Post on 23-Dec-2015
Frequency Modulation in Wireless Microphone
System
EECE 252 ProjectSpring 2012
Presented by:Haolin Wang, Muhamad Fuad Harun, Baihaqis Bahran
Introduction
IntroductionOne of the applications of frequency modulation in real life is the usage of a wireless microphone. Wireless Microphone System requires a wireless transmitter, and a wireless receiver.
The wireless transmitter can be built into the microphone itself or connected by a short cable to a body pack transmitter. The wireless receiver is tuned to the same electromagnetic wavelength as the transmitter and is physically attached to an output device. Wireless Microphone System uses FM to transmit the signal.
Project Goals To apply the knowledge about
Frequency Modulation learned in class To investigate the possible modes of
failure for Wireless FM Microphone System
To reach to a conclusion of how we can Reduce The Modes of Failure
System Block Diagram
Assumption and Focus We assume that the channel is an all pass
channel We focus on two parameters of the system,
the sampling rate and the frequency deviation (important parameter in FM transmission) and how they affect the output of the system (original voice signal)
Problems we ignore: Interference, random noise from the surrounding
Method and Procedure
Matlab:
Matlab was used to record a 5 seconds long sound through the built-in microphone on the laptop and the sound was sampled at frequency 44100 Hz.
Figure 1.1: The plot of the original sound in time.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-4
-3
-2
-1
0
1
2
3
4x 10
4
t (s)
y -
a.u
.
Original Sound in Time(s)
Resampling and Modulation We resampled the sound signal with a
higher sampling rate to provide more frequency spaces for modulation and demodulations.
The signal was modulated through direct FM modulation with a carrier frequency of 100 kHz.
Resampled Signal in Frequency Domain
-5 -4 -3 -2 -1 0 1 2 3 4 5
x 105
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5x 10
9
Freq (Hz)
|Orig
inal
Sig
nal I
n F
requ
ency
Dom
ain|
Modulated Signal in Frequency Domain
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x 105
0
0.5
1
1.5
2
2.5
3
3.5x 10
4
Freq (Hz)
|Gen
erat
ion
of a
FM
Wav
e us
ing
Dire
ct F
M M
odul
ator
|
Demodulation We demodulated the signal by using
envelope detector First we differentiate the modulated
signal by using differentiator Then we use a low pass filter to get the
signal that we need
Derivative of The Modulated Signal
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x 105
0
5
10
15x 10
9
Freq (Hz)
Am
plitu
de (
a.u.
)Derivative of the FM Signal in Frequency Domain
Low Pass Filter
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2000
-1500
-1000
-500
0
Normalized Frequency ( rad/sample)
Pha
se (
degr
ees)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-200
-100
0
100
Normalized Frequency ( rad/sample)
Mag
nitu
de (
dB)
This is our LP filter in normalized freq space
Demodulated Signal
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x 105
0
5
10
15x 10
9
Freq (Hz)
Am
plitu
de (
a.u.
)Demodulated Signal in Frequency Domain
Demodulated Signal In Time Domain
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x 106
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t (s)
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Demodulated Signal in Time
We repeat the same procedures with different values of Fs (sampling rate) and deltaF (frequency deviation) to observe the behavior of the system
Results and Conclusion
ResultsSettings
Fs_hi Fc DeltaF
Adjustment
Observation
1 10*100e3
100e3
10e3 Original Demodulated signal heard clearly
2 50*100e3
100e3
10e3 Increase Fs Demodulated signal contain noise
3 5*100e3
100e3
10e3 Decrease Fs Demodulated signal heard clearly
4 10*100e3
100e3
50e3 Increase Delta F
Demodulated signal heard clearly
5 10*100e3
100e3
1e3 Decrease Delta F
Demodulated signal cannot be heard clearly
Default Configuration
Settings
Fs_hi Fc DeltaF
Adjustment
Observation
1 10*100e3
100e3
10e3 Original Demodulated signal heard clearly
Audio Signal Graph
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4x 10
4
t (s)
y -
a.u.
Original Sound in Time(s)
Default Setting vs Setting 2
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x 106
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-0.5
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0.5
1
1.5x 10
5
t (s)
y - a
.u.
Demodulated Signal in Time
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x 107
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
5
t (s)
y - a
.u.
Demodulated Signal in Time
Default Setting vs Setting 3
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x 106
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-1
-0.5
0
0.5
1
1.5x 10
5
t (s)
y - a
.u.
Demodulated Signal in Time
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x 106
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
5
t (s)
y - a
.u.
Demodulated Signal in Time
Default Setting vs Setting 4
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x 106
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
5
t (s)
y - a
.u.
Demodulated Signal in Time
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x 106
-3
-2
-1
0
1
2
3
4x 10
5
t (s)
y -
a.u
.
Demodulated Signal in Time
Default Setting vs Setting 5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x 106
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
5
t (s)
y - a
.u.
Demodulated Signal in Time
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x 106
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5x 10
4
t (s)
y - a
.u.
Demodulated Signal in Time
Conclusion In summary, we used the radio and played around
with the parameters, Fs_Hi and DeltaF and compared the result (demodulated signal) with the original signal.
Apparently, an increase in Fs_Hi will amplify the arbitrary noise that appeared due to the approximation process of the differentiation.
A lower Fs_Hi will decrease the noise in the demodulated signal. An increase in DeltaF will decrease the noise significantly and a decrease in DeltaF results in an increase in noise and destruction of the original signal (i.e., when the demodulated signal is played, the original message cannot be heard).