[M4IF Technotes] Re: Non meaningful Window sequence

[Ralph Sperschneider]

Dear Apoorva,
there seems to be no simple way of correcting non-meaningful window sequence 
transitions (which might e.g. occur if cut opperations are performed on an AAC 
bitstream). Thus, the standard requires just to decode the frames and perform 
the overlap-and-add procedure as usual. That is what the reference software is 
intended to do and how the conformance test sequences are generated.
Best regards,
Ralph
Apoorva Ankad wrote:
> Dear Experts,
> 
>  
> 
> When we get some non meaningful sequence in the bit stream, then are we 
> suppose to correct the window sequence and, then decode or we have to 
> decode the sequence as we obtain?
> 
>  
> 
> *According to the standard,*
> 
>  
> 
> The meaningful block transitions are as follows:
> 
>  
> 
> from ONLY_LONG_SEQUENCE to { LONG_START_SEQUENCE
> 
>                                                                        
> ONLY_LONG_SEQUENCE
> 
>  
> 
> from LONG_START_SEQUENCE to { LONG_STOP_SEQUENCE
> 
>                                                                 
>           EIGHT_SHORT_SEQUENCE
> 
>  
> 
> from LONG_STOP_SEQUENCE to { LONG_START_SEQUENCE
> 
>                                                                        
> ONLY_LONG_SEQUENCE
> 
>  
> 
> from EIGHT_SHORT_SEQUENCE to { LONG_STOP_SEQUENCE
> 
>                                                                 
>            EIGHT_SHORT_SEQUENCE
> 
>  
> 
> In addition to the meaningful block transitions the following 
> transitions are possible:
> 
>  
> 
> from ONLY_LONG_SEQUENCE to { LONG_STOP_SEQUENCE
> 
>                                                                        
> EIGHT_SHORT_SEQUENCE
> 
>  
> 
> from LONG_START_SEQUENCE to { LONG_START_SEQUENCE
> 
>                                                                          
> ONLY_LONG_SEQUENCE
> 
>  
> 
> from LONG_STOP_SEQUENCE to { LONG_STOP_SEQUENCE
> 
>                                                                        
> EIGHT_SHORT_SEQUENCE
> 
>  
> 
> from EIGHT_SHORT_SEQUENCE to { LONG_START_SEQUENCE
> 
>                                                                         
>    ONLY_LONG_SEQUENCE
> 
>  
> 
> I am getting some difference with the reference PCM bit streams when I 
> consider streams with non meaningful window sequence (AL03).
> 
> If the non meaningful window sequence is encountered then what is to be 
> done?
> 
>  
> 
> Thanking you,
> 
> *Apoorva M. Ankad*
> 
>  
> 

-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
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