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FLAC made from iTunes AAC LC - how to detect?
EpicForever
post Jan 18 2013, 19:22
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Hello.

Since about 3 years I buy music at Beatport and Junodownload, sometimes at Trackitdown. I prefer CD quality, so mostly I buy lossless files (WAV/AIFF at Beatport and FLAC at Juno). Unfortunately several times I found that some of files that I bought were just transcodes from some kind of lossy format. I assume that were transcodes from MP3, as it's most popular and bandwidth of audio signal was cut off at specific frequencies like 16,5 kHz (128kbps), 18,6 kHz (192kbps) or 20,1 (320kbps) kHz. It is easy to find such thing - I can just see spectrogram in foobar2k (with linear frequency scale selected) or use auCDtectTaskManager. But about month ago I bought one album at iTunes - just because of iTunes bonus track available only with full album. I investigated how it is encoded (I've never tried AAC at higher bitrates before) and I found that iTunes albums have full frequency scale, despite they are lossy... There's no low pass filtering known to me from MP3. I've transcoded whole album into FLAC with foobar and then I passed FLACs through auCDtectTaskManager. All files were concluded as 100% CDDA... I was shocked. So my question is - is there any other method to determine authenticity of lossless file? Any software tool that uses better algorithm than auCDtect, or just different algorithm tuned specifically for detecting HQ AAC transcodes?

Thank you in advance for any help.
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EpicForever
post Jan 20 2013, 10:58
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Thank you for your help. I see it's really tricky question... I thought that it is possible to find it using computations, because lossy coding is mathematical operation - thus it could be mathematically detected (with specific values of some advanced signal parameters for example). I see I was completely wrong... Fortunately it's very unlikely to happen, that Beatport, Juno, Audiojelly, Trackitdown or DJDownload will have transcode made from AAC (as it's much rarer format in music industry than MP3). Besides the fact that transcodes in webshops are generally very rare (but unfortunately occurs sometimes). MP3 transcodes are very easy to find, because no one uses specifically tuned parameters in industry scale coding of MP3 files for webshops. Standard presets for LAME (as "-h"/"-q2") always expose transcoding in spectrograms.
Because someone revoked TOS and I think he said about TOS#8 (ABX-ing) I have to explain something. I am not trying to find transcodes because of my "golden ears" (which I don't own - I hear quiet but constant hiss above ca. 14~15kHz). It's all about money. If I pay 1$ more at Beatport for AIFF then I have to get what I paid for - full frequency scale and not flat cut at 18,6 kHz. 192mp3 kbps mp3 can be easily obtained from the Net, without paying for it. If I pay 1,49GBP for 320kbps MP3 then I don't want quality of 128kbps MP3 encoded with "-fast" preset. That's why I asked for sotware tool - objective algorithm based on scientific knowledge. I needed easy to obtain and easy to recreate evidence wheather something is transcoded or not, WITHOUT judging weather it is audible or not.
My conclusion is that the answer for question from the topic is: as for today it is impossible to objectively detect fake lossless file in case it was transcoded from iTunes AAC LC files - currently there's no such software tool/algorithm that could find this.

This post has been edited by EpicForever: Jan 20 2013, 11:01
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db1989
post Jan 20 2013, 16:50
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QUOTE (EpicForever @ Jan 20 2013, 09:58) *
Because someone revoked TOS and I think he said about TOS#8 (ABX-ing) I have to explain something. I am not trying to find transcodes because of my "golden ears" (which I don't own - I hear quiet but constant hiss above ca. 14~15kHz).
I suspect Dynamic’s reference to the TOS was just (A) to note that samples of copyrighted material >30 s are not allowed by TOS #9, and (B) to suggest that others try ABXing, rather than saying that you had said anything controversial. smile.gif

Your reasoning that retailers should not be able to sell ‘lossless’ content that is transcoded from lossy is totally correct, of course!
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Dynamic
post Jan 21 2013, 11:22
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Yes, I did mean TOS#9 that if you post a sample it should be no more than 30 seconds for copyright reasons.


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Dynamic – the artist formerly known as DickD
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