aac的格式, 以及faad解码raw数据解码
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ADTS AAC
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| ADTS_header | AAC ES | ADTS_header | AAC ES |
...
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ADTS_header | AAC ES |
ADTS内容及结构
ADTS头中相对有用的信息 采样率、声道数、帧长度。想想也是，我要是解码器的话，你给我一堆得AAC音频ES流我也解不出来。每一个带ADTS头信息的AAC流会清晰的告送解码器他需要的这些信息。
一般情况下ADTS的头信息都是7个字节，分为2部分：
adts_fixed_header();
adts_variable_header();
syncword ：同步头 总是0xFFF, all bitsmust be 1，代表着一个ADTS帧的开始
ID：MPEG Version: 0 for MPEG-4, 1 for MPEG-2
Layer：always: '00'
profile：表示使用哪个级别的AAC，有些芯片只支持AAC LC 。在MPEG-2AAC中定义了3种：
sampling_frequency_index：表示使用的采样率下标，通过这个下标在 SamplingFrequencies[ ]数组中查找得知采样率的值。
There are 13 supported frequencies:
- 0: 96000 Hz
- 1: 88200 Hz
- 2: 64000 Hz
- 3: 48000 Hz
- 4: 44100 Hz
- 5: 32000 Hz
- 6: 24000 Hz
- 7: 22050 Hz
- 8: 16000 Hz
- 9: 12000 Hz
- 10: 11025 Hz
- 11: 8000 Hz
- 12: 7350 Hz
- 13: Reserved
- 14: Reserved
- 15: frequency is written explictly
- 0: Defined in AOT Specifc Config
- 1: 1 channel: front-center
- 2: 2 channels: front-left, front-right
- 3: 3 channels: front-center, front-left, front-right
- 4: 4 channels: front-center, front-left, front-right,back-center
- 5: 5 channels: front-center, front-left, front-right,back-left, back-right
- 6: 6 channels: front-center, front-left, front-right,back-left, back-right, LFE-channel
- 7: 8 channels: front-center, front-left, front-right,side-left, side-right, back-left, back-right, LFE-channel
- 8-15: Reserved
frame_length :一个ADTS帧的长度包括ADTS头和AAC原始流.
adts_buffer_fullness：0x7FF 说明是码率可变的码流
将AAC打包成ADTS格式
通过对ADTS格式的了解，很容易就能把AAC打包成ADTS。我们只需得到封装格式里面关于音频采样率、声道数、元数据长度、aac格式类型等信息。然后在每个AAC原始流前面加上个ADTS头就OK了。
附上ffmpeg添加ADTS头的代码:
static int adts_write_packet(AVFormatContext*s, AVPacket *pkt)
{
ADTSContext *adts = s->priv_data;
AVIOContext *pb = s->pb;
uint8_tbuf[ADTS_HEADER_SIZE];
if(!pkt->size)
return 0;
if(adts->write_adts) {
ff_adts_write_frame_header(adts, buf, pkt->size,adts->pce_size);
avio_write(pb, buf,ADTS_HEADER_SIZE);
if (adts->pce_size){
avio_write(pb, adts->pce_data,adts->pce_size);
adts->pce_size = 0;
}
}
avio_write(pb, pkt->data, pkt->size);
avio_flush(pb);
return0;
}
int ff_adts_write_frame_header(ADTSContext*ctx,
uint8_t *buf, int size, intpce_size)
{
PutBitContext pb;
init_put_bits(&pb, buf, ADTS_HEADER_SIZE);
put_bits(&pb, 12, 0xfff);
put_bits(&pb, 1, 0);
put_bits(&pb, 2, 0);
put_bits(&pb, 1, 1);
put_bits(&pb, 2, ctx->objecttype);
put_bits(&pb, 4, ctx->sample_rate_index);
put_bits(&pb, 1, 0);
put_bits(&pb, 3, ctx->channel_conf);
put_bits(&pb, 1, 0);
put_bits(&pb, 1, 0);
put_bits(&pb, 1, 0);
put_bits(&pb, 1, 0);
put_bits(&pb, 13, ADTS_HEADER_SIZE + size +pce_size);
put_bits(&pb, 11, 0x7ff);
put_bits(&pb, 2, 0);
flush_put_bits(&pb);
return0;
}
二.ADIF：
Audio Data InterchangeFormat 音频数据交换格式。这种格式的特征是可以确定的找到这个音频数据的开始，不需进行在音频数据流中间开始的解码，即它的解码必须在明确定义的开始处进行。故这种格式常用在磁盘文件中。
AAC的ADIF格式见下图：
三.faad解码aac
iRet= pcmRender.init(2, 44100, 16, NULL);
static unsigned charframe[FRAME_MAX_LEN];
unsigned longsamplerate;
unsigned charchannels;
NeAACDecHandle decoder =0;
size_t data_size =0;
size_t size =0;
NeAACDecFrameInfoframe_info;
unsigned char* input_data= buffer;
unsigned char* pcm_data =NULL;
static int iFlag =0;
int iRead =0;
while (m_iThreadFlag&& (data_size = ReadData(NULL, buffer+iRead,BUFFER_MAX_LEN-iRead)))
{
#if1
data_size +=iRead;
if (0 ==iFlag)
{
if(get_one_ADTS_frame(buffer,data_size, frame, &size, &iRead) <0)
{
continue;
}
decoder = NeAACDecOpen();
//initializedecoder
NeAACDecInit(decoder,frame, size, &samplerate, &channels);
printf("samplerate %d,channels %d\n", samplerate, channels);
iFlag =1;
}
input_data =buffer;
while(m_iThreadFlag&& get_one_ADTS_frame(input_data, data_size, frame,&size, &iRead) == 0)
{
//decode ADTSframe
pcm_data = (unsignedchar*)NeAACDecDecode(decoder, &frame_info, frame,size);
if(frame_info.error >0)
{
printf("%s\n",NeAACDecGetErrorMessage(frame_info.error));
}
else if(pcm_data&& frame_info.samples > 0)
{
static FILE *fp1 =NULL;
if (NULL ==fp1)
{
fp1 = fopen("F:\\6.pcm","wb");
}
if(fp1)
{
fwrite(pcm_data, 1,frame_info.samples * frame_info.channels,fp1);
fflush(fp1);
}
Player((char*)pcm_data,frame_info.samples * frame_info.channels);
}
data_size -=size;
input_data +=size;
}
#endif
}
NeAACDecClose(decoder);