[M4IF Technotes] audio help

[Bernhard Grill]

Harisha wrote:
> 
>  Hi All
>  
> I have some question on MPEG-4 audio
>  
> 1 what is the use program config element in GASpecificConfig,Wheter 
> these information is necessary for decoding raw data block.As standard 
> says that any PCE in raw data block can be ignored.

Hallo,
Yes, in a MPEG-4 AAC System you must use the PCE in the GASpecificConfig (Part 
of AudioSpecificConfig). Legacy MPEG-2 AAC streams might carry PCEs in the 
stream. These PCEs can be parsed by an MPEG-4 AAC decoder but the decoder is not 
required to evaluate these PCEs for any purpose.
PCEs in MPEG-2 Streams might be carried for several reasons.
1.) Allow to tune into a running (ADTS) stream.
In MPEG-4 you would solve this by repeating the AudioSpecificConfig in whatever 
transport format you are using (e.g. MPEG-4 over MPEG-2 Systems) or you dou 
don't need that (e.g. ISMA, MPEG-4 over IP, 3GPP, RFC 3016) because 
AudioSpecificConfig is transmitted over different means (e.g. SDP at the time 
the session is initiated on a reliable channel).
2.) Dynamic reconfiguration of the number audio channels (e.g. changing from 2 
channels to 5 channels or change of sampling rate. AFAIK this is not used 
anywhere. In MPEG-4 you would use MPEG-4 concepts to do this (instantiate a new 
object and remove the old one).
Best regards,
Bernhard Grill
>  
>  
> 
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