Lab 0: hardware introduction  (Page 3/5)

Set the amplitude on the function generator to 1.0 V peak-to-peak and the pulse shape to sinusoidal. Observethe frequency response of the filter by sweeping the input signal through the relevant frequency range. What is therelevant frequency range for a DSP system with a sample rate of 44.1 kHz?

Based on the frequency response you observe, characterize the filter in terms of its type (e.g., low-pass,high-pass, band-pass) and its -6 dB (half-amplitude) cutoff frequency (or frequencies). It may help to set thetrigger on channel 2 of the oscilloscope since the signal on channel 1 may go to zero.

Step 5: re-assemble and re-run with new filter

Once you have determined the type of filter the DSP is implementing, you are ready to repeat the process with adifferent filter by including different coefficients during the assembly process. The different coefficientsare in the file coef2.asm . Make a copy of coef2.asm and call it coef.asm .

You can now repeat the assembly and testing process with the new filter using the asm instruction atthe DOS prompt and repeating the steps required to execute the code discussed in Step 4 .

Just as you did in Step 4 , determine the type of filter you are running and thefilter's -6 dB point by testing the system at various frequencies.

Step 6: check filter response in matlab

In this step, you will use MATLAB to verify the frequency response of your filter by copying the coefficients fromthe DSP to MATLAB and displaying the magnitude of the frequency response using the MATLAB command freqz .

The FIR filter coefficients included in the file coef.asm are stored in memory on the DSP starting at location (in hex) 0x1000 , and each filter you have assembled and run has eightcoefficients. To view the filter coefficients as signed integers, select the Memory option from the View menu to bring up a Memory Window Options box. In the appropriate fields, set the starting address to 0x1000 and the format to 16-Bit Signed Int . Click "OK" to open a memory window displaying the contents of the specifiedmemory locations. The numbers along the left-hand side indicate the memory locations.

In this example, the filter coefficients are placed in memory in decreasing order; that is, the last coefficient, $h(7)$ , is at location 0x1000 and the first coefficient, $h(0)$ , is stored at 0x1007 .

Now that you can find the coefficients in memory, you are ready to use the MATLAB command freqz to view the filter's response. You must create a vector in MATLABwith the filter coefficients to use the freqz command. For example, if we want to view the response of the three-tap filter with coefficients -10, 20, -10 we canuse the following commands in MATLAB:

• h = [-10, 20, -10];
• plot(abs(freqz(h)))

Note that you will have to enter eight values, the contents of memory locations 0x1000 through 0x1007 , into the coefficient vector, $h$ .

Does the MATLAB response compare with your experimental results? What might account for any differences?