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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 18 2013, 21:47
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Group: Members
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I know. But lowpass filtering used as element of production/mastering is mostly gradual - slopes aren't steep, transition band is wide. Spectrum peaks on spectrogram just diffuse. But if I see sharp edge on spectrogram, at 16,5 kHz it must be lossy compression artifact... Some cut off frequencies are specific for certain bitrates. Other thing is that I knew tracks that turned out to be transcodes before I bought them - from mixed CDs (holded in my hands) or vinyl rips and I knew that they got full frequency band.
I also tried "-k" switch in LAME 3.96/3.97beta and I know that MP3 can be done without filtering. But I didn't know that frequency response is the only factor used in analysis of losslessness of audio files... Are you sure that there isn't anything else that can be... computed? extracted? from signal that could tell us weather file is lossy or lossless (with some probability of course)?

This post has been edited by EpicForever: Jan 18 2013, 21:48
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