一个MP3帧由帧头和帧数据构成。Madlib分别填充mad_herder和mad_frame这两个结构，然后进行解码。mad_herder里包含mp3数据的描述信息，这个结构在frame.h头文件中定义：
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struct mad_header {
enum mad_layer layer; /* audio layer (1, 2, or 3) */
enum mad_mode mode; /* channel mode (see above) */
int mode_extension; /* additional mode info */
enum mad_emphasis emphasis; /* de-emphasis to use (see above) */
unsigned long bitrate; /* stream bitrate (bps) */
unsigned int samplerate; /* sampling frequency (Hz) */
unsigned short crc_check; /* frame CRC accumulator */
unsigned short crc_target; /* final target CRC checksum */
int flags; /* flags (see below) */
int private_bits; /* private bits (see below) */
mad_timer_t duration; /* audio playing time of frame */
};
struct mad_header {
enum mad_layer layer; /* audio layer (1, 2, or 3) */
enum mad_mode mode; /* channel mode (see above) */
int mode_extension; /* additional mode info */
enum mad_emphasis emphasis; /* de-emphasis to use (see above) */
unsigned long bitrate; /* stream bitrate (bps) */
unsigned int samplerate; /* sampling frequency (Hz) */
unsigned short crc_check; /* frame CRC accumulator */
unsigned short crc_target; /* final target CRC checksum */
int flags; /* flags (see below) */
int private_bits; /* private bits (see below) */
mad_timer_t duration; /* audio playing time of frame */
};
layer成员的类型是enum mad_layer，这个枚举类型有3个取值（1，2，3），分别对应MPEG音频的1、2、3层；mode成员描述音频的声道数和立体声类型，取值为MAD_MODE_SINGLE_CHANNEL（单声道）、MAD_MODE_DUAL_CHANNEL（双声道）、MAD_MODE_JOINT_STEREO（联合立体声）、MAD_MODE_STEREO（普通立体声）；接下来的比特率、采样率、CRC校验、播放时间等信息直接来自mp3帧。
mad_frame包含一个帧头（mad_herder）和一帧中的采样数据。该结构同样定义于frame.h头文件中：
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struct mad_frame {
struct mad_header header; /* MPEG audio header */
int options; /* decoding options (from stream) */
mad_fixed_t sbsample[2][36][32]; /* synthesis subband filter samples */
mad_fixed_t (*overlap)[2][32][18]; /* Layer III block overlap data */
};
struct mad_frame {
struct mad_header header; /* MPEG audio header */
int options; /* decoding options (from stream) */
mad_fixed_t sbsample[2][36][32]; /* synthesis subband filter samples */
mad_fixed_t (*overlap)[2][32][18]; /* Layer III block overlap data */
};
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static
enum mad_flow output(void *data,
struct mad_header const *header,
struct mad_pcm *pcm)
{
unsigned int nchannels, nsamples;
mad_fixed_t const *left_ch, *right_ch;
/* pcm->samplerate contains the sampling frequency */
nchannels = pcm->channels;
nsamples = pcm->length;
left_ch = pcm->samples[0];
right_ch = pcm->samples[1];
while (nsamples--) {
signed int sample;
/* output sample(s) in 16-bit signed little-endian PCM */
sample = scale(*left_ch++);
putchar((sample >> 0) & 0xff);//输出低8位
putchar((sample >> 8) & 0xff);//输出高8位
if (nchannels == 2) {
sample = scale(*right_ch++);
putchar((sample >> 0) & 0xff);//输出低8位
putchar((sample >> 8) & 0xff);//输出高8位
}
}
static
enum mad_flow output(void *data,
struct mad_header const *header,
struct mad_pcm *pcm)
{
unsigned int nchannels, nsamples;
mad_fixed_t const *left_ch, *right_ch;
/* pcm->samplerate contains the sampling frequency */
nchannels = pcm->channels;
nsamples = pcm->length;
left_ch = pcm->samples[0];
right_ch = pcm->samples[1];
while (nsamples--) {
signed int sample;
/* output sample(s) in 16-bit signed little-endian PCM */
sample = scale(*left_ch++);
putchar((sample >> 0) & 0xff);//输出低8位
putchar((sample >> 8) & 0xff);//输出高8位
if (nchannels == 2) {
sample = scale(*right_ch++);
putchar((sample >> 0) & 0xff);//输出低8位
putchar((sample >> 8) & 0xff);//输出高8位
}
}
其中的option字段来自mad_stream结构，sbsample[2][36][32]中保存的就是从mp3文件中取得的采样数据：2个声道，每声道36个采样（可播放26ms的音频，每秒帧速率大约为38fps）；overlap指针成我员不了解它的用途，希望有高手不吝赐教！
Madlib每次解码循环结束时解码完成一个帧，将1152个PCM采样数据保存在数组里传递给output回调函数作输出前的处理。压缩包里的参考示例minimad.c中的output回调函数只是简单地将PCM数据在屏幕上打印显示.
责任编辑：小草
