ADTS內容及結構
ADTS 頭中相對有用的信息 採樣率、聲道數、幀長度。想想也是,我要是解碼器的話,你給我一堆得AAC音頻ES流我也解不出來。每一個帶ADTS頭信息的AAC流會清晰的告送解碼器他需要的這些信息。
一般情況下ADTS的頭信息都是7個字節,分爲2部分:
adts_fixed_header();
adts_variable_header();
syncword :同步頭 總是0xFFF, all bits must be 1,代表着一個ADTS幀的開始
ID:MPEG Version: 0 for MPEG-4, 1 for MPEG-2
Layer:always: '00'
profile:表示使用哪個級別的AAC,有些芯片只支持AAC LC 。在MPEG-2 AAC中定義了3種:
sampling_frequency_index:表示使用的採樣率下標,通過這個下標在 Sampling Frequencies[ ]數組中查找得知採樣率的值。
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
rame_length : 一個ADTS幀的長度包括ADTS頭和AAC原始流.
adts_buffer_fullness:0x7FF 說明是碼率可變的碼流