[Live-devel] AAC (ADTS and ADIF) Header {Newbe}

[ranjeet gupta]

Dear Readers,

I have some doubts/querries Regarding the ADTS Headers for AAC files.

As we know that their are two Formats(ADTS And ADIF) regarding the AAC 
files. When we Extract the raw data (Compressed Data) from the AAC
files then you need to place the Headers in them so that we can play 
in the any software tool (Such as FOOBAR in my case).

At Present I have doubts about the ADTS headers, Please check below

ADTS HEADER FORMAT:

FIXED ADTS HEADERS VARAIBLE. 
(I am pendowning those variables in which I have the doubts)

1. syncword   12 bit  (always: '111111111111'), 
   
   What is the use of this and why it is constant ?? (Just to check in 
   run time that it is ADTS Header ?)

2. layer       2 bit  (always: '00') 

    What are the other significance of the numbers such as ( 1, 2, 3) 
    Why it is always 0 ? (what do these numbers such as 1, 2, 3 Maps 
    to ?)
 
3.  protection_absent 1 bit 
  
    when we take this as one the  it means that we dont need to 
    perform the CRC Check, But the question comes that when we
    want to set the value as 1, and value as 0;

 i.e
 if (protection_absent == 1) //  NO CRC Check is required
 if (protection_absent == 0) //  CRC Check is required

    So when to fixed the value. as ZERO or ONE for the 
    protection_absent ?

4. private_bit 1 bit     // What does this indicate ??
  
5. home 1 bit            // What this indicates ?


ADTS Variable header: these can change from frame to frame 

1. aac_frame_length  13 bits 
   length of the frame including header (in bytes) 

What does the line means "including header" in bytes, As what i
suppose that when I see the M4a file then I see all the AU units having the length as the 1024 bytes, So which headers they are 
talking about ?

2. adts_buffer_fullness 11 bits  (0x7FF indicates VBR) 

But When I see for the bit rate for the m4a file then i get the audio
bit rate as the 128kbps, means it is fixed ... but what eaxactly 
(0x7FF indicates VBR) this represent as I am having the (I suppose
Fixed Bit Rate .. So Why they are saying as the Variable Bit Rate)

3. no_raw_data_blocks_in_frame 2 bit 

what does this mean ?   

Refrence Link 

http://www.audiocoding.com/modules/wiki/?page=ADTS&PHPSESSID=d0c503be4481eb5d92e0ed51cd8637de

Note: Above link may be broken so please take the consideration of 
      this

I regret if I mis understood some thing as I am Newbe, please do 
correct me.

Thanks In Advance.
Ranjeet



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