Detecting whether a 24-bit file has been upconverted from 16-bit? Options

[vincefalks]

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

[2Bdecided]

An even smarter analysis can figure out which 24-bit (or even 16-bit) values are used, and how often. If there are some values which are unused (e.g. 0 is, 1,2,3,4,5,6 aren't, 7 is, 8,9,10,11,12,13 aren't, etc etc), you have a clear case of undithered conversion and scaling. If values (other than zero) are used disproportionately to those around them, you probably have some strange distortion or scaling. If values at or near full scale are over-used compared with those around them, you probably have clipping.

This technique still won't catch 16>24-bit conversions which added noise in the 8 LSBs, and will often miss problems if the occur before a final stage of (especially noise-shaped) dither. But it's surprising what it does catch.

It's hard to automate the detection of something wrong via this analysis - you mostly just have to eyeball the distribution. For 24-bit audio, it's only meaningful for long pieces of music, given the 16M different possible sample values. With 16-bit audio, you can get meaningful results with more than a minute or so of music.

Cheers,
David.