[Mp4-tech] Re: [AAC] Is this a legal AAC syntax sequence?

[Ralph Sperschneider]

Andy Quan wrote:
> Hi all,
> In MPEG4/MPEG2 AAC, is it a legal case if the 1st raw data block in
> certain track consists of PCE+TERMINATOR only? If this is legal, then
> the risk behind is that parsing PCE will not result in any channel
> information. Thanks.
>
>   

Hi Andy,
this is not a legal case. The PCE describes the the channel 
configuration by means of information on how many SCEs, CPEs etc. are 
within any subsequent frame, starting with the frame the PCE is sent. 
Thus, if a PCE is sent and it claims for any SCE(s), CPE(s) and so 
forth, those elements need to be available in the current frame and in 
any subsequent frame until another PCE is sent which changes the channel 
configuration.
It reads in the standard text:
"The configuration indicated by the PCE takes effect at the 
raw_data_block() containing the PCE. The number
of front, side and back channels as specified in the PCE must be present 
in that block and all subsequent raw_data_block()'s until a 
raw_data_block() containing a new PCE is transmitted.".
Having said this, one could think of a PCE which does not claim for any 
element. However, while this seems to be rather meaningless, it does 
also not obey the requirement that a raw_data_block() is a "block of raw 
data that contains audio data for a time period of
1024 samples".
Please note further:
"In the case of implicit channel mapping the number and order of SCEs, 
CPEs and LFEs and the resulting configuration may not change within the 
bitstream without sending a program_config_element(), i.e. an implicit 
reconfiguration is not allowed."
This again is not fulfilled if there is a PCE+TERMINATOR in one frame, 
but the following frame contains other elements - this change would 
require the availability of another PCE.
All text in quotation marks is available in ISO/IEC 13818-7:2005.
Hope this helps,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 6167
Fraunhofer IIS                  | Fax:   +49 9131 776 6099
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