AAC音频格式分析

关于AAC音频格式基本情况，可参考维基百科http://en.wikipedia.org/wiki/Advanced_Audio_Coding

 

AAC音频格式分析

AAC音频格式有ADIF和ADTS：

ADIF：Audio Data Interchange Format 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。

ADTS：Audio Data Transport Stream 音频数据传输流。这种格式的特征是它是一个有同步字的比特流，解码可以在这个流中任何位置开始。它的特征类似于mp3数据流格式。

简单说，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。且这两种的header的格式也是不同的，目前一般编码后的和抽取出的都是ADTS格式的音频流。

语音系统对实时性要求较高，基本是这样一个流程，采集音频数据，本地编码，数据上传，服务器处理，数据下发，本地解码

ADTS是帧序列，本身具备流特征，在音频流的传输与处理方面更加合适。

 

 

ADTS帧结构：

header

body

ADTS帧首部结构：

序号	域	长度（bits）	说明
1	Syncword	12	all bits must be 1
2	MPEG version	1	0 for MPEG-4, 1 for MPEG-2
3	Layer	2	always 0
4	Protection Absent	1	et to 1 if there is no CRC and 0 if there is CRC
5	Profile	2	the MPEG-4 Audio Object Type minus 1
6	MPEG-4 Sampling Frequency Index	4	MPEG-4 Sampling Frequency Index (15 is forbidden)
7	Private Stream	1	set to 0 when encoding, ignore when decoding
8	MPEG-4 Channel Configuration	3	MPEG-4 Channel Configuration (in the case of 0, the channel configuration is sent via an inband PCE)
9	Originality	1	set to 0 when encoding, ignore when decoding
10	Home	1	set to 0 when encoding, ignore when decoding
11	Copyrighted Stream	1	set to 0 when encoding, ignore when decoding
12	Copyrighted Start	1	set to 0 when encoding, ignore when decoding
13	Frame Length	13	this value must include 7 or 9 bytes of header length: FrameLength = (ProtectionAbsent == 1 ? 7 : 9) + size(AACFrame)
14	Buffer Fullness	11	buffer fullness
15	Number of AAC Frames	2	number of AAC frames (RDBs) in ADTS frame minus 1, for maximum compatibility always use 1 AAC frame per ADTS frame
16	CRC	16	CRC if protection absent is 0

 

 

http://wiki.multimedia.cx/index.php?title=ADTS

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 bit, guaranteed never to be used by MPEG, 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 id bit, the next bit of a centrally registered copyright identifier, set to 0 when encoding, ignore when decoding
L	1	copyright id start, signals that this frame's copyright id bit is the first bit of the copyright id, 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.

Category: 

• Container Formats

Channel Configurations

These are the channel configurations:

• 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