ADTS包头解析

ADTS全称是(Audio Data Transport Stream)，是AAC的一种十分常见的传输格式。

ADTS AAC
ADTS_header	AAC ES	ADTS_header	AAC ES	...	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();

syntax
adts_fixed_header(){
    syncword; 12 bslbf
    ID; 1 bslbf
    layer;2 uimsbf
    protection_absent;1 bslbf
    profile;2 uimsbf
    sampling_frequency_index;4 uimsbf
    private_bit;1 bslbf
    channel_configuration;3 uimsbf
    original_copy;1 bslbf
    home;1 bslbf
}

syncword ：同步头 总是0xFFF, all bits must be 1，代表着一个ADTS帧的

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-2 AAC中定义了3种：
    0:Main profile
    1:Low Complexity profile(LC)
    2:Scalable sampling Rate Profile(SSR)
    3:Reserved

sampling_frequency_index：表示使用的采样率下标，通过这个下标在 Sampling Frequencies[ ]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

channel_configuration: 表示声道数
    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

syntax
adts_variable_header(){
    copyright_identiflaction_bit;1 bslbf
    copyright_identification_start;1 bslbf
    acc_frame_length;13 bslbf;
    adts_buffer_fullness; 11 bslbf
    number_of_raw_data_blocks_in_frame;2 uimsfb
}

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

//---------------------------------------------------------------------------------------------------------------------------------------------------------------//

下面是来自一个国外网站的介绍。  个人觉得这个网站的这个表结构的说明更加的形象一些。很好理解。1byte = 8 bits ， 每个字段占用多少bits，各个字段的意义。

Audio Data Transport Stream (ADTS) is a format, used by MPEG TS or Shoutcast to stream audio, usually AAC.

Structure{
AAAAAAAA AAAABCCD EEFFFFGH HHIJKLMM MMMMMMMM MMMOOOOO OOOOOOPP (QQQQQQQQ QQQQQQQQ)
Header consists of 7 or 9 bytes (without or with CRC).
}

Letter Length (bits) Description
A     12     syncword 0xFFF, all bits must be 1
B     1       MPEG Version: 0 for MPEG-4, 1 for MPEG-2
C     2       Layer: always 0
D     1       protection absent, Warning, set to 1 if there is no CRC and 0 if there is CRC
E     2       profile, the MPEG-4 Audio Object Type minus 1
F     4       MPEG-4 Sampling Frequency Index (15 is forbidden)
G     1       private stream, set to 0 when encoding, ignore when decoding
H     3       MPEG-4 Channel Configuration (in the case of 0, the channel configuration is sent via an inband

PCE)
I      1       originality, set to 0 when encoding, ignore when decoding
J      1       home, set to 0 when encoding, ignore when decoding
K     1       copyrighted stream, set to 0 when encoding, ignore when decoding
L     1        copyright start, set to 0 when encoding, ignore when decoding
M   13      frame length, this value must include 7 or 9 bytes of header length: FrameLength =

(ProtectionAbsent == 1 ? 7 : 9) + size(AACFrame)
O    11       Buffer fullness
P     2        Number of AAC frames (RDBs) in ADTS frame minus 1, for maximum compatibility always use 1 AAC frame

per ADTS frame
Q     16     CRC if protection absent is 0

Usage in MPEG-TS
ADTS packet must be a content of PES packet. Pack AAC data inside ADTS frame, than pack inside PES packet, then mux

by TS packetizer.

Usage in Shoutcast
ADTS frames goes one by one in TCP stream. Look for syncword, parse header and look for next syncword after.

//---------------------------------------------------------------------------------------------------------------------------------------------------------------//

将AAC打包成ADTS格式

如果是通过嵌入式高清解码芯片做产品的话，一般情况的解码工作都是由硬件来完成的。所以大部分的工作是把AAC原始流打包成ADTS的格式，然后丢给硬件就行了。

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。

贴上ffmpeg中添加ADTS头的代码，就可以很清晰的了解ADTS头的结构：

int ff_adts_write_frame_header(ADTSContext *ctx,
                               uint8_t *buf, int size, int pce_size)
{
    PutBitContext pb;

    init_put_bits(&pb, buf, ADTS_HEADER_SIZE);

    /* adts_fixed_header */
    put_bits(&pb, 12, 0xfff);   /* syncword */
    put_bits(&pb, 1, 0);        /* ID */
    put_bits(&pb, 2, 0);        /* layer */
    put_bits(&pb, 1, 1);        /* protection_absent */
    put_bits(&pb, 2, ctx->objecttype); /* profile_objecttype */
    put_bits(&pb, 4, ctx->sample_rate_index);
    put_bits(&pb, 1, 0);        /* private_bit */
    put_bits(&pb, 3, ctx->channel_conf); /* channel_configuration */
    put_bits(&pb, 1, 0);        /* original_copy */
    put_bits(&pb, 1, 0);        /* home */

    /* adts_variable_header */
    put_bits(&pb, 1, 0);        /* copyright_identification_bit */
    put_bits(&pb, 1, 0);        /* copyright_identification_start */
    put_bits(&pb, 13, ADTS_HEADER_SIZE + size + pce_size); /* aac_frame_length */
    put_bits(&pb, 11, 0x7ff);   /* adts_buffer_fullness */
    put_bits(&pb, 2, 0);        /* number_of_raw_data_blocks_in_frame */

    flush_put_bits(&pb);

    return 0;
}