Elec 3305 Lab 3 Report

ELEC3305 LAB 3 REPORT

Introduction

The aim of this lab is to design and test two infinite impulse response band-pass filters created using Matlab and implement them on DSP board. These filters are intended to filter out specific row and column frequencies of a DTMF signal and thereby obtain the tones for digit ‘4’. Also, the frequency response of filters is measured using scope and function generator.

Methods and procedures

Two elliptic IIR band-pass filters should be designed with a sampling frequency of 8000Hz, pass band ripple of 0.1dB and stop band attenuation of 30dB. To filter out a row frequency of 770Hz, the pass band edge frequencies can be 760Hz and 780Hz and its stop band edge frequencies can be 700Hz and 850Hz. When filtering out a column frequency of 1209Hz, the pass band edge frequencies can be 1200Hz and 1220Hz and its stop band edge frequencies can be 950Hz and 1330Hz. Figure 1 shows the required frequencies to obtain tones for digit ‘4’.

[pic 1]

Figure1: Required row and column frequencies

• First, open the Filter Design & Analysis Toolbox by typing “fdatool” in Matlab command window. When the fdatool window pops up, the parameters shown in Figure 2 should be entered so as to design a row filter which meets the requirements described above. Column filter is also designed similarly using parameters according to requirements.

[pic 2]

Figure2: Parameters for row filter

When finished entering the parameters for a filter, press the design filter button and convert to single section. Then the coefficients of filters are exported to Matlab workspace.  The magnitude responses are provided below in Figure 3 and Figure 4.

[pic 3]

Figure3: Magnitude response for row filter

[pic 4]

Figure4: Magnitude response for column filter

It can be observed from the magnitude responses that the filters have the same shape but column filter is shifted to the right in a higher frequency band.

• The designed IIR band-pass filters can be tested by using the Simulink model by typing “iir_test” in the Matlab command window.
• Next, the IIR band-pass filters are implemented on the DSP board for real time testing. To do this, line-in of DSP is connected to the function generator as well as channel 1 of scope by using a T-junction. Then, line-out of DSP is connected to the channel 2 of scope. Also connect the DSP to PC and then in the iir_test window, press CTRL+B to compile the model and load it to the DSP board.
• Keep the DIP switch 0 OFF and DIP switch 1 ON in order to measure the row frequency filter magnitude response. Calculate the filter gain and relative gain with an input signal of 640mV peak-to-peak. A screen shot of scope is shown below and the results are illustrated in Table 1.

[pic 5]

Screenshot of scope

• Afterwards, the previous step is repeated but this time keeping DIP switch 0 ON and DIP switch 1 OFF. Results are demonstrated in Table 2.
• Lastly, turn both DIP switches 1 and 2 ON to produce a combinational output of row filter output and column frequency output. This output signal can be heard when a headphone is connected to the DSP.

Results

From the last step a tone in DRMF signal for the digit ‘4’ can be heard and this signal is very small for other digits.

Table 1: Measurements for IIR band-pass filter designed for row 2 frequency of 770Hz

Table 2: Measurements for IIR band-pass filter designed for column 1 frequency of 1209Hz