libmad解码mp3生成pcm文件
在之前的一篇文章中介绍了如何在linux下，使用libmad解码mp3并且播放，将源码编译了，可以正常的播放MP3文件，但是在项目里面需要将mp3解码成pcm文件，所以需要将之前的代码进行修改一下，修改是在原来的基础上的，这样可以测试一下。
/*
* 本程序是从 minimad 改进而来，如要更详细的说明请参看 minimad.c
*
* MyMinimad.c , 2009/12/25 , SiChuan University , China
*
* 编译： gcc MyMinimad.c -o MyMinimad -lmad -lasound
* 运行： ./MyMinimad filename.mp3
*
* 修改:
* 使用libmad解码mp3然后保存成pcm文件。
* 编译: gcc MyMinimad.c -o MyMinimad -lmad
* 运行: ./MyMinimad filename.mp3 outputfilename.pcm
*
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/mman.h>
//#include <sys/soundcard.h>
#include <sys/ioctl.h>
#include <sys/fcntl.h>
#include <sys/types.h>
#include <mad.h>
//#include <alsa/asoundlib.h>
#define SAMPLE_RATE 44100
#define CHANNELS 2
#define PCM_DEVICE "plughw:0,0"
static snd_pcm_hw_params_t *hwparams = NULL;
static snd_pcm_t *pcm_handle = NULL;
struct buffer
{
unsigned char const *start;
unsigned long length;
};
static int decode (unsigned char const *, unsigned long);
//static int init_alsa ();
FILE *outFile;
int main (int argc, char *argv[])
{
struct stat stat;
void *fdm;
char const *file;
int fd;
if(argc != 3 )return -1;
outFile = fopen(argv[2], "w+");
file = argv[1];
fd = open (file, O_RDONLY);
if (fstat (fd, &stat) == -1 || stat.st_size == 0)
return -1;
fdm = mmap (0, stat.st_size, PROT_READ, MAP_SHARED, fd, 0);//将文件内容拷贝到内存里面
if (fdm == MAP_FAILED)
return -1;
// if (init_alsa () == -1)
// {
// fprintf (stderr, "init_alsa() error/n");
// return -1;
// }
decode (fdm, stat.st_size);//进行解码
fclose(outFile);//关闭文件指针
if (munmap (fdm, stat.st_size) == -1)
return -1;
return 0;
}
//static int init_alsa ()
//{
// snd_pcm_stream_t stream = SND_PCM_STREAM_PLAYBACK;
// char *pcm_name;
// int rate = SAMPLE_RATE; /* Sample rate */
// int exact_rate; /* Sample rate returned by */
// pcm_name = strdup (PCM_DEVICE);
// snd_pcm_hw_params_alloca (&hwparams);
// if (snd_pcm_open (&pcm_handle, pcm_name, stream, 0) < 0)
// {
// fprintf (stderr, "Error opening PCM device %s/n", pcm_name);
// return -1;
// }
// if (snd_pcm_hw_params_any (pcm_handle, hwparams) < 0)
// {
// fprintf (stderr, "Can not configure this PCM device./n");
// return -1;
// }
// if (snd_pcm_hw_params_set_access (pcm_handle, hwparams, SND_PCM_ACCESS_RW_INTERLEAVED) < 0)
// {
// fprintf (stderr, "Error setting access./n");
// return -1;
// }
// if (snd_pcm_hw_params_set_format (pcm_handle, hwparams, SND_PCM_FORMAT_S16_LE) < 0)
// {
// fprintf (stderr, "Error setting format./n");
// return -1;
// }
// exact_rate = rate;
// if (snd_pcm_hw_params_set_rate_near (pcm_handle, hwparams, &exact_rate, 0) < 0)
// {
// fprintf (stderr, "Error setting rate./n");
// return -1;
// }
// if (rate != exact_rate)
// {
// fprintf (stderr, "The rate %d Hz is not supported by your hardware./n==> Using %d Hz instead./n", rate, exact_rate);
// }
// if (snd_pcm_hw_params_set_channels (pcm_handle, hwparams, CHANNELS) < 0)
// {
// fprintf (stderr, "Error setting channels./n");
// return -1;
// }
// if (snd_pcm_hw_params (pcm_handle, hwparams) < 0)
// {
// fprintf (stderr, "Error setting HW params./n");
// return -1;
// }
// return 0;
//}
static enum mad_flow input (void *data, struct mad_stream *stream)
{
struct buffer *buffer = data;
if (!buffer->length)
return MAD_FLOW_STOP;
mad_stream_buffer (stream, buffer->start, buffer->length);
buffer->length = 0;
return MAD_FLOW_CONTINUE;
}
/*这一段是处理采样后的pcm音频 */
static inline signed int scale (mad_fixed_t sample)
{
sample += (1L << (MAD_F_FRACBITS - 16));
if (sample >= MAD_F_ONE)
sample = MAD_F_ONE - 1;
else if (sample < -MAD_F_ONE)
sample = -MAD_F_ONE;
return sample >> (MAD_F_FRACBITS + 1 - 16);
}
static enum mad_flow output (void *data, struct mad_header const *header, struct mad_pcm *pcm)
{
unsigned int nchannels, nsamples, n;
mad_fixed_t const *left_ch, *right_ch;
unsigned char Output[6912], *OutputPtr;
int fmt, wrote, speed, exact_rate, err, dir;
nchannels = pcm->channels;
n = nsamples = pcm->length;
left_ch = pcm->samples[0];
right_ch = pcm->samples[1];
// fmt = AFMT_S16_LE;
// speed = pcm->samplerate * 2; /*播放速度是采样率的两倍 */
OutputPtr = Output;//将OutputPtr指向Output
while (nsamples--)
{
signed int sample;
sample = scale (*left_ch++);
*(OutputPtr++) = sample >> 0;
*(OutputPtr++) = sample >> 8;
if (nchannels == 2)
{
sample = scale (*right_ch++);
*(OutputPtr++) = sample >> 0;
*(OutputPtr++) = sample >> 8;
}
}
OutputPtr = Output;//由于之前的操作将OutputPtr的指针指向了最后，这时需要将其指针移动到最前面
fwrite(OutputPtr, 1, n*2*nchannels, outFile);
// snd_pcm_writei (pcm_handle, OutputPtr, n);
OutputPtr = Output;//写完文件后，OutputPtr的指针也移动到了最后，这时需要将其指针移动到最前面
return MAD_FLOW_CONTINUE;
}
static enum mad_flow error (void *data, struct mad_stream *stream, struct mad_frame *frame)
{
return MAD_FLOW_CONTINUE;
}
static int decode (unsigned char const *start, unsigned long length)
{
struct buffer buffer;
struct mad_decoder decoder;
int result;
buffer.start = start;
buffer.length = length;
mad_decoder_init (&decoder, &buffer, input, 0, 0, output, error, 0);
mad_decoder_options (&decoder, 0);
result = mad_decoder_run (&decoder, MAD_DECODER_MODE_SYNC);
mad_decoder_finish (&decoder);
return result;
}
在终端运行gcc MyMinimad.c -o MyMinimad -lmad就可以生成解码mp3文件生成pcm的工具.