Modulate broadcast FM signal
System object™ pre-emphasizes an audio signal and modulates it
onto a baseband FM signal. If the
Stereo property is set to
true, the object modulates the audio input
(L–R) in the 38 kHz band, in addition to
modulating it in the baseband (L+R). If the
RBDS property is set to
true, the object
modulates a baseband RDS/RBDS signal at 57 kHz. For more details, see Algorithms.
To FM modulate an audio signal:
Starting in R2016b, instead of using the
to perform the operation defined by the System
object, you can
call the object with arguments, as if it were a function. For example,
= step(obj,x) and
y = obj(x) perform
fmbMod = comm.FMBroadcastModulator creates
a modulator System
fmbMod, that frequency
modulates an input signal.
fmbMod = comm.FMBroadcastModulator(demod) creates
a broadcast FM modulator object whose properties are determined by
the corresponding broadcast FM demodulator object,
fmbMod = comm.FMBroadcastModulator( creates
a broadcast FM modulator object with each specified property
to the specified
Value. You can specify additional
name-value pair arguments in any order as (
Output signal sample rate (Hz)
Specify the sample rate of the output signal in Hz as a positive
real scalar. The default value is
Peak deviation of the output signal frequency (Hz)
Specify the frequency deviation of the FM modulator in Hz as
a positive real scalar. The default value is
Filter time constant (s)
Specify the pre-emphasis highpass filter time constant as a
positive real scalar. FM broadcast standards specify a value of 75
μs in the United States and 50 μs in Europe. The default
Sample rate of the input audio signal (Hz)
Specify the audio sample rate as a positive real scalar. The
default value is
Flag to set stereo operations
Set this property to
Flag to modulate RDS/RBDS waveform
Oversampling factor of RDS/RBDS input
Specify the number of samples per RDS/RBDS symbol as a positive integer. The RDS/RBDS sample
rate is given by
This property applies only when you set
The default is 10.
|info||Filter information about FM broadcast modulator|
|reset||Reset states of the FM broadcast modulator object|
|step||Apply FM broadcast modulation|
|Common to All System Objects|
Allow System object property value changes
Modulate and demodulate a streaming audio signal with the FM broadcast modulator and demodulator objects. Play the audio signal using a default audio device.
Note: This example runs only in R2016b or later. If you are using an earlier release, replace each call to the function with the equivalent
step syntax. For example, myObject(x) becomes step(myObject,x).
Create an audio file reader System object™ and read the file
audio = dsp.AudioFileReader('guitartune.wav','SamplesPerFrame',4410);
Create FM broadcast modulator and demodulator objects. Set the
AudioSampleRate property to match the sample rate of the input signal. Set the
SampleRate property of the demodulator to match the specified sample rate of the modulator. Set the
PlaySound property of the demodulator to
true to enable audio playback.
fmbMod = comm.FMBroadcastModulator('AudioSampleRate',audio.SampleRate, ... 'SampleRate',240e3); fmbDemod = comm.FMBroadcastDemodulator( ... 'AudioSampleRate',audio.SampleRate, ... 'SampleRate',240e3,'PlaySound',true);
Read the audio data in frames of length 4410, apply FM broadcast modulation, demodulate the FM signal and playback the audio input.
while ~isDone(audio) audioData = audio(); modData = fmbMod(audioData); demodData = fmbDemod(modData); end
Generate a basic RBDS waveform, FM modulate it with an audio signal, and then demodulate it.
Note: This example runs only in R2017a or later.
Create a 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; 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 = dsp.TimeScope('SampleRate',outRate,'YLimits',10^-2*[-1 1]);
Get the current audio input. Generate RBDS information at the same configured rate as audio. FM modulate the stereo audio with RBDS information. Add additive white Gaussian noise. FM demodulate the audio 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
The FM Broadcast modulator includes the functionality of the
baseband FM modulator, pre-emphasis filtering, and the ability to
transmit stereophonic signals. The algorithms which govern basic FM
modulation and demodulation are covered in
FM amplifies high-frequency noise and degrades the overall signal-to-noise ratio. To compensate, FM broadcasters insert a pre-emphasis filter prior to 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.
The pre-emphasis filter has a highpass characteristic transfer function given by
where τs is the filter time constant. The time constant is 50 μs in Europe and 75 μs in the United States. Similarly, the transfer function for the lowpass de-emphasis filter is given by
Irrespective of the audio sampling rate, the signal is converted to a 152 kHz output sampling rate. For an audio sample rate of 44.1 kHz, the pre-emphasis filter has the following response.
The FM broadcast modulator supports stereophonic and monophonic operations. To support stereo transmission, the left (L) and right (R) channel information (L+R) is assigned to the mono portion of the spectrum (0 to 15 kHz). The (L-R) 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/RBDS signals. Here is the spectrum of the multiplex baseband signal.
Here is the block diagram of the FM broadcast modulator, which is used to generate the multiplex baseband signal. L(t) and R(t) denote the time-domain waveforms from the left and right channels. RBDS(t) denotes the time-domain waveform of the RDS/RBDS signal.
The multiplex message signal, m(t) is given by
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/RBDS subcarrier, respectively.
true, both the audio and RDS/RBDS inputs
must satisfy the following equation:
The input length of the audio signal must be an integer multiple of the
AudioDecimationFactor property. The input length of the
RDS/RBDS signal must be an integer multiple of the
RBDSDecimationFactor property. For more information on
these two properties, see the
 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.
 Taub, Herbert, and Donald L. Schilling. Principles of Communication Systems. New York: McGraw-Hill, 1971, pp. 142–155.
 Der, Lawrence. “Frequency Modulation (FM) Tutorial”. FM Tutorial. Silicon Laboratories Inc., pp. 4–8.
Usage notes and limitations:
See System Objects in MATLAB Code Generation (MATLAB Coder).