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Fhg's "inverse Decoder" - Or Encoder Detector
Ivan Dimkovic
post Sep 8 2002, 17:48
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I've just read interesting AES article (#5256) regarding FhG's "Inverse Decoder" tool..

It seems that this tool is able to:

- Analyse the encoding window size (1152 / 1024 / ... samples)
- Analyse the short block usage
- Analyse the bit reservoir usage
- Analyse the M/S and I/S usage
- Analyse the quantization step size
- Analyse the TNS usage

With this information, they could detected what algorithm was used, and with database of known properties - what encoder and version, as well as encoding settings were used only by analysing of decoded PCM file!!

This is very interesting and straightforward algorithm for transcoding without losses, and they claim that they can even reconstruct MP3/AAC bitstream out of decoded WAV without degradation of sound quality!

But, there are some other usages, like this:

QUOTE
The increasing popularity of low bitrate audio coding schemes comes along with a significant number of undesirable activities, such as the illegal distribution of copyrighted audio over the Internet and the availability of encoders illicitly using patented algorithms. In this context, the inverse decoder can be an interesting tool both for forensic purposes as well as for investigations about violations of IPRs.



So, the tool could be used for detecting unlicensed MP3 and AAC encoders, or illegally obtained MP3 encoders smile.gif
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Frank Klemm
post Sep 8 2002, 19:11
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QUOTE (Ivan Dimkovic @ Sep 8 2002 - 06:48 PM)
I've just read interesting AES article (#5256) regarding FhG's "Inverse Decoder" tool..

It seems that this tool is able to:

- Analyse the encoding window size (1152 / 1024 / ... samples)
- Analyse the short block usage
- Analyse the bit reservoir usage
- Analyse the M/S and I/S usage
- Analyse the quantization step size
- Analyse the TNS usage

With this information, they could detected what algorithm was used, and with database of known properties - what encoder and version, as well as encoding settings were used  only by analysing of decoded PCM file!!

This is very interesting and straightforward algorithm for transcoding without losses,  and they claim that they can even reconstruct MP3/AAC bitstream out of decoded WAV without degradation of sound quality!

But, there are some other usages, like this:

QUOTE

The increasing popularity of low bitrate audio coding schemes comes along with a significant number of undesirable activities, such as the illegal distribution of copyrighted audio over the Internet and the availability of encoders illicitly using patented algorithms. In this context, the inverse decoder can be an interesting tool both for forensic purposes as well as for investigations about violations of IPRs.



So, the tool could be used for detecting unlicensed MP3 and AAC encoders, or illegally obtained MP3 encoders smile.gif

IMHO a tool is possible which determines the used encoder from the decoded PCM data
(which is much more difficukt than determine this from the data stream).

But to determine the used encoder is much much much more simple than a inverse decoder.
To my mind an inverse decoder is not possible for current computers, and actually not necessary.
If you need an inverse decoder you have actually a coordination problem which can also
be solved by solving this corrdination problem.

BTW there are also aproaches for inverse decoders using watermarking in the decoder.
Especially with current pop music this works fine, because you don't need to 1...2 LSBs of the
decoded PCM signal.


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-- Frank Klemm
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