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Lossless Audio Checker says mp3-derived files are clean

Hi all,

I have a set of mp3s that I converted both to WAV and FLAC (using dbpoweramp). I then used Lossless Audio Checker 2.0.6 to analyze the WAV and FLAC files. My results are below. Basically, 8 of the 11 files are identified as clean! Interestingly, the same 3 songs are detected as upsampled in both sets. Shouldn't Lossless Audio Checker say "upsampled" for ALL of these files, not just 3 of 8, considering they started out as mp3 files?

I am using Windows through Parallels on a Mac. I don't know if this matters but I thought I should mention it anyway.

Mike


Re: Lossless Audio Checker says mp3-derived files are clean

Reply #2
WAV and FLAC versions should always produce identical results, because the FLAC file is first converted back to WAV (PCM) before analyzing.

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #3
Quote from website. Emphasis is mine
Quote
The audio signal may have been upscaled (increasing the resolution), upsampled (increasing the sampling rate) or even transcoded from a lossy to a lossless format. Lossless Audio Checker analyzes lossless audio tracks and detects upscaling, upsampling and transcoding.

In other words, It doesn't seem that lossless audio checker has detected at all that they are transcodes. The only thing that it detected was that the bandwidth of some of them looks suspicious as being lower than what the sampling rate allows.  Of course, that can also be the consequence of lossy coding, but it is not a direct consequence.

More so, later in the same webpage
Quote
Work in progress
[...]
MP3 Transcoding: MP3 tracks transcoded to a lossless format
[...]

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #4
Lossy conversion can't be really detected simply by looking at the average power of frequency bands.
And it's perfectly normal for records to have almost nothing above ~20 kHz (sometimes even lower) to begin with.
Anyways, given this rate of false negatives, this tool can be confidently dismissed as worthless.

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #5
@magicgoose The algorithm in this program does not work that way and trying to detect mp3 transcoding is a misuse of this application.

I didn't know about the software, but it took less than 2 minutes to locate this file linked from their website: http://www.aes.org/tmpFiles/elib/20180405/17972.pdf

It details why It cannot detect random transcodes and why MP3 is a "work in progress". What it tries to detect is traces of quantization and "blockiness", and in order to do so, it needs to know the format that it is comparing it to.
In simple words: if it is not told how an MP3 file looks like, it will not know how to detect it.

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #6
[JAZ], very informative article. Thanks for that.

It would seem to me that detecting mp3 transcoding is the PRIMARY reason most people would use a tool like Lossless Audio Checker. However, it appears that this task may be easier said than done...I for sure can't do it so I'll just be patient and hope that this program develops to the point where it can reliably do this.

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #7
A couple years ago I experimented a bit with this program
Lossless rips of my CDs where labeled "upsampled"
In the context of LAC this simply means the the highest frequency on the recording is well below 1/2 the sample rate.
As I do have a lot of older acoustical (classical) recordings, you don't have much live above say 16 kHz

In your case (MP3) results will probably related to the bit rate. Say convert to 128 kbs, maybe 192 and they will probably all marked as upsampled as the heights are rolled off. At 256 or 320 LAC is probably fooled (just like us humans when we try to differentiate between high bit rate MP3 and CD)
TheWellTemperedComputer.com

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #8
@Roseval: If it is labeling lossless rips that you know for sure are good as upsampled, then that is worrying.

You brought up an interesting question, namely, is the false negative rate affected by the original bitrate of the mp3s?
The mp3s I used in my initial test were encoded with LAME using the -V2 switch (~192 kbps VBR). I used dbpoweramp to go from mp3 -V2 to mp3 -V5. I then converted the -V5 set of mp3s to WAV and analyzed the WAV files using LAC. The results are attached. As you can see, now ALL of the files are correctly detected as upsampled.

So, your hypothesis was correct Roseval! Nice job. This probably works in the other direction as well. That is, if the starting set of mp3s were encoded using -V0, then the WAV files made from that set of mp3s would probably all report clean on LAC and I probably wouldn't even be able to see the 3 upsampled hits I obtained using the WAVs derived from the -V2 set.

Re: Lossless Audio Checker says mp3-derived files are clean

Reply #9
I have read in that .pdf that I attached, that the upsampling detector was having a detection rate of 91%, so meaning that it wasn't working correctly in all cases, and that it could be difficult to fix.
Anyway, what is true is that it does not need to be half the bandwidth to be detected as upsampled.

Note: I see the link i posted above is not working now. Here is the page that has the actual link: http://www.aes.org/e-lib/browse.cfm?elib=17972

Anyaway, seeing that all the development of this application happened in 2016, I am not sure if it is going to evolve much more than what it is. (Noteworthy is that one of the version says that it fixed the upsampling detector)

 
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