关键数据结构之帧结构：mad_herder和mad_frame计算机等级考试
文章作者 100test 发表时间 2009:07:08 02:56:37
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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.