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FLAC made from iTunes AAC LC - how to detect?
post Jan 18 2013, 19:22
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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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post Jan 19 2013, 12:50
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Sometimes listening to the SIDE channel signal of mid-side encoding (i.e Left - Right sample values), will reveal lossy encoding effects, as can using very clean but extreme EQ to try to de-mask the varying quantization noise at certain frequencies. However, even that isn't definitive, especially with the number of legitimate DSP effects around, some of which will leave signatures that would be fully masked in normal listening (especially de-noise algorithms).

If you think you've found one, I guess you COULD post a short (<30s) lossless sample from a certain time in a certain track and ask for people with the CD version to bit-compare or upload the equivalent section of a CD release for comparison in compliance with the TOS.
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