[Mp4-tech] Re: [audio] AAC bitstream - headers

[Ralph Sperschneider]

Anubrata Chakarabarti wrote:
> Hello Ralph
> 
> Could you pl clarify the following when you have time:
> 
> 1. What is meant by levels within a profile and how do they affect the decoding process?

Levels specify the complexity of the decoding process. This might be due to the 
number of channels, due to the sampling frequency, due to the number of objects, 
due to the permitted amount of FEC etc.
> 2. If CRC check is present in ADTS frame then what measures will have to be taken at the decoder side?

You don´t need to do anything but just skipt the crc word. If you wonna make 
sure that the received data is integer, you can derive the crc on decoder side 
and compare it with the transmitted crc word.
> 3. Can I safely assume that 1 Raw data block = 1 AU?

Usually yes. Exceptions are related to epConfig=1.
> If yes then what should be the treatment for Data Stram Elemnts and/or Fill elements (from the perspective of an analyzer - do
> they have any significance?) My question is based on the basic AU definition that all values in an AU are associated with time data.

Data transmitted in the extension payload (I guess you talk about those data) 
might be of different kind. It might be directly related to the transmitted 
audio data (e.g. SBR, DRC) or just cover any kind of not specifically defined 
ancillary data.
Best regards,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
Fraunhofer IIS                  | Fax:   +49 9131 776 398
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