Demodulate using broadcast FM method
Modulation > Analog Baseband Modulation
The FM Broadcast Demodulator Baseband block demodulates a complex baseband FM signal by using the conjugate delay method, and filters the signal by using a de-emphasis filter. To demodulate stereo audio using 38 kHz, enable stereo demodulation. To demodulate RBDS signals from the 57 kHz band, enable RBDS demodulation.
Specify the input signal sample rate as a positive real scalar.
Specify the frequency deviation of the modulator in Hz as a positive real scalar. The system bandwidth is equal to twice the sum of the frequency deviation and the message bandwidth. FM broadcast standards specify a value of 75 kHz in the United States and 50 kHz in Europe.
Specify the de-emphasis lowpass filter time constant in seconds as a positive real scalar. FM broadcast standards specify a value of 75 μs in the United States and 50 μs in Europe.
Specify the output audio sample rate as a positive real scalar.
Select this check box to play sound from a default audio device.
Specify the buffer size the block uses to communicate with an audio device as a positive integer scalar. This parameter is available only when the Play audio device check box is selected.
Select this check box to enable demodulation of a stereo audio signal. If not selected, the audio signal is assumed to be monophonic.
Select this check box to demodulate the RBDS signal from the input complex baseband FM signal. By default, this check box is not selected.
Specify the number of samples of the RBDS output as a positive integer.
The RBDS sample rate is given by Number of samples per RBDS
symbol × 1187.5
Hz. According to the
RBDS standard, the sample rate of each bit is 1187.5 Hz.
This parameter appears when you select the RBDS demodulation check box.
The default is 10.
Specify whether a Costas loop is used to recover the phase of the RBDS
signal. Select this check box for radio stations that do not lock the
57
kHz RBDS signal in phase with the third harmonic
of the 19
kHz pilot tone.
This parameter appears when you select the RBDS demodulation check box.
By default, this check box is not selected.
Select the type of simulation to run.
Code generation
. Simulate model using
generate C code. The first time you run a simulation, Simulink
generates C code for the block. The C code is reused for subsequent
simulations, as long as the model does not change. This option
requires additional startup time but provides faster simulation
speed than Interpreted execution
.
Interpreted execution
. Simulate model
using the MATLAB interpreter. This option shortens startup time but
has slower simulation speed than Code
generation
.
The FM Broadcast demodulator includes the functionality of the
baseband FM demodulator, de-emphasis filtering, and the ability to
receive stereophonic signals. The algorithms which govern basic FM
modulation and demodulation are covered in comm.FMDemodulator
.
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
For an audio sample rate of 44.1 kHz, the de-emphasis filter has the following response.
The FM broadcast demodulator 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, m(t).
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.
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/RBDS signals, respectively. To generate a scaled version of the left and right channels that produce the stereo sound, the (L + R) and (L – R) signals are added and subtracted. The RDS/RBDS signal is recovered by mixing with the 57 kHz signal.
Here is the block diagram of the FM broadcast demodulator.
Load an audio input file, modulate and demodulate using the FM broadcast blocks. Compare the input signal spectrum with the demodulated signal spectrum.
Open the doc_fmbroadcast model.
Run the model. The spectrum of the baseband FM signal is attenuated at the higher frequencies relative to the original waveform.
Experiment with the model by changing the Frequency deviation (Hz) and the Pre-emphasis filter time constant (s) parameters on the modulator and demodulator and observe the impact on the FM signal spectrum.
The input length must be an integer multiple of the audio decimation factor. If the RBDS demodulation check box is selected, the input length in addition must be an integer multiple of the RBDS decimation factor.
Port | Supported Data Types |
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Signal Input |
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Signal Output |
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[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.
[3] Der, Lawrence. “Frequency Modulation (FM) Tutorial”. FM Tutorial. Silicon Laboratories Inc., pp. 4–8.