[Mp4-tech] a few questions with regard to MPEG-2 AAC

[Yueshi Shen]

Dear AAC experts,
I wish to ask your advices on a few questions with regard to MPEG-2 AAC
here, and the MPEG-2 AAC standard I've got is the ISO/IEC 13818-7:2004 (also
the Australian Standard ISO/IEC13818.7-2005).
1) crc_check
   In page 42, the spec says "CRC error detection data generated as
described in ISO/IEC 11172-3, subclause 2.4.3.1".  However, in the
corresponding section of the MPEG-1 Audio, apart from the CRC-check
calculation algorithm, the protected fields are also defined (and they
differ for Layer I, II, and III).  So I am wondering what is the CRC-check
protected range for MPEG-2 AAC?
2) adts_buffer_fullness
   In page 45, the mechanism of generating adts_buffer_fullness is
described.  I guess it's a similar idea as VBV buffer in MPEG video, so it's
a control mechanism for output's timing.  My question is how this
information is used in a typical AAC decoder?  If AAC is carried in the
MPEG-2 transport stream, is the adts_buffer_fullness' functionality
completely overridden by PTS/DTS/PCR/SCR?
3) padding audio frames
   One of the fill_element()'s usages is to keep every audio frame same
length.  In order to pad an audio frame to a particular length, whose value
is encoded in the frame header, I image there are two ways: a) write an
id_syn_ele of ID_END and pad all the rest bytes to 0; b) write a proper
fill_element() (i.e., fill_nibble, fill_byte, other_bits) and write ID_END
until the last byte.  Is the first approach legal, and has the same effect
as the second one?
I apologise if you find the above questions are too naive, and I will be
greatly appreciated if you can help me with any information.
Sincerely yours
Yueshi
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