Baseband System object™ to demodulate the received signal. The block also converts the sampling rate of 960 kHz to 48 kHz, a native sampling rate for your host computer's audio device. According to the FM broadcast standard in the United States, the deemphasis lowpass filter time constant is set to 75 microseconds.

fmBroadcastDemod = comm.FMBroadcastDemodulator(...
    'SampleRate', stereoFMRxParams.RadioSampleRate, ...
    'FrequencyDeviation', stereoFMRxParams.FrequencyDeviation, ...
    'FilterTimeConstant', stereoFMRxParams.FilterTimeConstant, ...
    'AudioSampleRate', stereoFMRxParams.AudioSampleRate, ...
    'PlaySound', true, ...
    'BufferSize', stereoFMRxParams.BufferSize, ...
    'Stereo', true);

To perform stereo decoding, the FM Broadcast Demodulator Baseband block uses a peaking filter which picks out the 19 kHz pilot tone from which the 38 kHz carrier is created. Using the obtained carrier signal, the FM Broadcast Demodulator Baseband block downconverts the L-R signal, centered at 38 kHz, to baseband. Afterwards, the L-R and L+R signals pass through a 75 microsecond deemphasis filter . The FM Broadcast Demodulator Baseband block separates the L and R signals and converts them to the 48 kHz audio signal.

Refer to the Simulink® model in the FM Stereo Receiver Using Analog Devices™ AD9361/AD9364 example for a block diagram view of the system.

Stream Processing Loop

FM signals are captured and stereo FM demodulation is applied for 10 seconds, which is specified by stereoFMRxParams.StopTime. The SDR Receiver System object returns a column vector, rxSig. The data from one of the channels is captured in rxSigCh1 for further processing. Since the MATLAB script may run faster than the hardware, the object also returns the actual size of the valid data in rxSigCh1 using the second output argument, len. If len is zero, then there is no new data for the demodulator code to process. The actual reception and processing of the data is enclosed in a try/catch block. This means that if there is an error, the system objects still get released properly.

try
    % The first time the SDR receiver object is used to receive data, it
    % performs some initial setup and takes about 4s. The data received
    % during this initial setup is discarded. This means the setup time is
    % not included as part of the desired run time.

    [~, ~] = sdrReceiver();
    display('Starting reception')
    timeCounter = 0;
    while timeCounter < stereoFMRxParams.StopTime
        [rxSig, len] = sdrReceiver();
        rxSigCh1 = rxSig(:,1); % Receive the data on Channel 1
        if len > 0
            % FM demodulation
            fmBroadcastDemod(rxSigCh1);
            % Update counter
            timeCounter = timeCounter + stereoFMRxParams.AudioFrameTime;
        end
    end
    disp('Finished reception')
catch ME
    rethrow(ME)
end

% Release all System objects
release(fmBroadcastDemod);
release(sdrReceiver);

Alternative Implementations

The example describes the MATLAB implementation of a receiver for demodulating and processing an FM stereo signal transmitted by the stereo FM broadcast radio. The example uses Communications System Toolbox™ System objects and Zynq-based Hardware support package to build a stereo FM receiver using SDR hardware with Analog Devices AD9361 and AD9364.

You can also view a Simulink® implementation of these examples in FM Stereo Receiver Using Analog Devices AD9361/AD9364 using Simulink

Further Exploration

To further explore the example, you can vary the center frequency of the SDR Hardware and listen to other radio stations.

Appendix

The following scripts are used in this example.

References