layer3.c

MP3解码源代码,能够对于MP3进行解码,效率不错

/* layer3.c  layer3 音频解码
 *
  */
#include "amp.h"
#include "audio.h"
#include "dump.h"
#include "getbits.h"
#include "getdata.h"
#include "huffman.h"
#include "misc2.h"
#include "rtbuf.h"
#include "transform.h"

#define LAYER3
#include "layer3.h"

/* 该程序对层3的一帧进行解码.*/
int layer3_frame(struct AUDIO_HEADER *header,int cnt)
{
static struct SIDE_INFO info;

int gr,ch,sb,i;
int mean_frame_size,bitrate,fs,hsize,ssize;

/* hsize 是帧头＋附加信息的大小*/
	if (header->ID) 
		if (header->mode==3) {
			nch=1;
			hsize=21;
		} else {
			nch=2;
			hsize=36;
		}
	else
		if (header->mode==3) {
			nch=1;
			hsize=13;
		} else {
			nch=2;
			hsize=21;
		}

		/* 加上crc 位hsize 的位数增加2*/
	if (header->protection_bit==0) hsize+=2;

/* 读取层3 特定的附加信息*/
        getinfo(header,&info);


	bitrate=t_bitrate[header->ID][3-header->layer][header->bitrate_index];
	fs=t_sampling_frequency[header->ID][header->sampling_frequency];
        if (header->ID) mean_frame_size=144000*bitrate/fs;
        else mean_frame_size=72000*bitrate/fs;


/* 判断mdb 对前几帧是否太大，如果是将意味着数据流的一部分将丢失**/
	if (f_bdirty) 
		if (info.main_data_begin > bclean_bytes) {
			fillbfr(mean_frame_size + header->padding_bit - hsize);
			bclean_bytes+=mean_frame_size + header->padding_bit - hsize;
			return 0;
		} else {
			f_bdirty=FALSE;
			bclean_bytes=0;
		}
		
/* 下面根据main_data_begin 的信息将数据指针重新定向*/
        data = 8 * ((append - info.main_data_begin) & (BUFFER_SIZE-1));


/* 将所有的字节都读入缓存器直到下一帧的开始*/
        fillbfr(mean_frame_size + header->padding_bit - hsize);



	t_l=&t_b8_l[header->ID][header->sampling_frequency][0];
	t_s=&t_b8_s[header->ID][header->sampling_frequency][0];


	if (A_DUMP_BINARY) dump((int *)info.part2_3_length);

/* 解码出比例因子和huffman编码的数据*/
	for (gr=0;gr < ((header->ID) ? 2 : 1);gr++) {
		for (ch=0;ch<nch;ch++) {
			ssize=decode_scalefactors(&info,header,gr,ch);
			decode_huffman_data(&info,gr,ch,ssize);
		}
		
/* 逆量化，立体声处理*/

		if (A_DOWNMIX && nch==2) requantize_downmix(gr,&info,header);
		else 
			if (header->mode!=1 || (header->mode==1 && header->mode_extension==0))
				for (ch=0;ch<nch;ch++) requantize_mono(gr,ch,&info,header);
			else requantize_ms(gr,&info,header);
		if (A_DOWNMIX) nch=1;

		for (ch=0; ch < (A_DOWNMIX ? 1:nch) ;ch++) {
		int win_type; /* same as in the standard, long=0, start=1 ,.... */
		int window_switching_flag = info.window_switching_flag[gr][ch];
		int block_type = info.block_type[gr][ch];
		int mixed_block_flag = info.mixed_block_flag[gr][ch];

	/* 混叠蝶形图*/
			if (!(window_switching_flag &&
			      block_type==2))
                                alias_reduction(ch);

			if (window_switching_flag &&
			    block_type==2 &&
			    mixed_block_flag)
				win_type=0;
			else
			        if (!window_switching_flag) win_type=0;
				else win_type=block_type;

	/* 逆MDCT*/
			for (sb=0;sb<2;sb++)
				imdct(win_type,sb,ch);

			if (window_switching_flag &&
			    block_type==2 &&
			    mixed_block_flag)
				win_type=2;
			for (sb=2;sb<no_of_imdcts[ch];sb++)
				imdct(win_type,sb,ch);

			for (;sb<32;sb++) 
				for (i=0;i<18;i++) {
					res[sb][i]=s[ch][sb][i];
					s[ch][sb][i]=0.0f;
				}
	
/* 多项滤波器组*/
				for (i=0;i<18;i++)
				        poly(ch,i);
		}

		printout();

		if (A_DOWNMIX && header->mode!=3) nch=2;

	}    

/* 如果解码正确，则返回0*/
	return 0;
}