[M4IF Technotes] Re: Question on AAC object type

Ralph Sperschneider sps iis.fhg.de
Wed Jul 2 11:53:17 EDT 2003


Dear Anil,
Anil Kumar wrote:
> Hi Ralph,
> 	Thanks for the info. I was going through the meeting output documents, and there seems to have been a significant change in the test sequences from the time I started working on AAC (1.5 yrs ago) and rightly so, as I understand, MPEG is a living thing!!.
That is indeed true.
> The doc w5373 (study on MPEG-2 AAC Conformance) written by you has a detailed description of all the MPEG-2 test cases. Is there any doc similar to this one, but on the MPEG-4 test cases?? 
Yes, the most recent MPEG-4 Conformance document is N5457.
> 	I also found a MPEG-2 reference softwere (refSoft_mpeg2aac_20030616.tgz) on the ftp://ftp.tnt.uni-hannover.de/incoming/cvs/ directory, but this code unfortunately doesn't work.
What exactly is not working? I am quite sure that this software works properly.
> Where can I find a reference MPEG-2 AAC decoder source (I think the MPEG-4 source is supposed to decode the MPEG-2 streams also, but it doesn't seem to recognize a .aac extension at all!).
This is since the aac extension doesn´t mean anything. AAC data are usually 
stored into a container. This might be
  - adif or mp4ff for storage
  - adts or latm/loas for transmission
> 	Also, is there a latest doc collating all the significant changes that has happened so far on MPEG-2/4? 
For MPEG-2 AAC, the latest documentation is N5374.
For MPEG-4 Audio, there is currently another merge activity going on to 
integrate ISO/IEC14496-3:2001, ISO/IEC14496-3:2001/Cor.1:2002 (N4760) and 
ISO/IEC 14496-3:2001 FDAM 1 (the new Extension toward Spectral Band Replication, 
N5570) and you can expect an updated document after the next MPEG meeting.
> I could not find the CRC polynomial in the 13818-7 doc, but there seems to be test sequences to test this.
The CRC polynomial is reused from MPEG-1 Audio. It is originally specified in 
ISO/IEC 11172-3 (MPEG-1 Audio), subclause 2.4.3.1 as G(X)=X16+X15+X2+1.
> I think most of my problems will be solved once I get hold of a reference software that can decode these sequences (Or am I wrong??) 
The CRC derivation in the multichannel decoder (mp4audec_mc   mp4mcDec) of the 
MPEG-4 audio reference software works properly.
> 
> Hoping for some inputs on these questions,
> Anil

Hope my comments help.
Best regards,
Ralph
PS: In the future, please avoid to ask similar questions several times in 
different e-mails.
> -----Original Message-----
> From: Ralph Sperschneider [mailto:sps   iis.fhg.de]
> Sent: Friday, June 27, 2023 10:22 PM
> To: Anil Kumar
> Cc: technotes   lists.m4if.org
> Subject: Re: Question on AAC object type
> 
> 
> Dear Anil,
> 
> there is unfortunately no such thing as a newsletter. The only possibly way I am 
> aware of is to stay tuned to what is going on during the MPEG meetings. This can 
> be done by participation, by screening the output documents of each meeting and 
> in joining the interested ad-hoc groups.
> 
> All published standards are available here: 
> http://www.iso.org/iso/en/CatalogueListPage.CatalogueList (for MPEG-4, just type 
> the standard number 14496). Non-publicated standards are only available as 
> meeting output documents, thus only available to MPEG members.
> 
> Hope this helps,
> 
> Ralph
> 
> 
> Anil Kumar wrote:
> 
> 
>>Hi Ralph, All,
>>	Regarding my previous question, most of my doubts were clarified thanks 
> 
>  > to Mr. Hans-Jürgen who pointed me to the audiocoding.com's wiki... apparently,
>  > the ADTS header format was changed a little in Dec 2002 (according to the
>  > wiki). However, I have one question... how do i keep track of such changes??
>  > being an ISO member, is there any way we can get a newsletter or something
>  > which intimates us of all these changes?? where can I find the latest
>  > corrigendums/ammendments of ISO??
> 
>>Thanks,
>>Anil
>>
>>-----Original Message-----
>>From: Ralph Sperschneider [mailto:sps   iis.fhg.de]
>>Sent: Friday, June 27, 2023 12:24 AM
>>To: Hans-Juergen Bardenhagen
>>Cc: technotes   lists.m4if.org
>>Subject: [M4IF Technotes] Re: Question on AAC object type
>>
>>
>>
>>
>>Hans-Juergen Bardenhagen wrote:
>>
>>
>>
>>>On Wednesday, 25.06.03, 10:56 Anil wrote:
>>>
>>>
>>>
>>>
>>>>1. Can I get an ADTS or ADIF as the transport layer for an MPEG-4
>>>>stream?? (i guess it is yes, but just to confirm).
>>>
>>>
>>>Not quite, because ADTS and ADIF are the header types of an AAC file  
>>>(not a "transport layer" in the usual sense like e.g. HTTP or RTSP) that  
>>>has not been multiplexed to a MP4 container yet. Inside this container  
>>>the raw AAC bitstream does not have any headers anymore, because these  
>>>informations (MPEG-x version, bitrate etc.) are handled by the MP4 file  
>>>header then.
>>
>>
>>Maby some more information:
>>- adts ( audio data transport stream ) is intended for transport. It was 
>>originally considered as become a "layer 4" (beside layer[123] as specified in 
>>ISO/IEC13818-3).
>>- adif ( audio data interchange format ) is intended for storage (it has just 
>>one header at the beginning, followed by the raw_data_stream())
>>
>>Both formats are specified in MPEG-2 and MPEG-4. There are some extensions in 
>>MPEG-4 to allow the signalling of AAC LTP in addition to AAC Main, AAC LC, AAC 
>>SSR and to distinguish between MPEG-2 and MPEG-4. However, these are AAC 
>>specific formats.
>>
>>For MPEG-4 Audio transport it is recommended to use LOAS/LATM (low overhead 
>>audio stream, low overhead audio transport multiplex).
>>
>>All these formats are specified in ISO/IEC14496-3.
>>
>>Best regards,
>>
>>Ralph
>>
>>
>>
>>>>2. What is the object type index for AAC LC in case of MPEG-2 and
>>>>MPEG-4. Does this change depending on the transport layer(ADTS or ADIF
>>>>or GA). 3. How do I distinguish b/w MPEG-2 and MPEG-4 in case of ADIF/
>>>>ADTS (Is it the bitstream_type and adts_id) 4. Where can I get a
>>>>complete documentation of all these things??
>>>
>>>
>>>A short overview of the ADTS (and ADIF) header structure is available in  
>>>the Wiki of Audiocoding.com:
>>>
>>>http://www.audiocoding.com/wiki/index.php?page=ADTS
>>>
>>>If you need more informations than this, you can download the  
>>>specifications of MPEG-2 and/or MPEG-4 AAC from the MPEG homepage or the  
>>>MPEG Audio Subgroup (links are mentioned in the Wiki, too). This also  
>>>refers to the question of the MP4 file format, by the way.
>>>
>>>ZZee ya, Hans-Jürgen
>>>
>>>_______________________________________________
>>>Technotes mailing list
>>>Technotes   lists.m4if.org
>>>http://lists.m4if.org/mailman/listinfo/technotes
>>
>>
> 

-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
FhG IIS                         | Fax:   +49 9131 776 398
Am Wolfsmantel 33               | mailto:sps   iis.fhg.de
D 91058 Erlangen                | http://www.iis.fhg.de/amm/


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