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You will implement a multirate system that includes three fininte impulse response filters.

Implementation

As this is your first experience with the C environment, most of the programming for the assignment is to be done directlyin assembly. A C skeleton will provide access to input samples and a way to output samples. From the C skeleton, anassembly module for implementing the complete multirate system (for a single sample) is called. In the assembly module, thedownsampling and upsampling blocks are implemented by using a loop or counter to determine which samples to keep and when toinsert zeros.

As there was a core file for working in the assembly environment (Labs 0-2), there is a core file for the Cenvironment ( V:\ece320\54x\dspclib\core.asm ) which handles the interrupts from the CODEC (A/D and D/A) andthe serial port. Here, we will describe the important aspects of the core code necessary to complete the assignment. Thecomplete documentation on the core code developed for the C environment will be made available soon.

C skeleton

Let's examine the following C main program lab3main.c which calls assembly FIR filter functions init_filter and filter .

1 #include "v:/ece320/54x/dspclib/core.h" /* Declarations for core file */ 2 void init_filter(void); /* Prototypes for assembly functions */ 3 int filter(int sample); 4 extern int dec_rate; /* Default decimation rate is 4 */ 5 6 main() 7 { 8 int *Rcvptr,*Xmitptr; /* pointers to Xmit & Rcv Bufs */ 9 int i, sample1, sample2; 10 11 init_filter(); /* Initialize the filter */ 12 13 while( 1 ) 14 { 15 /* Wait for a new block of samples */ 16 WaitAudio(&Rcvptr,&Xmitptr); 17 18 /* Process a block of samples */ 19 for( i = 0; i < BlockLen; i ++ ) 20 { 21 sample1 = Rcvptr[4*i]; /* Ch1 input sample */ 22 sample2 = Rcvptr[4*i+2]; /* Ch2 input sample */ 23 24 Xmitptr[6*i] = sample1; /* First output is input */ 25 Xmitptr[6*i+1] = filter(sample1); /* Secound output is result */ 26 } 27 28 i = SerialRX(); /* Check serial port */ 29 if( i > 0 ) 30 dec_rate = i; /* Save new decimation rate if we got it */ 31 } 32 }

In the main program, an infinite loop operates over the input samples accessed by the pointer Rcvptr and writes the output samples via the pointer Xmitptr . In C, pointers may be used as array names so that Xmitptr[0] is the first word pointed to by Xmitptr . The function WaitAudio is a assembly function in the core code which handles the CODEC interrupts. It returns a blockof BlockLen samples and writes BlockLen samples to each of the six channels. As in the assembly core, the input samples are not inconsecutive order. The right and left inputs are offset from Rcvptr respectively by 4 i and 4 i 2 , i 0 ,, BlockLen 1 . The six output channels are accessed consecutively as offsets from Xmitptr .

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Source:  OpenStax, Ece 320 - spring 2003. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10096/1.2
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