aac adts 格式分析

AAC音频格式在MPEG-2（ISO-13318-7 2003）中有定义。AAC后来又被采用到MPEG-4标准中。

AAC 有两种格式 ADIF(Audio Data Interchange Forma) 和 ADTS(Audio Data Transport Stream)

ADIF: ADIF只有一个统一的头，所以必须得到所有的数据后解码
ADTS: 每一帧都有头信息, 可以在任意帧解码

ADIF

adif头 + 对齐字节 + 数据块

adif_sequence() {
    adif_header();
    byte_alignment();
    raw_data_stream();
}

ADTS

数据流是一帧一帧的adts，可以在任何位置解码。

adts_sequence() {
    while (nextbits() == syncword) 
    { 
        adts_frame();
    } 
}

adts_frame

adts_frame() {
    adts_fixed_header();
    adts_variable_header();
    if (number_of_raw_data_blocks_in_frame == 0) {
        adts_error_check();
        raw_data_block(); 
    }
    else {
        adts_header_error_check();
        for (i = 0; i <= number_of_raw_data_blocks_in_frame; i++) {
            raw_data_block();
            adts_raw_data_block_error_check();
        }
    }
}

每一帧都有28位的固定header + 28位的可变header组成的，共7字节。

syncword

The bit string 1111 1111 1111.

0 for MPEG-4, 1 for MPEG-2

layer

Indicates which layer is used. Set to 00.

protection_absent

Indicates whether error_check() data is present or not.
0 存在，1 不存在。

profile

sampling_frequency_index

private_bit

Bit for private use. This bit will not be used in the future by ISO/IEC.
set to 0 when encoding, ignore when decoding ？？

channel_configuration

声道信息，大于0 根据下表来决定，等于0？

original/copy

This bit equals ‘O if the bitstream is a copy, ‘1’ if it is an original.

编码时设置为0，解码时忽略?

home

编码时设置为0，解码时忽略?

adts_variable_header

copyright_identification_bit

编码时设置为0，解码时忽略?

copyright_identification_start

编码时设置为0，解码时忽略?

frame_length

Length of the frame including headers and error_check in bytes

adts_buffer_fullness

state of the bit reservoir in the course of encoding the ADTS frame, up to and including the first raw_data_block(). It is transmitted as the number of available bits in the bit reservoir divided by NCC divided by 32 and truncated to an integer value (Table 9). A value of hexadecimal 7FF signals that the bitstream is a variable rate bitstream. In this case, buffer fullness is not applicable

0x7FF 代表码率是可变的。

number_of_raw_data_blocks_in_frame

Number of raw_data_block()’s that are multiplexed in the adts_frame() is equal to
raw_data_block_position[i]
number_of_raw_data_blocks_in_frame + 1. The minimum value is 0 indicating 1 raw_data_block()

根据 number_of_raw_data_blocks_in_frame 是否等于 0 做区分

等于0，该帧里面中只有一个raw_data_block
大于0，该帧中有number_of_raw_data_blocks_in_frame + 1个raw_data_block
大于 0 的情况较少见?
每个raw_data_block对应于1024个采样?

adts_error_check

是否开启循环冗余校验，开启后header总长（28 + 28 + 16 = 72）9个字节。

adts_header_error_check

raw_data_block_position[i]

Start position of the i-th raw_data_block() in the adts_frame(), measured as an offset in bytes from the start position of the first raw_data_block() in the adts_frame().

如果开启循环冗余校验，并且number_of_raw_data_blocks_in_frame > 0, 记录每个raw_data_block开始位置，添加crc校验。