程序说明:其实本来应该是在output函数中设置采样率和声道数的,但有莫名奇妙的问题。
所以定了个一般化的
#define SAMPLE_RATE 44100
#define CHANNELS 2
#define PCM_DEVICE "plughw:0,0"
即:mp3的采样率为44100Hz,声道数为2(立体声)
源代码:
- /*
- * 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
- */
- #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 ();
- int main (int argc, char *argv[])
- {
- struct stat stat;
- void *fdm;
- char const *file;
- int fd;
- 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);
- if (fdm == MAP_FAILED)
- return -1;
- if (init_alsa () == -1)
- {
- fprintf (stderr, "init_alsa() error/n");
- return -1;
- }
- decode (fdm, stat.st_size);
- 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;
- 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;
- snd_pcm_writei (pcm_handle, OutputPtr, n);
- OutputPtr = Output;
- 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;
- }
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