[Matroska-devel] CodecSpecs

Santiago Jimeno sjimeno at ya.com
Thu Dec 30 12:38:27 CET 2010

I am writing a program that reads information of audio files. Also it reads and writes the diverse Tags. It is centered specifically in the audio information. Now I am working with Matroska.

As for audio information, specifications in CodecSpecs are quite clear in some cases (MPEG, Vorbis, MS/ACM, Flac, WavPack, etc)
But in another cases it is not so well explained, although I have been able to write code:

AAC-MP4: there it says that it is stripped from ADTS header. Really, in both cases of 2 or 5 bytes size it follows the specifications ISO-IEC 14496-1 Systems (AudioSpecificConfig). Another thing is that the first 2 bytes are common for ADTS and other formats.

Real Audio: there is not information. Here, CodecPrivate follows the specifications of "MDPR" Media Properties -> Specific Data (audio/x-pn-realaudio or audio/x-pn-multirate-realaudio). Although in this case is contemplated the possibility of including "REAL/RALF" (Real Lossles), the specifications are possibly different: "audio/x-ralf-mpeg4-generic." But I have not been able to get the sample to study it with any program that encodes Matroska that I have been able to find.

Maybe it would be interesting for other people to add this information in CodecSpecs

Now the problems:
AC3:  I have not been able to find in Block or CodecPrivate the mentioned Byte with fscod and frmsizecod data. 
QuickTime:  In spite of their resemblance with AAC-MP4, I have not found the applied format to CodecPrivate data and CodecSpecs don't says anything about this.

Can somebody inform me about these 2 problems?

Now the suggestions:
I miss the data of Bitrate and AudioSize. Apart of case when the codec specifies it (MPEG, MS/ACM) there is not bitrate information in any place. AudioSize can be easily calculated from bitrate. Also to the reverse, but then it would be necessary parsing all blocks.
I know that it is not outstanding to decode the file but it is important for the information. There is not a place to write this information In the future? This can fill one byte or less inside of the big space filled for container Matroska 

In a next message I will speak of my experience in Matroska Taging.

Regards. Santiago
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