AAC⾳频格式：Advanced Audio Coding(⾼级⾳频解码)，是⼀种由MPEG-4
标准定义的有损⾳频压缩格式，由Fraunhofer发展，Dolby, Sony和AT&T是主
要的贡献者
AAC分两种格式：
ADIF（Audio Data Interchange Format）⾳频数据交换格式。
ADTS（Audio Data Transport Stream） 音频数据传输流。
他们之间的主要区别是：ADTS可以在任意帧解码，也就是说它每⼀帧都有头信息。ADIF只有⼀个统⼀的头，所以必须得到所有的数据后解码。
这篇文章主要讲解ADTS这种格式。
AAC⾳频⽂件的每⼀帧由ADTS Header和AAC Audio Data组成，每⼀帧的ADTS的头⽂件都包含了⾳频的采样率，声道，帧⻓度等信息，这样解码器才能解析读取。
ADTS的头信息分两部分：其⼀为固定头信息，紧接着是可变头信息。固定头信息中的数据每⼀帧都相同，⽽可变头信息则在帧与帧之间可变。
ADTS Header的长度可能为7字节或9字节，protection_absent=0时占9个字节，protection_absent=1时占7个字节。
固定头部分说明：
| 位域 | 占位数 | 说明 |
|---|---|---|
| synword | 12 | 同步头，总是0xFFF，代表一个ADTS帧的开始 |
| ID | 1 | MPEG标识符，0标识MPEG-4，1标识MPEG-2 |
| Layer | 2 | 总为00 |
| protection_absent | 1 | 表示是否做校验，0表示做校验，1表示不做校验 |
| profile | 2 | 表示使⽤哪个级别的AAC |
| sampling_frequency_index | 4 | 表示使⽤的采样率下标 |
| private_bit | 1 | 保护位 |
| channel_configuration | 3 | 表示声道数 |
| original_copy | 1 | 原始数据位 |
| home | 1 | home位 |
可变头部部分说明：
| copyright_identifcation_bit | 1 | 拷贝权限识别位 |
|---|---|---|
| copyright_identification_start | 1 | 拷贝权限识别开始 |
| aac_frame_length | 13 | ⼀个ADTS帧的⻓度包括ADTS头和AAC原始流,注意头部可能为7或9个字节大小 |
| adts_buffer_fullness | 11 | 0x7FF 说明是码率可变的码流 |
| number_of_raw_data_blocks_in_frame | 2 | 表示ADTS帧中有number_of_raw_data_blocks_in_frame + 1个AAC原始帧，number_of_raw_data_blocks_in_frame一般都是0 |
解析aac代码：
#include
#include
#include
#include
#define ADTS_HEADER_LEN 7;
const int sampling_frequencies[] = {
96000, // 0x0
88200, // 0x1
64000, // 0x2
48000, // 0x3
44100, // 0x4
32000, // 0x5
24000, // 0x6
22050, // 0x7
16000, // 0x8
12000, // 0x9
11025, // 0xa
8000 // 0xb
// 0xc d e f是保留的
};
int adts_header(char * const p_adts_header, const int data_length,
const int profile, const int samplerate,
const int channels)
{
int sampling_frequency_index = 3; // 默认使用48000hz
int adtsLen = data_length + 7;
int frequencies_size = sizeof(sampling_frequencies) / sizeof(sampling_frequencies[0]);
int i = 0;
for(i = 0; i < frequencies_size; i++)
{
if(sampling_frequencies[i] == samplerate)
{
sampling_frequency_index = i;
break;
}
}
if(i >= frequencies_size)
{
printf("unsupport samplerate:%d\n", samplerate);
return -1;
}
p_adts_header[0] = 0xff; //syncword:0xfff 高8bits
p_adts_header[1] = 0xf0; //syncword:0xfff 低4bits
p_adts_header[1] |= (0 << 3); //MPEG Version:0 for MPEG-4,1 for MPEG-2 1bit
p_adts_header[1] |= (0 << 1); //Layer:0 2bits
p_adts_header[1] |= 1; //protection absent:1 1bit
p_adts_header[2] = (profile)<<6; //profile:profile 2bits
p_adts_header[2] |= (sampling_frequency_index & 0x0f)<<2; //sampling frequency index:sampling_frequency_index 4bits
p_adts_header[2] |= (0 << 1); //private bit:0 1bit
p_adts_header[2] |= (channels & 0x04)>>2; //channel configuration:channels 高1bit
p_adts_header[3] = (channels & 0x03)<<6; //channel configuration:channels 低2bits
p_adts_header[3] |= (0 << 5); //original：0 1bit
p_adts_header[3] |= (0 << 4); //home：0 1bit
p_adts_header[3] |= (0 << 3); //copyright id bit：0 1bit
p_adts_header[3] |= (0 << 2); //copyright id start：0 1bit
p_adts_header[3] |= ((adtsLen & 0x1800) >> 11); //frame length：value 高2bits
p_adts_header[4] = (uint8_t)((adtsLen & 0x7f8) >> 3); //frame length:value 中间8bits
p_adts_header[5] = (uint8_t)((adtsLen & 0x7) << 5); //frame length:value 低3bits
p_adts_header[5] |= 0x1f; //buffer fullness:0x7ff 高5bits
p_adts_header[6] = 0xfc; //11111100 //buffer fullness:0x7ff 低6bits
// number_of_raw_data_blocks_in_frame：
// 表示ADTS帧中有number_of_raw_data_blocks_in_frame + 1个AAC原始帧。
return 0;
}
int main(int argc, char *argv[])
{
int ret = -1;
char errors[1024];
char *in_filename = NULL;
char *aac_filename = NULL;
FILE *aac_fd = NULL;
int audio_index = -1;
int len = 0;
AVFormatContext *ifmt_ctx = NULL;
AVPacket pkt;
// 设置打印级别
av_log_set_level(AV_LOG_DEBUG);
if(argc < 3)
{
av_log(NULL, AV_LOG_DEBUG, "the count of parameters should be more than three!\n");
return -1;
}
in_filename = argv[1]; // 输入文件
aac_filename = argv[2]; // 输出文件
if(in_filename == NULL || aac_filename == NULL)
{
av_log(NULL, AV_LOG_DEBUG, "src or dts file is null, plz check them!\n");
return -1;
}
aac_fd = fopen(aac_filename, "wb");
if (!aac_fd)
{
av_log(NULL, AV_LOG_DEBUG, "Could not open destination file %s\n", aac_filename);
return -1;
}
// 打开输入文件
if((ret = avformat_open_input(&ifmt_ctx, in_filename, NULL, NULL)) < 0)
{
av_strerror(ret, errors, 1024);
av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
in_filename,
ret,
errors);
return -1;
}
// 获取解码器信息
if((ret = avformat_find_stream_info(ifmt_ctx, NULL)) < 0)
{
av_strerror(ret, errors, 1024);
av_log(NULL, AV_LOG_DEBUG, "failed to find stream information: %s, %d(%s)\n",
in_filename,
ret,
errors);
return -1;
}
// dump媒体信息
av_dump_format(ifmt_ctx, 0, in_filename, 0);
// 初始化packet
av_init_packet(&pkt);
// 查找audio对应的steam index
audio_index = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
if(audio_index < 0)
{
av_log(NULL, AV_LOG_DEBUG, "Could not find %s stream in input file %s\n",
av_get_media_type_string(AVMEDIA_TYPE_AUDIO),
in_filename);
return AVERROR(EINVAL);
}
// 打印AAC级别
printf("audio profile:%d, FF_PROFILE_AAC_LOW:%d\n",
ifmt_ctx->streams[audio_index]->codecpar->profile,
FF_PROFILE_AAC_LOW);
if(ifmt_ctx->streams[audio_index]->codecpar->codec_id != AV_CODEC_ID_AAC)
{
printf("the media file no contain AAC stream, it's codec_id is %d\n",
ifmt_ctx->streams[audio_index]->codecpar->codec_id);
goto failed;
}
// 读取媒体文件，并把aac数据帧写入到本地文件
while(av_read_frame(ifmt_ctx, &pkt) >=0 )
{
if(pkt.stream_index == audio_index)
{
char adts_header_buf[7] = {0};
adts_header(adts_header_buf, pkt.size,
ifmt_ctx->streams[audio_index]->codecpar->profile,
ifmt_ctx->streams[audio_index]->codecpar->sample_rate,
ifmt_ctx->streams[audio_index]->codecpar->channels);
fwrite(adts_header_buf, 1, 7, aac_fd); // 写adts header , ts流不适用，ts流分离出来的packet带了adts header
len = fwrite( pkt.data, 1, pkt.size, aac_fd); // 写adts data
if(len != pkt.size)
{
av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
len,
pkt.size);
}
}
av_packet_unref(&pkt);
}
failed:
// 关闭输入文件
if(ifmt_ctx)
{
avformat_close_input(&ifmt_ctx);
}
if(aac_fd)
{
fclose(aac_fd);
}
return 0;
}
输入：
believe.flv out.aac
输出：
[NULL @ 00dd8540] Opening 'believe.flv' for reading
[file @ 00dc3680] Setting default whitelist 'file,crypto'
[flv @ 00dd8540] Format flv probed with size=2048 and score=100
[flv @ 00dd8540] Before avformat_find_stream_info() pos: 13 bytes read:32768 seeks:0 nb_streams:0
[NULL @ 00dd4100] nal_unit_type: 7(SPS), nal_ref_idc: 3
[NULL @ 00dd4100] nal_unit_type: 8(PPS), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 7(SPS), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 8(PPS), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 6(SEI), nal_ref_idc: 0
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] nal_unit_type: 5(IDR), nal_ref_idc: 3
[h264 @ 00dd4100] Format yuv420p chosen by get_format().
[h264 @ 00dd4100] Reinit context to 1920x1088, pix_fmt: yuv420p
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[h264 @ 00dd4100] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2
[flv @ 00dd8540] All info found
[flv @ 00dd8540] rfps: 14.833333 0.017658
[flv @ 00dd8540] rfps: 14.916667 0.004437
[flv @ 00dd8540] rfps: 15.000000 0.000018
[flv @ 00dd8540] rfps: 15.000000 0.000017
[flv @ 00dd8540] rfps: 15.083333 0.004399
[flv @ 00dd8540] rfps: 15.083333 0.004399
[flv @ 00dd8540] rfps: 15.166667 0.017581
[flv @ 00dd8540] rfps: 29.833333 0.017749
[flv @ 00dd8540] rfps: 29.916667 0.004509
[flv @ 00dd8540] rfps: 29.916667 0.004509
[flv @ 00dd8540] rfps: 30.000000 0.000070
[flv @ 00dd8540] rfps: 30.000000 0.000070
[flv @ 00dd8540] rfps: 45.000000 0.000157
[flv @ 00dd8540] rfps: 45.000000 0.000157
[flv @ 00dd8540] rfps: 60.000000 0.000280
[flv @ 00dd8540] rfps: 60.000000 0.000280
[flv @ 00dd8540] rfps: 120.000000 0.001120
[flv @ 00dd8540] rfps: 120.000000 0.001120
[flv @ 00dd8540] rfps: 240.000000 0.004480
[flv @ 00dd8540] rfps: 240.000000 0.004480
[flv @ 00dd8540] rfps: 29.970030 0.000653
[flv @ 00dd8540] rfps: 29.970030 0.000653
[flv @ 00dd8540] rfps: 59.940060 0.002612
[flv @ 00dd8540] rfps: 59.940060 0.002612
[flv @ 00dd8540] rfps: 14.985015 0.000163
[flv @ 00dd8540] rfps: 14.985015 0.000163
[flv @ 00dd8540] After avformat_find_stream_info() pos: 177501 bytes read:198684 seeks:0 frames:170
Input #0, flv, from 'believe.flv':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
comment : www.ieway.cn
encoder : Lavf58.29.100
Duration: 00:03:42.53, start: 0.000000, bitrate: 286 kb/s
Stream #0:0, 41, 1/1000: Video: h264 (Constrained Baseline), 1 reference frame, yuv420p(progressive, left), 1920x1080 (1920x1088), 0/1, 150 kb/s, 14.46 fps, 15 tbr, 1k tbn, 30 tbc
Stream #0:1, 129, 1/1000: Audio: aac (LC), 48000 Hz, stereo, fltp, 128 kb/s
audio profile:1, FF_PROFILE_AAC_LOW:1
[AVIOContext @ 00dca700] Statistics: 7972715 bytes read, 0 seeks
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