//This is a decmating FIR filter made with my knowledge of sdr so far... don't laugh ;-)

//Reads samples from standard input and writes decimated samples to standard output

//

// to compile: gcc decfir.c -o decfir

//

//rob@m0dts.co.uk

//

//September 2015



#include <stdlib.h>

#include <stdio.h>





#define taps 199





float coeff[taps] ={0.000063, 0.000075, 0.000088, 0.000102, 0.000117,

0.000133, 0.000151, 0.000170, 0.000190, 0.000212, 0.000235, 0.000260

, 0.000286, 0.000314, 0.000343, 0.000374, 0.000407, 0.000442, 

0.000478, 0.000517, 0.000557, 0.000599, 0.000643, 0.000689, 0.000736,

0.000786, 0.000838, 0.000891, 0.000947, 0.001004, 0.001064, 0.001125

, 0.001189, 0.001254, 0.001321, 0.001390, 0.001461, 0.001533, 

0.001608, 0.001684, 0.001762, 0.001841, 0.001922, 0.002005, 0.002088,

0.002174, 0.002260, 0.002348, 0.002437, 0.002526, 0.002617, 0.002709

, 0.002801, 0.002894, 0.002988, 0.003082, 0.003176, 0.003271, 

0.003365, 0.003460, 0.003554, 0.003648, 0.003742, 0.003835, 0.003928,

0.004020, 0.004111, 0.004201, 0.004289, 0.004377, 0.004463, 0.004548

, 0.004630, 0.004712, 0.004791, 0.004868, 0.004943, 0.005016, 

0.005086, 0.005154, 0.005219, 0.005282, 0.005342, 0.005399, 0.005453,

0.005504, 0.005552, 0.005596, 0.005638, 0.005676, 0.005710, 0.005741

, 0.005769, 0.005793, 0.005813, 0.005830, 0.005843, 0.005852, 

0.005858, 0.005859, 0.005858, 0.005852, 0.005843, 0.005830, 0.005813,

0.005793, 0.005769, 0.005741, 0.005710, 0.005676, 0.005638, 0.005596

, 0.005552, 0.005504, 0.005453, 0.005399, 0.005342, 0.005282, 

0.005219, 0.005154, 0.005086, 0.005016, 0.004943, 0.004868, 0.004791,

0.004712, 0.004630, 0.004548, 0.004463, 0.004377, 0.004289, 0.004201

, 0.004111, 0.004020, 0.003928, 0.003835, 0.003742, 0.003648, 

0.003554, 0.003460, 0.003365, 0.003271, 0.003176, 0.003082, 0.002988,

0.002894, 0.002801, 0.002709, 0.002617, 0.002526, 0.002437, 0.002348

, 0.002260, 0.002174, 0.002088, 0.002005, 0.001922, 0.001841, 

0.001762, 0.001684, 0.001608, 0.001533, 0.001461, 0.001390, 0.001321,

0.001254, 0.001189, 0.001125, 0.001064, 0.001004, 0.000947, 0.000891

, 0.000838, 0.000786, 0.000736, 0.000689, 0.000643, 0.000599, 

0.000557, 0.000517, 0.000478, 0.000442, 0.000407, 0.000374, 0.000343,

0.000314, 0.000286, 0.000260, 0.000235, 0.000212, 0.000190, 0.000170

, 0.000151, 0.000133, 0.000117, 0.000102, 0.000088, 0.000075, 

0.000063};

    

    

void main(void){



	int n,i;

	int bufptr,bufsize;

	float isample,qsample;	

	float *buf,inbuf[2],outbuf[2];

	

	bufptr=0;

	bufsize=1024;

	

	

	//allocate memory for circular buffer

	buf= malloc(bufsize * sizeof(float));



	//set buffer to zeroes

	for(n=0;n<bufsize;n++){

		buf[n] = 0;

	}



	

	//main loop, read some samples into buffer and then output some samples every so often

	n=0;

	while(1){

		//Read in 32 pairs of samples

		for(n=0;n<32;n++){	//decimation?

			fread(&inbuf,sizeof(float),2,stdin);

			buf[bufptr] = inbuf[0];

			bufptr++;

			if (bufptr > bufsize-1){bufptr=0;}

			buf[bufptr] = inbuf[1];

			bufptr++;

			if (bufptr > bufsize-1){bufptr=0;}

		}

		

		//reset sum

		outbuf[0]=0;	

		outbuf[1]=0;

		

		//pass on current buffer pointer

		n=bufptr;

		

		//apply coefficients to input samples

		//filter appears to have 10dB attenuation.	

		for(i=0;i<taps;i++){

			if(n>bufsize-1){n=0;}

			outbuf[0] +=buf[n]* coeff[i];

			n++;

			if(n>bufsize-1){n=0;}

			outbuf[1] +=buf[n]* coeff[i];

			n++;

		}

		

		//output one pair of decimated samples

		fwrite(&outbuf,sizeof(float),2,stdout);



	}



}