EEE997-HO3

The state array contains a pointer to the delay line as its first element, followed by the delay line values. The length of the state array is therefore 1 greater than the number of taps. Each filter has its own state array, which should not be modified by the calling program, only by the fir function. The state array should be initialized to zeros before the fir function is called for the first time.
School of Electrical, Electronic and Computer Engineering
4
University of Newcastle Upon Tyne
EEE997 MSc Coursework
• C implementation:
FUNCTION fir-finite impulse response (FIR) filter
• To learn how to implement the filtering in the ADSP 21061 SHARC DSP board.
Figure 1: Front-end of the receiver structure
Digital FIR Lowpass FIR Filter Specifications:
The fir() function implements a finite impulse response (FIR) filter defined by the coefficients and delay line that are supplied in the call of fir. The function produces the filtered response of its input data. This FIR filter is structured as a sum of products. The characteristics of the filter (passband, stop band, etc.) are dependent on the coefficient values and the number of taps supplied by the calling program.
Objectives:
• To learn how to design in Matlab a bandpass FIR filter that meets a given set of specifications,
• To learn how to transfer the coefficients of the designed FIR filter from Matlab into a C-like structure format,
fs2
=
fc
+
3 2
Rb
where fc = 4800 Hz is the carrier frequency, Rb = 2400 bps is the data rate and fS = 48000 Hz is the sampling frequency.
a( f ) Rp
Rs
f s1
f p1
EXAMPLE • The first step required is to include the header filters.h. At the top of your C
program, insert the following: #include <filters.h>
• For simplicity, it is constructive to define the preprocessor variable TAPS: #define N 61 where 61 is the number of taps used in the filter design.
Lower passband cutoff frequency:
f p1
=
fc
−
1 2
Rb
Upper passband cutoff frequency:
f p2
=
fc
+
1 2
Rb
Lower stopband cutoff frequency:
f s1
=
fc
−
3 2
Rb
Upper stopband cutoff frequency:
• Variable declarations required in the main():
float x0; /* Filter input */ float x1; /* Filter output */ int k;
Dr. C. Tsimenidis
School of Electrical, Electronic and Computer Engineering
EEE997 MSc Coursework
Aims:
Lab 3: Bandpass Filter Design
• To design and implement the front-end of the DPSK receiver architecture in the ADSP 21061 SHARC DSP board.
• Start Matlab and type sptool on the command prompt.
Lab 3: Bandpass Filter Design
Figure 3: Starting Matlab and SPTool
• Select New Design to begin with a new filter design.
EEE997 MSc Coursework
Lab 3: Bandpass Filter Design
Figure 7: Exporting filter variables to the Matlab workspace (b)
• The filter parameters are stored under the variable name filt1 in an object-oriented manner. You can access the FIR filter coefficients as illustrated in Fig. 8. The custom Matlab function coef2c.m converts the filter coefficient vector h into a C-like structure format which is stored in the text file bp.h for subsequent use. You need to download the Matlab function coef2c.m from my webpage:
SYNOPSIS
#include <filters.h> float fir(float sample, \ float pm coeffs[], \ float dm state[], int taps);
Lab 3: Bandpass Filter Design
DESCRIPTION This function is an Analog Devices extension to the ANSI standard.
Lab 3: Bandpass Filter Design
• Insert the FIR filter specifications and complete the filter design by pressing Apply.
Figure 5: Inserting the FIR filter specifications
The floating-point input to the filter is sample. The integer taps indicates the length of the filter, which is also the length of the array coeffs. The coeffs array holds one FIR filter coefficient per element. The coefficients are stored in reverse order; for example, coeffs[0] holds the (taps-1) coefficient. The coeffs array is located in program memory data space to use the single-cycle dual memory fetch of the processor.
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Figure 8: Converting the FIR filter coefficients to a C-like structure format
Dr. C. Tsimenidis
f p2
fs2
f
Figure 2: Bandpass specifications
Dr. C. Tsimenidis