Frequency Modulation in Wireless Microphone System EECE 252 Project Spring 2012 Presented by: Haolin Wang, Muhamad Fuad Harun, Baihaqis Bahran

Introduction

One 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 Hz.

Figure 1.1: The plot of the original sound in time.

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

Modulated Signal in Frequency Domain

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

Low Pass Filter

Demodulated Signal

Demodulated Signal In Time Domain

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

Results SettingsFs_hiFcDelta F AdjustmentObservation 1 10*100 e3 100e310e3OriginalDemodulated signal heard clearly 2 50*100 e3 100e310e3Increase FsDemodulated signal contain noise 3 5*100e 3 100e310e3Decrease FsDemodulated signal heard clearly 4 10*100 e3 100e350e3Increase Delta F Demodulated signal heard clearly 5 10*100 e3 100e31e3Decrease Delta F Demodulated signal cannot be heard clearly

Default Configuration Setting s Fs_hiFcDelta F AdjustmentObservation 1 10*100e 3 100e310e3OriginalDemodulated signal heard clearly

Audio Signal Graph

Default Setting vs Setting 2

Default Setting vs Setting 3

Default Setting vs Setting 4

Default Setting vs Setting 5

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).