本系列文章主要介绍的是音频相关的编程知识，首先从生成音频开始 。本系列的代码在我的仓库中，以供参考。这些代码大部分是The Audio Programming Book中的源代码。这本书从比较底层的地方——音频生成开始，逐步推进到高级的领域，算是音频领域不错的一本书。这第一篇文章就从音频生成开始讲起。
首先是一个sin函数生成的代码：
#include <stdio.h>
#include <stdlib.h>
#ifndef M_PI
#define M_PI (3.141592654)
#endif
/* define our program argument list */
enum
{
ARG_NAME,
ARG_NSAMPS,
ARG_FREQ,
ARG_SR,
ARG_NARGS
};
int main(int argc, char** argv)
{
int i, nsamps;
double samp, freq, srate;
double twopi = 2.0 * M_PI;
double angleincr;
if (argc != ARG_NARGS)
{
fprintf(stderr, "Usage: sinetext2 nsamps freq srate\n");
return 1;
}
nsamps = atoi(argv[ARG_NSAMPS]);
freq = atof(argv[ARG_FREQ]);
srate = atof(argv[ARG_SR]);
angleincr = twopi * freq / srate;
for (i = 0; i < nsamps; i++)
{
samp = sin(angleincr * i);
fprintf(stdout, "%.8lf\n", samp);
}
fprintf(stderr, "done.\n");
return 0;
}
这段代码很简单，根据输入的采样数量，sin波形频率和采样频率产生数据，编译完程序后，只需要将程序的输出重定向到文件，就可以得到结果。
但是仅有这个文本文件是不够的，需要将其转换为通用的音频文件，才可以播放出声音。书中给出了一个简单的portsf音频文件库，可以用于生成wav或者是aiff格式的音频文件。连带有一个简单的音频转换工具，可以将几个部分结合，用于输出实际使用的声音文件。
转换工具的代码如下：
/* Copyright (c) 2009 Richard Dobson
Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation
files (the "Software"), to deal in the Software without
restriction, including without limitation the rights to use,
copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following
conditions:
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.
*/
/* text2sf.c : convert text soundfile to binary using portsf library */
#include <stdio.h>
#include <stdlib.h>
#include "portsf.h"
enum
{
ARG_PROGNAME,
ARG_INFILE,
ARG_OUTFILE,
ARG_SR,
ARG_CHANS,
ARG_GAIN,
ARG_NARGS
};
/* read one multi-channel frame */
int readframe(FILE* fp, float* frame, int chans)
{
int i;
int samps = 0, got;
for (i = 0; i < chans; i++)
{
got = fscanf(fp, "%f", &frame[i]);
if (got == EOF)
return -1;
samps += got;
}
if (got == chans)
return 1;
else
return 0; /* error! */
}
int main(int argc, char* argv[])
{
PSF_PROPS props;
long framesread;
long totalread;
/* init all dynamic resources to default states */
int ifd = -1, ofd = -1;
int i, error = 0;
double gain = 1.0;
PSF_CHPEAK* peaks = NULL;
psf_format outformat = PSF_FMT_UNKNOWN;
FILE* fin = NULL;
float* frame;
printf("TEXT2SF: convert text audio data to soundfile\n");
if (argc < ARG_NARGS)
{
printf("insufficient arguments.\n"
"usage:\n\t"
"text2sf infile outfile srate chans gain\n");
return 1;
}
/* be good, and startup portsf */
if (psf_init())
{
printf("unable to start portsf\n");
return 1;
}
fin = fopen(argv[ARG_INFILE], "r");
/* we now have a resource, so we use goto hereafter on hitting any error */
/* tell user if source file is already floats */
if (fin == NULL)
{
printf("cannot open infile %s\n", argv[ARG_INFILE]);
return 1;
}
props.chans = atoi(argv[ARG_CHANS]);
if (props.chans <= 0)
{
printf("chans must be positive!\n");
fclose(fin);
return 1;
}
props.srate = atoi(argv[ARG_SR]);
if (props.srate <= 0)
{
printf("srate must be positive!\n");
fclose(fin);
return 1;
}
gain = atof(argv[ARG_GAIN]);
if (gain <= 0.0)
{
printf("gain value must be positive!\n");
fclose(fin);
return 1;
}
frame = (float*)malloc(props.chans * sizeof(float));
if (frame == NULL)
{
puts("No memoery!\n");
fclose(fin);
return 1;
}
/* fixed output to 16bit file; one alternative would be PSF_SAM_IEEE_FLOAT for 32bit float format */
props.samptype = PSF_SAMP_16;
props.format = PSF_STDWAVE;
props.chformat = STDWAVE;
/* check file extension of outfile name, so we use correct output file format*/
outformat = psf_getFormatExt(argv[ARG_OUTFILE]);
if (outformat == PSF_FMT_UNKNOWN)
{
printf("outfile name %s has unknown format.\n"
"Use any of .wav, .aiff, .aif, .afc, .aifc\n",
argv[ARG_OUTFILE]);
error++;
goto exit;
}
props.format = outformat;
ofd = psf_sndCreate(argv[ARG_OUTFILE], &props, 0, 0, PSF_CREATE_RDWR);
if (ofd < 0)
{
printf("Error: unable to create outfile %s\n", argv[ARG_OUTFILE]);
error++;
goto exit;
}
/* and allocate space for PEAK info */
peaks = (PSF_CHPEAK*)malloc(props.chans * sizeof(PSF_CHPEAK));
if (peaks == NULL)
{
puts("No memory!\n");
error++;
goto exit;
}
printf("copying....\n");
/* single-frame loop to do copy: report any read/write errors */
framesread = readframe(fin, frame, props.chans);
totalread = 0; /* count sample frames as they are copied */
/* note: readframe retval of -1 signifies EOF */
while (framesread == 1)
{
totalread++;
/* <--- do any processing here! ---------->*/
for (i = 0; i < props.chans; i++)
frame[i] *= gain;
if (psf_sndWriteFloatFrames(ofd, frame, 1) != 1)
{
printf("Error writing to outfile\n");
error++;
break;
}
framesread = readframe(fin, frame, props.chans);
}
if (framesread == 0)
{
printf("Error reading infile at line %d. Outfile may be incomplete.\n", totalread);
error++;
}
else
printf("Done. %d sample frames copied to %s\n", totalread, argv[ARG_OUTFILE]);
/* report PEAK values to user */
if (psf_sndReadPeaks(ofd, peaks, NULL) > 0)
{
long i;
double peaktime;
printf("PEAK information:\n");
for (i = 0; i < props.chans; i++)
{
peaktime = (double)peaks[i].pos / (double)props.srate;
printf("CH %d:\t%.4f at %.4f secs\n", i + 1, peaks[i].val, peaktime);
}
}
/* do all cleanup */
exit:
if (fin != NULL)
fclose(fin);
if (ofd >= 0)
psf_sndClose(ofd);
if (frame)
free(frame);
if (peaks)
free(peaks);
psf_finish();
/* return error status to caller: may be useful in a script etc */
return error;
}
为了输出不同的声音，书中还给出了一个声音衰减的sin波形生成器，代码如下：
/* tfork.c virtual tuning fork combining sinetext.c and * expdecay.c */
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#ifndef M_PI
#define M_PI (3.141592654)
#endif
enum
{
ARG_NAME,
ARG_OUTFILE,
ARG_DUR,
ARG_HZ,
ARG_SR,
ARG_SLOPE,
ARG_NARGS
};
int main(int argc, char** argv)
{
int i, sr, nsamps;
double samp, dur, freq, srate, k, a, x, slope;
double twopi = 2.0 * M_PI;
double angleincr;
double maxsamp = 0.0;
FILE* fp = NULL;
if (argc != ARG_NARGS)
{
printf("Usage: tfork outfile.txt dur freq srate slope\n");
return 1;
}
fp = fopen(argv[ARG_OUTFILE], "w");
if (fp == NULL)
{
printf("Error creating output file %s\n", argv[ARG_OUTFILE]);
return 1;
}
dur = atof(argv[ARG_DUR]);
freq = atof(argv[ARG_HZ]);
srate = atof(argv[ARG_SR]);
slope = atof(argv[ARG_SLOPE]);
nsamps = (int)(dur * srate);
angleincr = twopi * freq / srate;
k = dur / nsamps;
a = exp(-k / slope);
x = 1.0;
for (i = 0; i < nsamps; i++)
{
samp = sin(angleincr * i);
/* apply exp decay */
x *= a;
samp *= x;
fprintf(fp, "%.8lf\n", samp);
}
fclose(fp);
printf("done\n");
return 0;
}
通过这些代码，我们就可以简单的生成一些音频文件，查看效果了。
后续章节会介绍更多的生成音频以及对应的处理方法。