This example shows how to build an FM mono or stereo receiver using Simulink® and Communications Toolbox™. You can either use captured signals, or receive signals in real time using the RTL-SDR or ADALM-PLUTO.
To run this example using captured signals, you need the following software:
To receive signals in real time, you also need one of the following hardware:
RTL-SDR radio and the corresponding software Communications Toolbox Support Package for RTL-SDR Radio
ADALM-PLUTO radio and the corresponding software Communications Toolbox Support Package for ADALM-PLUTO Radio
For a full list of Communications Toolbox supported SDR platforms, refer to Supported Hardware section of Software Defined Radio (SDR).
For an introduction to the FM broadcasting technology and demodulation of these signals, refer to the FM Broadcast Receiver example.
To run the example using captured signals, select the
FM Broadcast Captured Signal block as the source using the
Signal Source Selector block. Then click the run button.
To run the example using the RTL-SDR radio or ADALM-PLUTO radio as the source, select the corresponding
RTL-SDR Receiver or
ADALM-PLUTO Radio Receiver block as the source using the
Signal Source Selector block. Double-click the
Center Frequency (MHz) block and select the value to the center frequency to a broadcast FM radio station near you.
If you hear some dropouts or delay in the sound, run the model in Accelerator mode. From the model menu, select Simulation->Accelerator, then click the run button. If you still experience dropouts or delay in Accelerator mode, try running the model in Rapid Accelerator mode.
The following block diagram summarizes the receiver structure. The processing has three main parts: signal source, FM broadcast demodulation, and audio output.
This example can use three signal sources:
''Captured Signal'': Over-the-air signals written to a file and sourced using a Baseband File Reader block at 228e3 samples/sec.
''RTL-SDR Radio'': RTL-SDR radio running at 228e3 samples/sec. Set the center frequency to a broadcast FM radio station near you.
''ADALM-PLUTO Radio Receiver'': ADALM-PLUTO radio running at 228e3 samples/sec. Set the center frequency to a broadcast FM radio station near you.
FM Broadcast Demodulation
The baseband samples received from the signal source are processed by the
FM Broadcast Demodulation Baseband block. This block converts the input sampling rate of 228 kHz to 45.6 kHz, the sampling rate for your host computer's audio device. According to the FM broadcast standard in the United States, the de-emphasis lowpass filter time constant is set to 75 microseconds. This example processes received mono signals. The demodulator can also process stereo signals.
To perform stereo decoding, the FM Broadcast Demodulator Baseband object uses a peaking filter which picks out the 19 kHz pilot tone from which the 38 kHz carrier is created. Using the resulting 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 de-emphasis filter. The FM Broadcast Demodulator Baseband block separates the L and R signals and converts them to the 45.6 kHz audio signal.
Audio Device Writer
Play the demodulated audio signals through your computer's speakers using the
Audio Device Writer block.
To further explore the example, you can vary the center frequency of the RTL-SDR radio or ADALM-PLUTO radio and listen to other radio stations using the
Center Frequency (MHz) block.
You can set the Stereo property of the
FM Broadcast Demodulator Baseband block to true to process the signals in stereo fashion and compare the sound quality.