comm.FMBroadcastDemodulator

Demodulate broadcast FM audio signal

Description

The comm.FMBroadcastDemodulator System object™ demodulates a complex broadcast FM-modulated signal and filters the signal with a de-emphasis filter to produce an audio signal. For more details, see Algorithms.

To demodulate a broadcast FM audio signal:

1. Create the comm.FMBroadcastDemodulator object and set its properties.

2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

Creation

Syntax

fmbdemodulator = comm.FMBroadcastDemodulator

fmbdemodulator = comm.FMBroadcastDemodulator(Name,Value)

fmbdemodulator = comm.FMBroadcastDemodulator(fmbmodulator)

Description

fmbdemodulator = comm.FMBroadcastDemodulator creates an FM broadcast demodulator System object.

fmbdemodulator = comm.FMBroadcastDemodulator(Name,Value) sets properties using one or more name-value arguments. For example, fmbdemodulator = comm.FMBroadcastDemodulator('SampleRate',400e3) specifies a sample rate of 400 kHz.

fmbdemodulator = comm.FMBroadcastDemodulator(fmbmodulator) sets properties based on the configuration of the input comm.FMBroadcastModulator System object, fmbmodulator.

Properties

Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

SampleRate — Sample rate

240e3 (default) | positive scalar

Sample rate of the input of the demodulator in Hz, specified as a positive scalar. The sample rate must be greater than twice the frequency deviation (that is, SampleRate > 2×FrequencyDeviation).

Data Types: double

FrequencyDeviation — Peak deviation of the input signal frequency

75e3 (default) | positive scalar

Peak deviation of the input signal frequency in Hz, specified a positive scalar. The frequency deviation must be less than half the sample rate (that is, FrequencyDeviation < SampleRate/2).

The system bandwidth B_T = 2×(FrequencyDeviation + B_M), where B_M is the message bandwidth in Hz. For more information, see Algorithms.

FM broadcast standards specify a value of 75 kHz in the United States and 50 kHz in Europe.

Data Types: double

FilterTimeConstant — De-emphasis highpass filter time constant

7.5e-05 (default) | positive scalar

De-emphasis highpass filter time constant in seconds, specified as a positive scalar. FM broadcast standards specify a value of 75 μs in the United States and 50 μs in Europe.

Data Types: double

Stereo — Option to enable stereo demodulation

false or 0 (default) | true or 1

Option to enable stereo demodulation, specified as a logical 0 (false) or 1 (true).

• false — The output is mono audio.

• true — The object performs stereo decoding and outputs stereo audio.

For more information, see Multiplexed Stereo and RDS (or RBDS) FM Signal.

Data Types: logical

PlaySound — Option to enable audio playback

false or 0 (default) | true or 1

Option to enable audio playback, specified as a logical 0 (false) or 1 (true). To playback the output signal on the default audio device connected to the computer, set this property to true.

Data Types: logical

AudioSampleRate — Sample rate of output audio signal

48000 (default) | positive scalar

Sample rate of the output audio signal in Hz, specified as a positive scalar.

Data Types: double

BufferSize — Size of buffer

4096 (default) | positive integer

Size of the buffer in samples, specified as a positive integer. This property specifies the size of the buffer used by the System object for communication with the audio device.

Dependencies

To enable this property, set the PlaySound property to true.

Data Types: double

RBDS — Option to enable RDS (or RBDS) waveform demodulation

false or 0 (default) | true or 1

Option to enable RDS (or RBDS) waveform demodulation, specified as a logical 0 (false) or 1 (true). If you set this property set to true, the object demodulates the RDS (or RBDS) waveform. For more information, see Multiplexed Stereo and RDS (or RBDS) FM Signal.

Data Types: logical

RBDSSamplesPerSymbol — Oversampling factor of RDS (or RBDS) output signal

10 (default) | positive integer

Oversampling factor of the RDS (or RBDS) output signal, specified as a positive integer. The sample rate of RDS (or RBDS) broadcast data is 1187.5 Hz. The RDS (or RBDS) signal sample rate is RBDSSamplesPerSymbol × 1187.5 Hz.

Dependencies

To enable this property, set the RBDS property to true.

Data Types: double

RBDSCostasLoop — Option to recover phase of RDS (or RBDS) signal

false or 0 (default) | true or 1

Option to recover phase of the RDS (or RBDS) signal, specified as a logical 0 (false) or 1 (true).

To recover the phase of the RDS (or RBDS) signal by using a Costas loop, set this property to true. For radio stations that do not lock the 57 kHz RDS (or RBDS) signal in phase with the third harmonic of the 19 kHz pilot tone, a Costas loop helps recover the phase of the RDS (or RBDS) signal.

Dependencies

To enable this property, set the RBDS property to true.

Data Types: logical

Usage

Syntax

audiosig = fmbdemodulator(insig)

[audiosig,rbdssig] = fmbdemodulator(insig)

Description

audiosig = fmbdemodulator(insig) demodulates an FM-modulated baseband audio signal and filters this signal with a de-emphasis filter to produce an audio signal.

[audiosig,rbdssig] = fmbdemodulator(insig) also demodulates a baseband RBDS signal at 57 kHz. To enable this syntax, set the RBDS property to true.

Input Arguments

insig — FM-modulated baseband audio signal

column vector

FM-modulated baseband audio signal, specified as a column vector. For information about signal length restrictions, see Limitations.

Data Types: double | single
Complex Number Support: Yes

Output Arguments

audiosig — Audio signal

N-element column vector | M-by-N matrix

Audio signal, returned as a column vector or an M-by-N matrix. M is the number of stereo channels. N is the number of samples in the audio signal per channel. The output has the same data type as the input signal, insig.

If you set the Stereo property to true, the audio signal must have at least two channels and the System object performs stereo encoding before de-emphasis filtering. The length of the output is N × (AudioSampleRate/SampleRate).

rbdssig — RBDS signal

column vector

RBDS signal, returned as a column vector with the same data type as the input signal.

Data Types: double | single
Complex Number Support: Yes

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

Specific to comm.FMBroadcastDemodulator

info	Information about FM broadcast modulator or demodulator

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

Examples

FM Broadcast Streaming Audio Signal

Play back an audio file after applying FM broadcast modulation and demodulation using System objects to process the data in streaming mode.

Load the audio file guitartune.wav by using an audio file reader System object™ with the samples per frame set to 4410.

audiofilereader = dsp.AudioFileReader('guitartune.wav', ...
    'SamplesPerFrame',4410);

Create FM broadcast modulator and demodulator objects. Set the sample rate of the output audio signal to match the sample rate of the input audio signal. Set the sample rate of the demodulator to match the specified sample rate of the modulator. Enable audio playback for the broadcast demodulator.

fmbMod = comm.FMBroadcastModulator( ...
    'AudioSampleRate',audiofilereader.SampleRate, ...
    'SampleRate',240e3);
fmbDemod = comm.FMBroadcastDemodulator( ...
    'AudioSampleRate',audiofilereader.SampleRate, ...
    'SampleRate',240e3,'PlaySound',true);

Read the audio data in frames of length 4410, apply FM broadcast modulation, demodulate the FM signal, and play back the demodulated signal (demodData).

while ~isDone(audiofilereader)
    audioData = audiofilereader();
    modData = fmbMod(audioData);
    demodData = fmbDemod(modData); % Demodulate and play signal
end

FM Broadcast Modulate and Demodulate an RBDS Waveform

Generate an RBDS waveform, FM broadcast modulate the RBDS waveform with an audio signal, and FM broadcast demodulate the FM signal.

Specify parameters for an RBDS waveform with 19 groups per frame and 10 samples per symbol. The sample rate of the RBDS waveform is given by 1187.5 x 10. Set the audio sample rate to 1187.5 x 40.

groupLen = 104;
sps = 10;
groupsPerFrame = 19;
rbdsFrameLen = groupLen*sps*groupsPerFrame;
afrRate = 40*1187.5;
rbdsRate = 1187.5*sps;
outRate = 4*57000;

Load the audio file guitartune.wav by using an audio file reader System object™ while setting the samples per frame. Create RBDS waveform generator, FM broadcast modulator, FM broadcast demodulator, and time scope System objects. Configure the modulator and demodulator objects to process a stereo audio file and an RBDS waveform.

afr = dsp.AudioFileReader( ...
    'rbds_capture_47500.wav', ...
    'SamplesPerFrame',rbdsFrameLen*afrRate/rbdsRate);
rbds = comm.RBDSWaveformGenerator( ...
    'GroupsPerFrame',groupsPerFrame, ...
    'SamplesPerSymbol',sps);

fmMod = comm.FMBroadcastModulator( ...
    'AudioSampleRate',afr.SampleRate, ...
    'SampleRate',outRate,...
    'Stereo',true, ...
    'RBDS',true, ...
    'RBDSSamplesPerSymbol',sps);
fmDemod = comm.FMBroadcastDemodulator( ...
    'SampleRate',outRate,...
    'Stereo',true, ...
    'RBDS',true, ...
    'PlaySound',true);
scope = timescope( ...
    'SampleRate',outRate, ...
    'YLimits',10^-2*[-1 1]);

Read the audio signal. Generate RBDS information at the same configured rate as audio. FM broadcast modulate the stereo audio signal with RBDS information. Add additive white Gaussian noise. FM-demodulate the audio signal and RBDS waveforms. View the waveforms in a time scope.

for idx = 1:7
    input = afr();                         
    rbdsWave = rbds();                     
    yFM = fmMod([input input],rbdsWave);  
    rcv = awgn(yFM,40);                   
    [audioRcv, rbdsRcv] = fmDemod(rcv);    
    scope(rbdsRcv);
end

Limitations

The length of the input signal, insig, must be an integer multiple of the AudioDecimationFactor property. If you set the RBDS property to true, the length of the input signal, insig, also must be an integer multiple of RBDSDecimationFactor. For more information on the AudioDecimationFactor and RBDSDecimationFactor properties, see the info object function.

Algorithms

The comm.FMBroadcastDemodulator System object includes the functionality of the comm.FMDemodulator System object, plus de-emphasis filtering and the ability to receive stereophonic signals.

Filtering

FM amplifies high-frequency noise and degrades the overall signal-to-noise ratio. To compensate, FM broadcasters insert a pre-emphasis filter before FM modulation to amplify the high-frequency content. The FM receiver has a reciprocal de-emphasis filter after the FM demodulator to attenuate high-frequency noise and restore a flat signal spectrum. This figure shows the order of processing operations.

The pre-emphasis filter has a highpass characteristic transfer function given by

$$H_{\text{p}}(f)=1+j2\pi f{\tau }_{\text{s}},$$

where τ_s is the filter time constant. The time constant is 75 μs in the United States and 50 μs in Europe. Similarly, the transfer function for the lowpass de-emphasis filter is given by

$$H_{\text{d}}(f)=\frac{1}{1+j2\pi f{\tau }_{\text{s}}}.$$

For an audio sample rate of 44.1 kHz, the de-emphasis filter has the response shown in this figure.

Multiplexed Stereo and RDS (or RBDS) FM Signal

FM broadcast supports stereophonic and monophonic operations. To support stereo transmission:

• The Left + Right channel information is assigned to the mono portion of the spectrum (0 to 15 kHz).

• The Left – Right channel information is amplitude modulated onto the 23 to 53 kHz region of the baseband spectrum using a 38 kHz subcarrier signal.

A pilot tone at 19 kHz in the multiplexed signal enables the FM receiver to coherently demodulate the stereo and RDS (or RBDS) signals.

This figure shows the spectrum of the multiplex baseband signal.

The multiplex message signal m(t) is given by

$$m(t)=C_0\left[L(t)+R(t)\right]+C_1\cos (2\pi \times 19kHz\times t)+C_0\left[L(t)-R(t)\right]\cos (2\pi \times 38kHz\times t)+C_{2}RBDS(t)\cos (2\pi \times 57kHz\times t),$$

where C₀, C₁, and C₂ are gains. To generate the appropriate modulation level, these gains scale the amplitudes of the L(t)±R(t) signals, the 19 kHz pilot tone, and the RDS (or RBDS) subcarrier, respectively.

The demodulator applies m(t) to three bandpass filters with center frequencies at 19, 38, and 57 kHz and to a lowpass filter with a 3 dB cutoff frequency of 15 kHz. The 19 kHz bandpass filter extracts the pilot tone from the modulated signal. The recovered pilot tone is doubled and tripled in frequency to produce the 38 kHz and 57 kHz signals, which demodulate the (L – R) and RDS (or RBDS) signals, respectively. To generate a scaled version of the left and right channels that produces the stereo sound, the object adds and subtracts the (L + R) and (L – R) signals. To recover the RDS (or RBDS) signal, m(t) is mixed with the 57 kHz signal.

This figure shows the multiplexing (MPX) decoder block diagram of the FM broadcast demodulator. L(t) and R(t) are the left and right audio signal components of the time-domain waveforms. RBDS(t) is the time-domain waveform of the RDS (or RBDS) signal.

References

[1] Hatai, I., and I. Chakrabarti. “A New High-Performance Digital FM Modulator and Demodulator for Software-Defined Radio and Its FPGA Implementation.” International Journal of Reconfigurable Computing (December 25, 2011): 1–10. https://doi.org/10.1155/2011/342532.

[2] Taub, H., and D. Schilling. Principles of Communication Systems. McGraw-Hill Series in Electrical Engineering, 142–55. New York: McGraw-Hill, 1971.

[3] Der, Lawrence. "Frequency Modulation (FM) Tutorial." Silicon Laboratories Inc.: 4–8.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

See System Objects in MATLAB Code Generation (MATLAB Coder).

Version History

Introduced in R2015a