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.

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4x 10

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t (s)

y -

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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

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x 105

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Freq (Hz)

|Orig

inal

Sig

nal I

n F

requ

ency

Dom

ain|

Modulated Signal in Frequency Domain

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x 105

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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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Freq (Hz)

Am

plitu

de (

a.u.

)Derivative of the FM Signal in Frequency Domain

Low Pass Filter

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Normalized Frequency ( rad/sample)

Pha

se (

degr

ees)

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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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Freq (Hz)

Am

plitu

de (

a.u.

)Demodulated Signal in Frequency Domain

Demodulated Signal In Time Domain

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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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Original Sound in Time(s)

Default Setting vs Setting 2

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Demodulated Signal in Time

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Demodulated Signal in Time

Default Setting vs Setting 3

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Demodulated Signal in Time

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Demodulated Signal in Time

Default Setting vs Setting 4

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Demodulated Signal in Time

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Demodulated Signal in Time

Default Setting vs Setting 5

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Demodulated Signal in Time

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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).