comedilib/demo/antialias.c

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/*
 * Antialiasing Analog Output Demo
 * Part of Comedilib
 *
 * Copyright (c) 1999,2000 David A. Schleef <ds@schleef.org>
 *
 * This file may be freely modified, distributed, and combined with
 * other software, as long as proper attribution is given in the
 * source code.
 */

/* Not functional */

/*
 * Requirements: an analog output subdevice that is capable
 *     of ansynchronous output.
 *
 * Normally, the resolution of analog output channels is limited by
 * the resolution of the D/A converter.  However, if you limit the
 * bandwith of the D/A converter by using a low-pass filter, you
 * can trade some of the bandwidth for additional resolution.  This
 * is done by changing the output rapidly between two adjacent
 * values: a signal of an alternating 0,1,0,1,0,1 sequence will
 * look like 0.5 after an appropriate low-pass filter.
 *
 * The disadvantage, of course, is that you lose bandwidth.  Worse,
 * the simple technique demonstrated here will cause predictable
 * noise in the stop band.  More complicated techniques will allow
 * you to tune the spectrum of the noise in the stop band.
 */

#include <stdio.h>
#include <comedilib.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <getopt.h>
#include <ctype.h>
#include "examples.h"

void ao_antialias(unsigned int data);

comedi_t *device;

int main(int argc, char *argv[])
{
        lsampl_t data;
        int ret;
        struct parsed_options options;

        init_parsed_options(&options);
        parse_options(&options, argc, argv);

        device = comedi_open(options.filename);
        if(!device){
                comedi_perror(options.filename);
                exit(0);
        }

        data = options.value;
        if(options.verbose){
                printf("writing %d to device=%s subdevice=%d channel=%d range=%d analog reference=%d\n",
                        data, options.filename, options.subdevice, options.channel, options.range, options.aref);
        }

        ret = comedi_data_write(device, options.subdevice, options.channel, options.range, options.aref, data);
        if(ret<0){
                comedi_perror(options.filename);
                exit(0);
        }

        printf("%d\n", data);

        ao_antialias((1000<<16)+1000);

        return 0;
}


void ao_antialias(unsigned int data)
{
        unsigned int hibits=data>>16;
        unsigned int lobits=data&0xffff;
        int i;
        unsigned int acc;

        acc=0;
        for(i=0;i<100;i++){
                acc+=lobits;
                printf("%d\n",hibits+(acc>>16));
                acc&=0xffff;
        }
}