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Detecting whether a 24-bit file has been upconverted from 16-bit?
vincefalks
post Nov 6 2012, 05:50
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I downloaded a FLAC album from online web store and it appears that the files I've downloaded are 24-bit. it's not a hd web store like hdtracks, it's generally a CD 16-bit 44.1kHz store with either FLAC or MP3 to choose from in its offerings.

I'm wondering whether the 24-bit files are real and were supplied as 24-bit from the label and they just didn't label it correctly on the download page, or whether there was an encoding error or just somehow it made its way from 16-bit files to 24-bit FLACs.

Is there a way I can definitively tell or analyze it (looking at spectral maybe) to see if it's been upconverted? Are there certain algorithms and 'good jobs' that can be done to make its 16->24 upconversion undetectable?

happy to upload a sample if it helps.

thanks
vince
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2Bdecided
post Nov 6 2012, 12:45
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There's no way of knowing for sure. Unlike low bitrate mp3 encoding, converting to 16-bits leaves no tell-tale signature.

Once "up-converted" to 24-bits, it's trivial to fill the bottom 8 bits with noise (any operation, even a tiny gain-change, will fill them with some non-zero values anyway). Noise-shaped dither at the 16-bit level could be a tell-tale, but it might not have been used, and if it was, you can use a filter to drop the level to something benign looking. A similar rising noise spectrum (though only above 20k, and at a lower level) is also found on DSD-sourced recordings - so you don't need to be that careful - just dropping it to DSD-like levels will do the trick.

A recording with analogue silence won't give anything away, but a recording with 16-bit dithered digital silence could alert some people. In this case, it's easy to replace it with 24-bit digital fades and silence.

So, as I said, there's no way to know.


If you had both the 24-bit and 16-bit versions, you could show that the 16-bit version was probably generated from the 24-bit version (if it was), rather than the other way around - but you couldn't prove that the 24-bit version itself wasn't generated from a previous 16-bit version.

So, if version A=24-bits and version B=16-bits, you could show that B was created from A, rather than A being created from B. But you cannot show that A was not created from U, a 16-bit version that you don't have access to. That possibility always exists, and you cannot disprove it.

(I am assuming a real recording in a normal recording studio with real microphones, which will almost always result in a noise floor that's far above the 16-bit dithered noise floor. If you had an entirely synthetic recording, it could have an arbitrarily low measurable noise floor, far below 16-bit dither, and here you could very easily prove whether it was native 24-bits, or upconverted from 16-bits).

Cheers,
David.
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- vincefalks   Detecting whether a 24-bit file has been upconverted from 16-bit?   Nov 6 2012, 05:50
- - nu774   Try something like the following to see if wasted ...   Nov 6 2012, 06:31
|- - skamp   QUOTE (nu774 @ Nov 6 2012, 06:31) Try som...   Nov 6 2012, 12:06
|- - julf   QUOTE (skamp @ Nov 6 2012, 12:06) I used ...   Nov 8 2012, 16:33
|- - Sparktank   QUOTE (nu774 @ Nov 6 2012, 06:31) CODEfla...   Apr 13 2013, 01:47
- - bandpass   QUOTE (vincefalks @ Nov 6 2012, 04:50) Ar...   Nov 6 2012, 09:11
- - spoon   >Are there certain algorithms and 'good job...   Nov 6 2012, 11:47
- - nu774   BTW, I think just a slight gain scaling from the o...   Nov 6 2012, 12:31
- - greynol   You can add noise or simply use Audacity which can...   Nov 6 2012, 12:42
- - 2Bdecided   There's no way of knowing for sure. Unlike low...   Nov 6 2012, 12:45
- - vincefalks   Thanks guys (and yeah thanks greynol for pointing ...   Nov 6 2012, 16:29
|- - skamp   QUOTE (vincefalks @ Nov 6 2012, 16:29) Th...   Nov 6 2012, 17:40
|- - vincefalks   QUOTE (skamp @ Nov 6 2012, 17:40) QUOTE (...   Nov 6 2012, 18:44
||- - skamp   QUOTE (vincefalks @ Nov 6 2012, 18:44) it...   Nov 6 2012, 18:58
||- - Porcus   QUOTE (vincefalks @ Nov 6 2012, 19:44) it...   Nov 8 2012, 17:33
|- - Destroid   Interesting discussion. I hope the market does not...   Nov 6 2012, 23:21
|- - Porcus   QUOTE (Destroid @ Nov 7 2012, 00:21) I ho...   Nov 8 2012, 17:38
- - greynol   Don't blame flac.exe. It will not take 16-bit...   Nov 6 2012, 17:23
|- - vincefalks   QUOTE (greynol @ Nov 6 2012, 17:23) Don...   Nov 6 2012, 17:47
- - greynol   6dB of gain was applied to the master with no furt...   Nov 6 2012, 17:42
- - greynol   You would really have to go out of your way to do ...   Nov 6 2012, 17:59
- - 2Bdecided   An even smarter analysis can figure out which 24-b...   Nov 8 2012, 17:16
- - pdq   Let's not forget that a file that was up-conve...   Nov 8 2012, 18:27
|- - 2Bdecided   QUOTE (pdq @ Nov 8 2012, 18:27) Let's...   Nov 9 2012, 10:33
|- - julf   QUOTE (pdq @ Nov 8 2012, 18:27) Let's...   Nov 9 2012, 13:39
- - pdq   ...and it makes sense that if the tracks had been ...   Nov 9 2012, 15:11
- - Arnold B. Krueger   QUOTE (vincefalks @ Nov 5 2012, 23:50) I ...   Nov 9 2012, 16:01
- - skamp   Maybe Greynol can help?   Apr 13 2013, 07:41
- - Case   Here's a quick Windows .cmd script trying to d...   Apr 13 2013, 09:39
|- - Sparktank   QUOTE (Case @ Apr 13 2013, 01:39) Here...   Apr 13 2013, 09:59
- - Case   It's meant to be used from command prompt when...   Apr 13 2013, 10:11
- - mjb2006   Thanks for the .cmd version. I suggest modifying ...   Apr 13 2013, 10:40
- - Arnold B. Krueger   QUOTE (vincefalks @ Nov 6 2012, 00:50) I ...   Apr 13 2013, 16:12
|- - skamp   QUOTE (Arnold B. Krueger @ Apr 13 2013, 17...   Apr 13 2013, 17:28
- - greynol   I don't see the point in adding dither when in...   Apr 13 2013, 16:18
|- - Arnold B. Krueger   QUOTE (greynol @ Apr 13 2013, 11:18) I do...   Apr 13 2013, 22:22
|- - Kees de Visser   QUOTE (Arnold B. Krueger @ Apr 13 2013, 22...   Apr 14 2013, 15:23
- - BearcatSandor   Anyone know how to adjust Skamp's script to wo...   Apr 14 2013, 01:00
- - MikeFord   How would this technique rate a file consisting of...   Apr 16 2013, 21:11


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