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comm.FMDemodulator System object

Demodulate using FM method

Description

The FMDemodulator System object™ demodulates an FM modulated signal.

To FM demodulate a signal:

  1. Define and set up the FMDemodulator object. See Construction.

  2. Call step to FM demodulate a signal according to the properties of comm.FMDemodulator. The behavior of step is specific to each object in the toolbox.

Note

Starting in R2016b, instead of using the step method to perform the operation defined by the System object™, you can call the object with arguments, as if it were a function. For example, y = step(obj,x) and y = obj(x) perform equivalent operations.

Construction

H = comm.FMDemodulator creates a demodulator System object, H, that frequency demodulates an input signal.

H = comm.FMDemodulator(mod) creates an FM demodulator object whose properties are determined by the corresponding FM modulator object, mod.

H = comm.FMDemodulator(Name,Value) creates an FM demodulator object with each specified property Name set to the specified Value. Name must appear inside single quotes. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

FrequencyDeviation

Peak deviation of the output signal frequency (Hz)

Specify the frequency deviation of the FM demodulator in Hz as a positive real scalar. The default value is 75e3. The system bandwidth is equal to twice the sum of the frequency deviation and the message bandwidth. This property is nontunable.

SampleRate

Sample rate of input signal (Hz)

Specify the sample rate in Hz as a positive real scalar. The default value is 240e3. The output sample rate is equal to the input sample rate. This property is nontunable.

Methods

resetReset states of the FM demodulator object
stepApplies FM baseband demodulation
Common to All System Objects
clone

Create System object with same property values

getNumInputs

Expected number of inputs to a System object

getNumOutputs

Expected number of outputs of a System object

isLocked

Check locked states of a System object (logical)

release

Allow System object property value changes

Examples

expand all

Modulate and demodulate a sinusoidal signal. Plot the demodulated signal and compare it to the original signal.

Set the example parameters.

fs = 100;              % Sample rate (Hz)
ts = 1/fs;             % Sample period (s)
fd = 25;               % Frequency deviation (Hz)

Create a sinusoidal input signal with duration 0.5s and frequency 4 Hz.

t = (0:ts:0.5-ts)';
x = sin(2*pi*4*t);

Create an FM modulator System object™.

MOD = comm.FMModulator('SampleRate',fs,'FrequencyDeviation',fd);

FM modulate the input signal and plot its real part. You can see that the frequency of the modulated signal changes with the amplitude of the input signal.

y = step(MOD,x);
plot(t,[x real(y)])

Demodulate the FM modulated signal.

DEMOD = comm.FMDemodulator('SampleRate',fs,'FrequencyDeviation',fd);
z = step(DEMOD,y);

Plot the input and demodulated signals. The demodulator output signal exactly aligns with the input signal.

plot(t,x,'r',t,z,'ks')
legend('Input Signal','Demod Signal')
xlabel('Time (s)')
ylabel('Amplitude')

Create an FM demodulator System object™ from an FM modulator object. Modulate and demodulate audio data loaded from a file and compare its spectrum with that of the input data.

Set the example parameters.

fd = 50e3;                               % Frequency deviation (Hz)
fs = 300e3;                              % Sample rate (Hz)

Create an FM modulator System object.

MOD = comm.FMModulator('FrequencyDeviation',fd,'SampleRate',fs);

Create a companion demodulator object based on the modulator.

DEMOD = comm.FMDemodulator(MOD);

Verify that the properties are identical in the two System objects.

MOD
DEMOD
MOD = 

  comm.FMModulator with properties:

            SampleRate: 300000
    FrequencyDeviation: 50000


DEMOD = 

  comm.FMDemodulator with properties:

            SampleRate: 300000
    FrequencyDeviation: 50000

Load audio data into structure variable, S.

S = load('handel.mat');
data = S.y;
fsamp = S.Fs;

Create a spectrum analyzer System object.

SA = dsp.SpectrumAnalyzer('SampleRate',fsamp,'ShowLegend',true);

FM modulate and demodulate the audio data.

modData = step(MOD,data);
demodData = step(DEMOD,modData);

Verify that the spectrum plot of the input data (Channel 1) is aligned with that of the demodulated data (Channel 2).

step(SA,[data demodData])

Playback an audio file after applying FM modulation and demodulation. The example takes advantage of the characteristics of System objects™ to process the data in streaming mode.

Load the audio file, guitartune.wav, using an audio file reader object.

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

Create an audio device writer object for audio playback.

AUDIOPLAYER = audioDeviceWriter;

Create modulator and demodulator objects having default properties.

MOD = comm.FMModulator;
DEMOD = comm.FMDemodulator;

Read audio data, FM modulate, FM demodulate, and playback the demodulated signal, z.

while ~isDone(AUDIO)
    x = step(AUDIO);                      % Read audio data
    y = step(MOD,x);                      % FM modulate
    z = step(DEMOD,y);                    % FM demodulate
    step(AUDIOPLAYER,z);                  % Playback the demodulated signal
end

Algorithms

You can represent a standard frequency modulated passband signal, Y(t), as

Y(t)=Acos(2πfct+2πfΔ0tx(τ)dτ),

where A is the carrier amplitude, fc is the carrier frequency, x(τ) is the baseband input signal, and fΔ is the frequency deviation in Hz. The frequency deviation is the maximum shift from fc in one direction, assuming |x(t)| ≤ 1.

A baseband FM signal can be derived from the passband representation by downconverting it by fc such that

ys(t)=Y(t)ej2πfct=A2[ej(2πfct+2πfΔ0tx(τ)dτ)+ej(2πfct+2πfΔ0tx(τ)dτ)]ej2πfct=A2[ej2πfΔ0tx(τ)dτ+ej4πfctj2πfΔ0tx(τ)dτ].

Removing the component at -2fc from ys(t) leaves the baseband signal representation, y(t), which is expressed as

y(t)=A2ej2πfΔ0tx(τ)dτ.

The expression for y(t) is rewritten as

y(t)=A2ejϕ(t),

where ϕ(t)=2πfΔ0tx(τ)dτ, which implies that the input signal is a scaled version of the derivative of the phase, ϕ(t).

A baseband delay demodulator is used to recover the input signal from y(t).

A delayed and conjugated copy of the received signal is subtracted from the signal itself.

w(t)=A24ejϕ(t)ejϕ(tT)=A24ej[ϕ(t)ϕ(tT)],

where T is the sample period. In discrete terms, wn=w(nT), consequently

wn=A24ej[ϕnϕn1],vn=ϕnϕn1.

The signal vn is the approximate derivative of ϕn such that vnxn.

Selected Bibliography

[1] Chakrabarti, I. H., and Hatai, I. “A New High-Performance Digital FM Modulator and Demodulator for Software-Defined Radio and Its FPGA Implementation.” International Journal of Reconfigurable Computing. Vol. 2011, No. 10.1155/2011, 2011, p. 10.

[2] Taub, Herbert, and Donald L. Schilling. Principles of Communication Systems. New York: McGraw-Hill, 1971, pp. 142–155.

Extended Capabilities

Introduced in R2015a

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