HydrogenAudio

Lossless Audio Compression => FLAC => Topic started by: XeR0 on 2011-06-29 19:58:59

Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-29 19:58:59
I know the topic title sounds absolutely absurd but hear me out. I've tested my FLAC collection by encoding them into V0 MP3. I then decoded the MP3 back to WAV and then compressed it in FLAC. My question is: unless you've ripped the files yourself, how would you know the FLAC file you have is ACTUALLY lossless instead of an MP3 converted into FLAC? I've used the TEST option in FLAC frontend and it doesn't give a result. I have used Audiotester and it does say the file failed because it's TRUNCATED.

Bottom-line: Is there a sure-fire way of knowing that a FLAC file is truly lossless and not a derivative of a lossy file?
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-29 20:00:50
Bottom-line on a "sure-fire way": no.
Title: True FLAC vs. Fake FLAC
Post by: gottogo99 on 2011-06-29 20:05:58
It's not guaranteed, but if you look at a file in spectral view and there is a sharp cutoff at say 15kHz, there's a very good chance the file was lossy.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-29 20:10:31
...recorded from the radio, or simply low-pass filtered.

There is also the real possibility that the CD was sourced from such data.  There have been examples of this happening with legitimate CDs posted on this forum and elsewhere.
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-29 20:19:10
It's not guaranteed, but if you look at a file in spectral view and there is a sharp cutoff at say 15kHz, there's a very good chance the file was lossy.

Yeah, I noticed the same thing and that was what I was using to make sure the FLAC's weren't lossy.

...recorded from the radio, or simply low-pass filtered.

There is also the real possibility that the CD was sourced from such data.  There have been examples of this happening with legitimate CDs posted on this forum and elsewhere.

Which is why I wanted to know if there was a way. In any case at least there are methods that work 90% of the time. Thanks for your replies.
Title: True FLAC vs. Fake FLAC
Post by: Rotareneg on 2011-06-29 21:04:35
Looking at a spectrogram of the audio is a good way to tell, but you need to look for more than just a low-pass filter.

Here are three spectrograms made with SoX that show what to look for:

First, the lossless original:

(http://farm6.static.flickr.com/5160/5885034567_dd919f4fd5_z_d.jpg) (http://www.flickr.com/photos/inghramjp/5885034567/in/photostream)

Next, Lame MP3 at -V5:

(http://farm7.static.flickr.com/6023/5885034361_e4e51ec1c8_z_d.jpg) (http://www.flickr.com/photos/inghramjp/5885034361/in/photostream/)

And lastly, Vorbis at -Q4:

(http://farm7.static.flickr.com/6011/5885034441_b49a8664a0_z_d.jpg) (http://www.flickr.com/photos/inghramjp/5885034441/in/photostream/)

The main thing I look for is the "holes" in the audio that the lossy compressors psycoacoustic model determined to be inaudible due to auditory masking.
Title: True FLAC vs. Fake FLAC
Post by: ExUser on 2011-06-29 21:29:29
There is also the real possibility that the CD was sourced from such data.  There have been examples of this happening with legitimate CDs posted on this forum and elsewhere.
My personal favourite anecdote was buying a music CD by Orbital from Hong Kong. It was silver and pressed, but had no catalog number and was audibly MP3 sourced. I later found the MP3s that they had sourced from. The artifacts matched.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-29 21:35:14
That's a good one.

I was referring to some of the titles on the Century Media label available on Amazon.
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-29 21:39:02
Rotareneg: Yeah, I use Sonic Visualizer and just go over to look for the holes you specified. Just ask greynol and he'll/she'll tell you how I was misguided by looking at spectrographs as the determining factor for audio quality instead of doing a proper ABX test

There is also the real possibility that the CD was sourced from such data.  There have been examples of this happening with legitimate CDs posted on this forum and elsewhere.
My personal favourite anecdote was buying a music CD by Orbital from Hong Kong. It was silver and pressed, but had no catalog number and was audibly MP3 sourced. I later found the MP3s that they had sourced from. The artifacts matched.

That sucks. The only only to really know (to my knowledge) if a CD is authentic is to buy it from the publisher themselves.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-29 21:46:29
Just ask greynol and he'll/she'll tell you how I was misguided by looking at spectrographs as the determining factor for audio quality instead of doing a proper ABX test

Nice non sequitur.

The only only to really know (to my knowledge) if a CD is authentic is to buy it from the publisher themselves.

I guess Century Media doesn't qualify as a publisher?
Title: True FLAC vs. Fake FLAC
Post by: DonP on 2011-06-29 21:51:00
My personal favourite anecdote was buying a music CD by Orbital from Hong Kong. It was silver and pressed, but had no catalog number and was audibly MP3 sourced. I later found the MP3s that they had sourced from. The artifacts matched.

That sucks. The only only to really know (to my knowledge) if a CD is authentic is to buy it from the publisher themselves.


Don't ya just hate it when a pirate publisher, doing enough volume for pressings,  is too cheap to buy the real CD for their source material?
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-29 22:12:10
The only only to really know (to my knowledge) if a CD is authentic is to buy it from the publisher themselves.

I guess Century Media doesn't qualify as a publisher?

What I meant was this: Let's say for instance I wanted to buy an album called "Tales of the Inexpressible". A trustworthy publisher would be "Twisted Records" instead of "World Music of New Age". A good amount of research would show that the composers of the album founded Twisted Records and sell their music there. Therefore, the only real way to make sure that you're getting an authentic CD, would be to buy it from Twisted Records and not from any other obscure publisher.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-29 22:31:30
You're operating under the assumption that "Twisted Records" is competent in providing a CD that was sourced from lossless data.
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-29 22:40:07
You're operating under the assumption that "Twisted Records" is competent in providing a CD that was sourced from lossless data.

Given the fact that "Twisted Records" is run by the very composers of the album in question, I can safely assume the CD's I buy from them are sourced from lossless data. As a matter of fact, the closest you can get to the source is by downloading/buying from the creators themselves. At the end of the day however, you can only do so much before it's out of your hand.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-29 22:48:11
I regret to say I don't find that all composers are very tech-savvy.  As such, I wouldn't be so certain that you assumption is all that safe.
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-29 22:55:36
I regret to say I don't find that all composers are very tech-savvy.  As such, I wouldn't be so certain that you assumption is all that safe.

I don't disagree at all. Like I said, you can only do so much before you reach a point where things are out of your hand. If the composer him/her-self doesn't know much about tech, there's no one else in the world that can do a better job given that the source file is with the composer and the only access to the "source" elsewhere is through the CD albums they release which is of questionable quality. Therefore, the best you can do is to buy it from the publisher/composer themselves.

To contribute to the original topic, I would like to add that Audiotester from vuplayer.com has indicated that the "fake" FLAC file failed the test. So, besides checking the spectrographs of each FLAC file, Audiotester is a much more convenient alternative.
Title: True FLAC vs. Fake FLAC
Post by: ExUser on 2011-06-29 23:09:29
I regret to say I don't find that all composers are very tech-savvy.  As such, I wouldn't be so certain that you assumption is all that safe.
Knowing a bit more about Twisted Records would be reassuring. Simon Posford, the founder of Twisted Records and the primary member of Shpongle, is known for deeply technical audio wizardry.
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-30 00:01:08
Knowing a bit more about Twisted Records would be reassuring. Simon Posford, the founder of Twisted Records and the primary member of Shpongle, is known for deeply technical audio wizardry.

My point exactly. However, I'm sure some people haven't had the pleasure of knowing Simon Posford as well as his expertise in audio production. In which case, I didn't feel was necessary to elaborate. The bottom-line: The best you can do to make sure you've got the best is to get it from the source or the closest thing to the source.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-30 00:28:46
A good amount of research would show that the composers of the album founded Twisted Records and sell their music there.

...and would apparently instill a high degree of confidence that a CD obtained from them directly would be lossless; I clearly hadn't conducted a good amount of research.

After a bit of googling, I'm sure knowing him would be pleasurable, but I'm a bit skeptical I would derive much pleasure in his expertise in audio production.
Title: True FLAC vs. Fake FLAC
Post by: d_headshot on 2011-06-30 05:05:47
I guess Century Media doesn't qualify as a publisher?


Not a very good publisher imo since they've consistently brickwalled all of their releases that I've bought 
Title: True FLAC vs. Fake FLAC
Post by: Goratrix on 2011-06-30 09:38:14
Also, when you think about it, the OP's question really doesn't make sense. The result of a FLAC compression is a lossless image of the source. In case you input a mp3-compressed audio, does that make the resulting FLAC "non-lossless" or "fake"? No, it doesn't. It makes it a perfect lossless version of the source material, regardless of what the source material is :-)))))

On another note, if we are talking about identifing "fake FLAC" in files obtained by...ehm, questionable means, then AccurateRip comes in quite handy 
Title: True FLAC vs. Fake FLAC
Post by: xnor on 2011-06-30 15:17:04
As well as creating flac files by ripping CDs you actually bought. 
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-30 16:50:00
On another note, if we are talking about identifing "fake FLAC" in files obtained by...ehm, questionable means, then AccurateRip comes in quite handy 

Yeah, 'cause there's no way anyone would ever submit downloads back to the AR database. 
Title: True FLAC vs. Fake FLAC
Post by: Goratrix on 2011-06-30 17:28:22
On another note, if we are talking about identifing "fake FLAC" in files obtained by...ehm, questionable means, then AccurateRip comes in quite handy 

Yeah, 'cause there's no way anyone would ever submit downloads back to the AR database. 


Did I say it's 100% accurate? No, I didn't. Is it helpful and useful most of the time? Yes it in fact is, as the volume of fake and poisoned entries is very small compared to the geniune entries.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-30 17:34:53
You get a match against a verified yet errant submission from a download and it doesn't matter how many submissions from legitimate sources are made.
Title: True FLAC vs. Fake FLAC
Post by: Goratrix on 2011-06-30 17:55:51
You get a match against a verified yet errant submission from a download and it doesn't matter how many submissions from legitimate sources are made.


Yes, I don't deny that. But it's a question of statistics and trust. What I've been doing for the last year or so is a little test. For every CD that I bought (around 50 albums in 2010, a mix of old and new releases) I also downloaded a lossless rip (before someone goes "pirate!", that's actually legal where I live). I ripped my original and compared it to my downloaded copy, and looked at the AR results, also in a few months time again, after AR got populated, if it was a new album. The result was that ALL of those downloaded rips were genuine (same as my original CD), except two, and even those could have been a US vs. EU pressing/mastering difference and not a fake.

Is that a conclusive scientific test? No it isn't. But it's enough for me to trust that statistically, the majority of illegal lossless rips are in fact geniune.

It's the same dilemma as when doing a rip yourself, you never know if it's 100% "perfect". But statistically, most of them are, so why worry about the 1% or 5% (or whatever) that aren't? If someone is so worried, they should buy the original, as always.
Title: True FLAC vs. Fake FLAC
Post by: XeR0 on 2011-06-30 17:56:30
Also, when you think about it, the OP's question really doesn't make sense. The result of a FLAC compression is a lossless image of the source. In case you input a mp3-compressed audio, does that make the resulting FLAC "non-lossless" or "fake"? No, it doesn't. It makes it a perfect lossless version of the source material, regardless of what the source material is :-)))))

On another note, if we are talking about identifing "fake FLAC" in files obtained by...ehm, questionable means, then AccurateRip comes in quite handy 

Technically speaking, you are correct. And while my original question may not make any sense, I did explain what I meant by True FLAC vs. Fake FLAC. MP3's discard audio data. You can decode it into WAV and then compress it into FLAC. My question was, how would I be able to know the difference between a FLAC file derived from an MP3 file vs. a FLAC file derived directly from WAV which was derived from a CD. But I've already gotten an answer to that question so I guess there's no need to continue that particular discussion any further.

You get a match against a verified yet errant submission from a download and it doesn't matter how many submissions from legitimate sources are made.

You'll probably get a couple of matches but assuming the album is popular, the amount of mismatches against correct rips would be larger.
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-06-30 18:01:11
Assuming you're using a tool configured to show you mismatches and exercise ample skepticism, sure.
Title: True FLAC vs. Fake FLAC
Post by: ChrisC7 on 2011-08-07 09:55:25
Looking at a spectrogram of the audio is a good way to tell, but you need to look for more than just a low-pass filter.

Here are three spectrograms made with SoX that show what to look for:

First, the lossless original:

http://farm6.static.flickr.com/5160/588503...19f4fd5_z_d.jpg (http://farm6.static.flickr.com/5160/5885034567_dd919f4fd5_z_d.jpg)

Next, Lame MP3 at -V5:

http://farm7.static.flickr.com/6023/588503...51ec1c8_z_d.jpg (http://farm7.static.flickr.com/6023/5885034361_e4e51ec1c8_z_d.jpg)

And lastly, Vorbis at -Q4:

http://farm7.static.flickr.com/6011/588503...a8664a0_z_d.jpg (http://farm7.static.flickr.com/6011/5885034441_b49a8664a0_z_d.jpg)

The main thing I look for is the "holes" in the audio that the lossy compressors psycoacoustic model determined to be inaudible due to auditory masking.


Rotareneg,

Thank you for the info. Can you please tell me how to make spectrograms like that? I went to get the SoX program but I see that it's all command-line and I'm not very good with that sort of thing. I use the Spectro program (http://spectro.enpts.com/) which is great and very easy to use by right-clicking a FLAC or mp3 and selecting "Analyze," but the spectrograms are very small and I'd like to make big ones like you did. Could you please tell me what you type at the command-line to make those? Thank you very much.
Title: True FLAC vs. Fake FLAC
Post by: Porcus on 2011-08-07 13:59:32
The only only to really know (to my knowledge) if a CD is authentic is to buy it from the publisher themselves.

I guess Century Media doesn't qualify as a publisher?


Seems to me that this should be an entry for the knowledgebase wiki.


unless you've ripped the files yourself, how would you know the FLAC file you have is ACTUALLY lossless instead of an MP3 converted into FLAC?


As others have pointed out to you, there is no bullet-proof way to tell whether you have gotten the source, because the source sold by the record company may have been mp3'ed along the way. Possibly even from the band themselves. There are ways to verify with fair confidence that the material has been through lossy processing though (falsifying this could possibly be harder, I don't know), but without a «better» source to compare with, there are basically no ways to tell that what you got, was not the «least lossy» copy there is.


I've used the TEST option in FLAC frontend and it doesn't give a result. I have used Audiotester and it does say the file failed because it's TRUNCATED.


That sounds like an encoder/decoder issue.
Title: True FLAC vs. Fake FLAC
Post by: astroidmist on 2011-10-08 06:41:20
There used to be a freeware DOS command line/console program that could examine a WAV and tell if it came from an MP3.  I tried it out on my personl files and it was NEVER WRONG even though I couldn't tell by listening and needed to double check the sources.  Unfortunately I can't remember what this program was called or where to find it.  It ran on Windows 98 SE too.  I am really bummed that I can't find it.
Title: True FLAC vs. Fake FLAC
Post by: Northpack on 2011-10-08 12:00:48
There used to be a freeware DOS command line/console program that could examine a WAV and tell if it came from an MP3.

It's called AuCDtect and does a spectrum analysis looking for patters introduced by lossy compression. It works very well, you'll hardly ever get a false negative. There a windows frontend called Tau Analyzer and even a foobar plugin, all avaiable here: http://en.true-audio.com (http://en.true-audio.com)


Title: True FLAC vs. Fake FLAC
Post by: dreamliner77 on 2011-10-08 20:15:54
The only only to really know (to my knowledge) if a CD is authentic is to buy it from the publisher themselves.

I guess Century Media doesn't qualify as a publisher?

What I meant was this: Let's say for instance I wanted to buy an album called "Tales of the Inexpressible". A trustworthy publisher would be "Twisted Records" instead of "World Music of New Age". A good amount of research would show that the composers of the album founded Twisted Records and sell their music there. Therefore, the only real way to make sure that you're getting an authentic CD, would be to buy it from Twisted Records and not from any other obscure publisher.



A great example of this is God Lives Underwater's Up Off The Floor.  The official release was very obviously sourced from MP3.
Title: True FLAC vs. Fake FLAC
Post by: frozenspeed on 2011-10-09 04:40:52
That's a good one.

I was referring to some of the titles on the Century Media label available on Amazon.



Care to mention which ones? I only ask because I have a bunch of Century Media cds and now I want to make sure they're not in my collection
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-09 19:01:48
Hi. I've read this topic with some interest especially since i'm now trying to buy my digital music in FLAC and i want to be sure i'm not fooled by some company selling mp3 recoded into flacs.
I've ran a little test comparing the SoX spectros of 2 flacs for the same song. Once was coming from such a store, the other one was coming from ... well, you don't want to know 
I got this
I don't think this album was re-mastered.

Oh, BTW, i can't tell wich one comes from the store because i don't know if SoX will put the first file at the top or bottom, and i can't analyse more, nor less than 2 files with the spectro. Any SoX guru out there who can help me clarify this?
Title: True FLAC vs. Fake FLAC
Post by: soviet123 on 2011-10-09 20:35:42
...From what I've read, you can use Goldwave (a piece of software) to check the frequency graphs (much like post #35 above). Since lossy never (or almost never) reaches above 20k, it should be easily discernible.
It's what they say in China, anyway, since there are so many "bad lossless" sources online there.
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-10 09:46:21
...From what I've read, you can use Goldwave (a piece of software) to check the frequency graphs (much like post #35 above). Since lossy never (or almost never) reaches above 20k, it should be easily discernible.
It's what they say in China, anyway, since there are so many "bad lossless" sources online there.

Most interesting is post #6 (http://www.hydrogenaudio.org/forums/index.php?s=&showtopic=89408&view=findpost&p=761351), Rotareneg clearly explained what to look for. I just tried to use the idea on my own files but i'm still unsure how to read the graphs.
And since i was analysing two files, i thought the 2 graphs were from both files, but now i wonder whether they are for left and right channels of a single file. Which one?
SoX really needs some tutorials in addition to the man pages
Title: True FLAC vs. Fake FLAC
Post by: Northpack on 2011-10-10 10:14:09
Can anyone give me a clue why i got so many differences. It doesn't look like the lossy paterns showed in a previous post so i guess both a re lossless, but it looks like they come from 2 different versions of the same song.  I don't think this album was re-mastered.

For me they look like sourced from two different lossy encodes, the latter being normalized to a higher level.

...From what I've read, you can use Goldwave (a piece of software) to check the frequency graphs (much like post #35 above). Since lossy never (or almost never) reaches above 20k, it should be easily discernible.

That's wrong. Lowpass filtering below 20kHz is often a feature of lossy encoding but not necessarily. On the other hand, lowpass filtering is often deliberately utilized during mastering, to get rid of high frequency noise and distortion, so there are many tracks showing a low pass filter which were not subject to lossy compression.

You should really try the AuCDtect tools I told you about. They are much more advanced than any spectrogram guesswork by eye.

Also note that if a file shows signs of lossy compression it isn't necessarily the distributor to blaim. Nowadays many musicians pretty carelessly use lossy formats to record and trade samples and whole tracks (in fact they do for quite a while, think of MiniDisc recorders). Some artists, like NIN, have used MP3-like artifacts as artistics means.
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-10 10:49:36
Quote
You should really try the AuCDtect tools I told you about. They are much more advanced than any spectrogram guesswork by eye.

Unfortunately, this application only seems to works on physical CDs. I was trying to analyse FLAC files i bought as digital downloads.

EDIT : what makes you tell they were lossy? I tried looking for "holes" in the spectrum as on post #6 rather than just a pass filter and didn't see those. Is there something else to look for?
Title: True FLAC vs. Fake FLAC
Post by: Northpack on 2011-10-10 11:04:19
Unfortunately, this application only seems to works on physical CDs. I was trying to analyse FLAC files i bought as digital downloads.

That's right, that Tau Analyzer frontend really should be updated to support analyzing lossless files. However, there's the command line tool and there's the foobar component to analyse files. Or, if you want to use the frontend, you could make a virtual image from your FLACs.
Quote
EDIT : what makes you tell they were lossy? I tried looking for "holes" in the spectrum as on post #6 rather than just a pass filter and didn't see those. Is there something else to look for?

Of course it doesn't look like the spectrum from post #6 because that shows a time scale of ~4 seconds whereas your spectrum shows a time scale of ~260 seconds. You wouldn't look at a sattelite image of the country you live in and wonder why you can't see your house on it, wouldn't you?
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-10 11:13:38
That's right, that Tau Analyzer frontend really should be updated to support analyzing lossless files. However, there's the command line tool and there's the foobar component to analyse files. Or, if you want to use the frontend, you could make a virtual image from your FLACs.

Ah. I'll try to get the command line tool as soon as their web page stops giving me errors.
Quote
Of course it doesn't look like the spectrum from post #6 because that shows a time scale of ~4 seconds whereas your spectrum shows a time scale of ~260 seconds. You wouldn't look at a sattelite image of the country you live in and wonder why you can't see your house on it, wouldn't you?

Well, of course i would first take a magnifying glass 
Silly me, i didn't notice the timescale on the first screenshots and wrongly guessed this was an entire track

EDIT : Northpack : Thanks a million times for pointing my sillyness.
I've re-analysed my files, selecting a 5 seconds portion, and here is the result :

File i got in torrent
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-10 13:28:27
Not sure why i can no longer edit my previous post, maybe it's too old.
OK, before i cause any harm, i got spectrograms for track from 3 other albums i bought at Qobuz (i had bought 4 already), and none of them shows the lossy pattern visible in the previous spectro. Unfortunately i don't have other files to compare.
It might be that the files they received from the publisher were lossy to start with. I won't stop buying flac as i said in my previous post, but i will keep checking them. Hopefully this one was an isolated accident.
Title: True FLAC vs. Fake FLAC
Post by: dhromed on 2011-10-10 13:42:50
Have you notified them?
Title: True FLAC vs. Fake FLAC
Post by: Northpack on 2011-10-10 14:57:00
I've re-analysed my files, selecting a 5 seconds portion, and here is the result :

The one from Qobuz is from a lossy source for sure. But the first one is definetely not from the same file as one of the spectrograms in your first post. Are you sure those spectrograms are not just left and right channels from the Quobuz file?
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-10 15:59:48
I havn't contacted them yet because i first wanted to check the .wav file they offer shows the same pattern. It does.
My first spectro was actually the 2 channels of the Qobuz file. I finally understood what i had to write in the command line to get the spectro of one file. I actually forgot to add a -n parameter for the output file, wich caused quite some havoc. 
The joys of command line tools 

BTW, since someone asked, here is what i used
Code: [Select]
sox <"Input file name"> -n trim <x> <y> spectrogram

Where <"Input file name"> is the name of the track you want to analyse. Better put it under quotes as chances are great you have command line unfriendly characters in the name (if only a simple space).
<x> is the time where you want to start analysing.
<y> is the time you want to analyse. In my case was 5 (seconds).
If you omit trim <x> <y> you will analyse the whole track. This can be useful to spot the area you want to check more precisely.

Edit : do i need to add i strongly suggest you run SoX on copies of you tracks, in a separate folder. Cause i should have done this
Title: True FLAC vs. Fake FLAC
Post by: _m²_ on 2011-10-11 12:59:09
Quote
You should really try the AuCDtect tools I told you about. They are much more advanced than any spectrogram guesswork by eye.

Sadly, auCDtect has MANY false positives.
Quote
Unfortunately, this application only seems to works on physical CDs. I was trying to analyse FLAC files i bought as digital downloads.

Just decode them to wav or, like suggested, use some frontend that will do it for you.
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-11 17:24:31
By "False positives" do you mean files wrongly identified as lossy or wrongly identified as lossless (or both)?
Quote
Just decode them to wav or, like suggested, use some frontend that will do it for you.

Decoding won't help me with Tau Analyser as it works only on CDs. I would have to burn the files to a disk to analyse them (or create an image and mount it), i can't just analyse files on the HD.

I found the command line tool, but it's in the "old" downloads so i guess it havn't been updated in a while. Didn't found the foobar plugin.

Well, at least i know how to use SoX now.
Title: True FLAC vs. Fake FLAC
Post by: Northpack on 2011-10-11 17:34:01
By "False positives" do you mean files wrongly identified as lossy or wrongly identified as lossless (or both)?

AFAIK wrongly identifies as lossy, not the other way around. I got false positives for tracks which had a heavy noise reduction filter applied to them - but it's really hard to discern such filters from lossy compression because they work on similar psychoacoustical principles.
Title: True FLAC vs. Fake FLAC
Post by: _m²_ on 2011-10-11 20:56:09
By false positive I mean that the file is fine, but detected as lossy.

Tau Analyser uses the same engine as auCDtect, so you can decode to wav and use auCDtect directly or automate it with fooCDtect or another frontend.
Title: True FLAC vs. Fake FLAC
Post by: Joseph93 on 2011-10-12 05:13:03
3 things:

1. hi I am new

2. I recently stumbled across a paper which details an algorithm that, with a very high success rate, guess the bit rate of an audio file just using data from the file's high-frequency spectrum. If developed further it could remove the need to visually inspect a spectrogram etc. and would be much faster.

http://www.fileden.com/files/2009/2/14/232...20Frequency.pdf (http://www.fileden.com/files/2009/2/14/2321055/My%20Documents/MP3%20Bit%20Rate%20Quality%20Detection%20through%20Frequency.pdf)

3. If anyone has Max, I threw together a patch the other day which allows you to compare the quality of two audio files in realtime (by looking at their spectrograms; they play simultaneously). I successfully used it to tell the difference between files of varying bitrate, even between 256kbps and 320kbps mp3s. you can also use vst spectrograms with it if you prefer. If anyone is interested I can upload it
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2011-10-12 05:22:01
I've seen examples of both false positives and false negatives.
Title: True FLAC vs. Fake FLAC
Post by: saratoga on 2011-10-12 05:44:27
3 things:

1. hi I am new

2. I recently stumbled across a paper which details an algorithm that, with a very high success rate, guess the bit rate of an audio file just using data from the file's high-frequency spectrum. If developed further it could remove the need to visually inspect a spectrogram etc. and would be much faster.

http://www.fileden.com/files/2009/2/14/232...20Frequency.pdf (http://www.fileden.com/files/2009/2/14/2321055/My%20Documents/MP3%20Bit%20Rate%20Quality%20Detection%20through%20Frequency.pdf)


Quote
In order to obtain the feature data, the source MP3 files were each
decompressed into a 1411 kbps WAV file using the Fraunhofer
IIS MP3 Surround Commandline Decoder V1.4 [2]. This was
done because audio files in this format can easily be read into
MATLAB, and as we have demonstrated, transcoding to a higher
bit rate does not affect the frequency characteristics of the audio
which we are observing.


It is, in my opinion, not a good sign when the author of a paper does not understand that WAV is a lossless format and so resorts to arguing that "transcoding" to PCM probably doesn't change the audio.  Regardless, all that paper demonstrates is that if you know that LAME 3.97 was used with default lowpass for each bitrate, you can figure out the source bitrate by looking at the lowpass setting. 

Title: True FLAC vs. Fake FLAC
Post by: Joseph93 on 2011-10-12 06:19:29
It is, in my opinion, not a good sign when the author of a paper does not understand that WAV is a lossless format and so resorts to arguing that "transcoding" to PCM probably doesn't change the audio.  Regardless, all that paper demonstrates is that if you know that LAME 3.97 was used with default lowpass for each bitrate, you can figure out the source bitrate by looking at the lowpass setting.

So does the paper actually not do what it claims it does? I don't see any reliance on prior knowledge concerning the "history" of the file in question.
Title: True FLAC vs. Fake FLAC
Post by: saratoga on 2011-10-12 06:49:36
So does the paper actually not do what it claims it does?


It does what they claim, take a known encoder and version and then determine what bitrate was used.  It doesn't do what people in this thread are interested in though.

I don't see any reliance on prior knowledge concerning the "history" of the file in question.


Suggest reading section 2, "procedure".  They train their model using the same encoder and settings they will then attempt to detect.  Without this the system is useless.
Title: True FLAC vs. Fake FLAC
Post by: Joseph93 on 2011-10-12 07:01:10
Quote
It doesn't do what people in this thread are interested in though.


once you have an algorithm which can estimate original bit rates of a transcoded lossless format, getting a yes/no answer to the question "are my flacs 'real'?" seems trivial.


Quote
Suggest reading section 2, "procedure". They train their model using the same encoder and settings they will then attempt to detect. Without this the system is useless.

Correct.

I still don't see any reliance on prior knowledge concerning the "history" of the file in question. Of course any algorithm of this nature needs information about what it is looking for!
Title: True FLAC vs. Fake FLAC
Post by: saratoga on 2011-10-12 07:09:59
Quote
It doesn't do what people in this thread are interested in though.


once you have an algorithm which can estimate original bit rates of a transcoded lossless format, getting a yes/no answer to the question "are my flacs 'real'?" seems trivial.


If you know that the file was encoded with a given lame version, then you already know the answer to the question  "are my flacs that I've created from my LAME mp3s 'real'" is "No". 

That said you are correct that determining if the output of a given LAME version will be lossy is quite trivial. 

Quote
Suggest reading section 2, "procedure". They train their model using the same encoder and settings they will then attempt to detect. Without this the system is useless.

Correct.

I still don't see any reliance on prior knowledge concerning the "history" of the file in question. Of course any algorithm of this nature needs information about what it is looking for!


As I said above, the prior knowledge is the encoder and settings (aside from bitrate) used to create the file in question.
Title: True FLAC vs. Fake FLAC
Post by: Joseph93 on 2011-10-12 07:28:01
Quote
If you know that the file was encoded with a given lame version, then you already know the answer to the question "are my flacs that I've created from my LAME mp3s 'real'" is "No".


Yes but surely the OP was referring to a situation where the file history is unknown. (Suggest reading OP.) Even in this case the algorithm should be able to take arbitrary WAVs and, if they are indeed transcodes, guess their original bitrate with a great deal of accuracy.


Quote
As I said above, the prior knowledge is the encoder and settings (aside from bitrate) used to create the file in question.

No. The prior knowledge is the information about the frequency characteristics of lame-encoded mp3s. What I am saying is that once you have trained the algorithm, you can then take arbitrary WAVs (or flacs, or whatever) and use the algorithm on them. This is pretty standard. Train an algorithm with a set of inputs, then give it arbitrary inputs and see how it does. 100% accuracy cannot be expected.
Title: True FLAC vs. Fake FLAC
Post by: saratoga on 2011-10-12 07:41:29
Yes but surely the OP was referring to a situation where the file history is unknown.


In this case you cannot use this software.  The authors have demonstrated identification of files that are known a priori to be transcoded by incorporating that knowledge into their algorithm.  Hence my point above that its not useful for what people in this thread want to do.

(Suggest reading OP.)


No need to get angry at me.  I'm not attacking you, I'm just trying to lead you towards an understanding of why what you are proposing does not work.

Even in this case the algorithm should be able to take arbitrary WAVs


It should?  The authors certainly haven't demonstrated that.  In fact they are quite clear that they have not chosen arbitrary wav files. 

Quote
As I said above, the prior knowledge is the encoder and settings (aside from bitrate) used to create the file in question.

No. The prior knowledge is the information about the frequency characteristics of lame-encoded mp3s.


I'm not being condescending, but read more carefully, there is a LOT more prior information being used here.  The procedure explains that the training set and the unknown set were encoded with identical settings and encoder.  This is not by chance. The authors have not accidentally made their problem extremely easy compared to the one you want to solve. 

What I am saying is that once you have trained the algorithm, you can then take arbitrary WAVs (or flacs, or whatever) and use the algorithm on them.


Ignoring for a moment what is actually going on, if this actually worked, why do you think the authors decided not to show that this was possible?  Perhaps they were concerned about making their paper too exciting
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-12 12:37:13
I've seen examples of both false positives and false negatives.

I guess when a full CD is identified as CDDA it's safe to consider it an original and not an MP3 reconstruct. This might be the reason why Tau Analyser only works for a full CD and not an individual file.

OK, i'll go a bit off topic for the last time (i hope  ). I have receive the answer from Qobuz. They have checked the file and think at has been through some MPA compression. They will ask the producer for a true original and offered me a free album to compensate. They were pretty quick to react too. Good point for them
Title: True FLAC vs. Fake FLAC
Post by: mjb2006 on 2011-10-12 22:03:45
I have receive the answer from Qobuz. They have checked the file and think at has been through some MPA compression. They will ask the producer for a true original and offered me a free album to compensate. They were pretty quick to react too. Good point for them

Many people, even many musicians, simply can't hear the difference between a lossy version and the original, which shouldn't be surprising, given the robustness of the lossy formats and all the listening tests we're familiar with. Some/many are also just not very technologically savvy. It really would not surprise me to find out, then, that artists or their representatives wouldn't necessarily even know that MP3/AAC/whatever is lossy at all, or recognize that once something is lossily encoded, there's no going back, even if they have a converter that turns their MP3s back into the WAVs or AIFFs needed by their labels and distributors. That's one of the reasons transcodes happen in general; people think "if higher bitrates mean higher quality, then I'll just convert this 128 kbps MP3 to a 320 kbps one! or maybe I'll just convert it to WAV and it'll be perfect!"
Title: True FLAC vs. Fake FLAC
Post by: testyou on 2011-10-13 09:52:20
Quote
Some/many are also just not very technologically savvy.

Unfortunately, this is very true.
I have contacted several artists that I follow on Soundcloud about this.
Knowledge of lossy/lossless encoding is not a prerequisite to creating music, and is sometimes not learned.
Title: True FLAC vs. Fake FLAC
Post by: r0k on 2011-10-13 15:31:57
And then, there are people who are not even musicians or technicians involved at some point.
Hopefully, it will change now that compagnies selling FLAC start to appear throughout the web. Unfortunately there is still too few poeple who are well aware of the issue to be careful about what they buy.
1 year ago i was still ripping to MP3 and burning those back on CDs when i wanted to copy a disc! 

It's up to us to spread the word now
Title: True FLAC vs. Fake FLAC
Post by: knutinh on 2011-10-16 00:20:07
I have noticed that mp3s commonly add a large error to the input signal - when the error is estimated visually by looking at waveforms, not if you are listening to the decoded file (what the format is made for really).

In my simplified understanding, this can be interpreted as signal-dependent narrow-band noise insertion (psy-model guided quantization of subbands), and perhaps phase-error? Are there no known mechanisms to guesstimate that such an error was inserted at one stage? I believe that natural music commonly contain spectrally sparse content (pure harmonic waveforms) or temporally coherent impulses (at least when rising in level). Can one not search for such things in a file, and find traces commonly attributed to mp3 encoding and with little chance of being generated via other means?

-k
Title: True FLAC vs. Fake FLAC
Post by: 2Bdecided on 2011-10-17 10:42:50
On exceptionally pure tone-like signals you can see the shape of the codec's noise skirting it.

I can't think of many recordings that contain spectrally sparse content. I went looking for some once when trying to assess the audibility of distortion. Even a solo flute or violin or piano is too rich to spot masking noise at high bitrates, and with most pop, rock, jazz etc you can forget it completely.

Pure impulses are a great test signal for ID-ing codecs, but only synthetic signals are known to be clean. With anything else, the pre-echo is usually lost in the other instruments. You could find it in isolation in some recordings, and maybe make a judgement that nothing else had caused it, but it doesn't sound like something you could automate.

If the recording you want to check contains no ultra-pure tone and no isolated impulse-like sounds, then this task is impossible IMO. You can't see the coding noise in the coded version when looking in the waveform view or the spectral view. Apart from the low pass, some of the common lossy distortions can be easier to hear than see.

Cheers,
David.
Title: True FLAC vs. Fake FLAC
Post by: astroidmist on 2011-10-21 23:00:18
There used to be a freeware DOS command line/console program that could examine a WAV and tell if it came from an MP3.

It's called AuCDtect and does a spectrum analysis looking for patters introduced by lossy compression. It works very well, you'll hardly ever get a false negative. There a windows frontend called Tau Analyzer and even a foobar plugin, all avaiable here: http://en.true-audio.com (http://en.true-audio.com)


OK thanks very much for that.  I haven't been able to find that program for years!
Title: True FLAC vs. Fake FLAC
Post by: djchristian on 2012-01-06 02:50:57
By false positive I mean that the file is fine, but detected as lossy.

Tau Analyser uses the same engine as auCDtect, so you can decode to wav and use auCDtect directly or automate it with fooCDtect or another frontend.


Where can i download fooCDtect?
Title: True FLAC vs. Fake FLAC
Post by: db1989 on 2012-01-06 15:26:21
Second result on Google (not sure which version): http://www.softpedia.com/get/Multimedia/Au...fooCDtect.shtml (http://www.softpedia.com/get/Multimedia/Audio/Audio-Plugins/fooCDtect.shtml)
Fourth result on Google (the same download is linked in a topic of our own, which is the first result): http://idle.netau.net/5099/foobar2000-fooc...i-for-aucdtect/ (http://idle.netau.net/5099/foobar2000-foocdtect2-frontend-gui-for-aucdtect/)
Title: True FLAC vs. Fake FLAC
Post by: Ubulord on 2013-09-01 11:27:14
I know the topic title sounds absolutely absurd but hear me out. I've tested my FLAC collection by encoding them into V0 MP3. I then decoded the MP3 back to WAV and then compressed it in FLAC. My question is: unless you've ripped the files yourself, how would you know the FLAC file you have is ACTUALLY lossless instead of an MP3 converted into FLAC? I've used the TEST option in FLAC frontend and it doesn't give a result. I have used Audiotester and it does say the file failed because it's TRUNCATED.

Bottom-line: Is there a sure-fire way of knowing that a FLAC file is truly lossless and not a derivative of a lossy file?


If the flacs you have correspond to a complete album you can test it with CUETools. If you have the flacs and a ".cue" file, CUETools will tell you if the whole rip correpsonds to a rip in the CTDB or AccurateRip databases or not. If it does, then I think you can be sure it's all genuine. If you don't have the ".cue" file, sometimes CUETools will also be able to check, sometimes it won't be able.
Title: True FLAC vs. Fake FLAC
Post by: claudiod on 2013-09-01 12:15:21
I have two J.S. Bach cds (Das Wohltemperierte Klavier I and II, Leonhardt, Harmonia Mundi/BMG Classics) which I believe to be legitimate, and sound perfectly.

Yet Audiochecker finds them to be 99% MPEG. It may have something to do with being a single instrument (harpsichord), so they may have used a lowpass filter to reduce noise in the higher frequencies where a cembalo is not supposed to be.

So we can't trust checking software completely.
Title: True FLAC vs. Fake FLAC
Post by: Nessuno on 2013-09-02 09:14:35
I have two J.S. Bach cds (Das Wohltemperierte Klavier I and II, Leonhardt, Harmonia Mundi/BMG Classics) which I believe to be legitimate, and sound perfectly.

Yet Audiochecker finds them to be 99% MPEG. It may have something to do with being a single instrument (harpsichord), so they may have used a lowpass filter to reduce noise in the higher frequencies where a cembalo is not supposed to be.

So we can't trust checking software completely.

None around here has ever told to thrust this kind of software completely, in the first place.
All the more, generally harpsichord is one of the most challenging instruments for lossy codec and very revealing for humans in ABX tests, but those are analog recordings from late sixties or early seventies, so I think they could hardly be considered a valid reference.
Title: True FLAC vs. Fake FLAC
Post by: Alexa on 2013-10-11 19:01:20
Can anybody tell if these are both fake:


but "aucdtect" reports mpeg 95% on the 2nd one too

I used "spectro" to make the screenshots
Title: True FLAC vs. Fake FLAC
Post by: saratoga on 2013-10-11 22:20:32
Can anybody tell if these are both fake:


I doubt it.  Compare the file you have to the lossless version from the CD. 
Title: True FLAC vs. Fake FLAC
Post by: Rescator on 2013-10-12 02:44:12
And as pointed out earlier by folks, lowpass does not equal lossy encoding, I tend to filter out any audio I do not find valuable to the work I'm creating, in that case the master could actually be flagged as being "lossily encoded" etc.
Title: True FLAC vs. Fake FLAC
Post by: Glenda on 2013-10-12 10:49:06
One other reason I like engineers who use the PM2 A-D,  you can't have an MP3 HDCD file.

But there is a program that was good at detecting lossy gens call Audio Checker v1.2 by dester.
Title: True FLAC vs. Fake FLAC
Post by: saratoga on 2013-10-12 17:52:06
you can't have an MP3 HDCD file.


Sure you can.

Edit: ah you mean mp3 transcoded to flag with hdcd.
Title: True FLAC vs. Fake FLAC
Post by: alekksander on 2013-10-26 17:19:29
use spek.cc and verify by spectrum. just consider date of the record
Title: True FLAC vs. Fake FLAC
Post by: incifinci on 2015-12-15 12:00:56
I downloaded a fake FLAC from a blog site, with exact cut-off on spectrogram at 16 kHz. I tell them about, and they gave me a new one, with a frequency range up to 22 kHz - but the spectrogram is very strange, in my opinion, and has a big peak exactly at 16 kHz, too. I would like to know, is there a way to create fake FLAC with so exotic spectrogram, or is the new one is true FLAC? My earthes (by the test) are good (till 16 kHz), and I could not hear a quality difference between two files, using good enough quality JVC headphones.

2 FLACs, 2 Spek images + Audacity frequency analysis image are here (http://users.atw.hu/piacter/fake-or-not.zip). Thank you in advance for help.

(This like peak seems on the 60% of songs of the full album. Disney Aladdin soundtrack, hungarian, 1992.)
Title: True FLAC vs. Fake FLAC
Post by: xnor on 2015-12-15 12:46:37
From a quick analysis I conclude it's just noise added on top. Maybe their source material just isn't better..
Title: True FLAC vs. Fake FLAC
Post by: mjb2006 on 2015-12-15 13:33:32
The first one is almost certainly transcoded from lossy, IMHO. The 13 to 16 kHz range is very selective, 16 kHz and up is just occasional noise. The spectrogram in Audition has the look of a piano roll / low-res brick-out game.

The second one has instrumental harmonics going well above 16 kHz. Everything is smooth, no blockiness. The noise at the high end looks like shaped dither, so probably it was converted down from a higher-res source. Not a transcode, in my assessment.

Here I have zoomed in on a 4.7-second section from about 10.5 kHz to 16.5 kHz and am toggling back and forth between the lossy and lossless. Levels are not matched, so ignore the difference in brightness. I am not showing the harmonics or dither noise above 16 kHz; it's just showing the blockiness. Click on the image and view it at full res if you can.

(https://skew.org/tmp/hydrogenaudio/is-it-fake.gif)

Anyway, if you can't hear a difference, the lossy encoder did a good job deciding what to throw away, and the quality levels are by definition the same. But of course you don't want to be paying for one thing (*cough*) and getting another, even if you can't hear the difference.
Title: True FLAC vs. Fake FLAC
Post by: incifinci on 2015-12-15 17:42:42
xnor, mjb2006, thank both of you for help. Especially was helpful for me the explanation of the high end noise's origin (dither).

About the peak at 16 kHz: this was not the first time, when I met this like peak. mjb2006, what do you think, is its cause the downconverting from a higher-res source, too? (As I read in wikipedia about dithering, if I understood it well.) Or is it simply because of the quantization (signal processing)?
Title: True FLAC vs. Fake FLAC
Post by: greynol on 2015-12-15 18:53:06
https://www.hydrogenaud.io/forums/index.php?showtopic=17212 (https://www.hydrogenaud.io/forums/index.php?showtopic=17212)
Title: True FLAC vs. Fake FLAC
Post by: incifinci on 2015-12-15 19:12:17
https://www.hydrogenaud.io/forums/index.php?showtopic=17212 (https://www.hydrogenaud.io/forums/index.php?showtopic=17212)

Thank you for a good link!

It's unfortunate, that the cause is not find, but anyway, it's good to know, that it's not because of transcoding or ripping.
Title: Re: True FLAC vs. Fake FLAC
Post by: herkow on 2016-04-02 20:35:40
Hi guys. After reading a million posts all over the internet , I've decided to run practical tests that help me realize if we actually can determine if a song or album is lossy or not.

The answer is...  NO. (Well, if someone uses a really bad enconder, and low bitrates, of course we can, but we must test the best possible "cheater")

There is absolutely NO WAY for music. We can hardly do it in some special situations, like when we work with a single tone, but music is NOT a tone. So, to cover all the audio spectrum, we use PINK NOISE. Pink Noise is NOT music, but is pretty close, and the best for this test.

First, the Pink Noise, created in this nice audio editor:

(http://i.imgur.com/8VTiZ7e.png)

(http://i.imgur.com/z47b89e.jpg)

Then, the same wav converted to Lame MP3 V1 (192-224 kbps). We can see the cutoff at the lowpass frequency.

(http://i.imgur.com/SmoDitM.png)

(http://i.imgur.com/leRpCCo.jpg)

And finally, the wav converted to Lame MP3 V0 (NO Lowpass filter) 224-256 kbps.

(http://i.imgur.com/mHse0PJ.png)

(http://i.imgur.com/j69L9lP.jpg)


If we compare the original wav vs. the -V1 mp3, we may say "dude, that's lossy". But when we compare the original vs. the -V0 all we can say is "what the f"·%!".

If we take a closer look, we can see some difference in the spectral image (bigger blue points in the image), but that difference is not useful at all if we don´t own an original sample to compare. The only parameter we can use is frequency, but as you can see, when a good compression is used, there's no lowpass filtering, and the file looks like a 100% LOSSLESS CDDA.

I hope this small contribution may help.

Title: Re: True FLAC vs. Fake FLAC
Post by: herkow on 2016-04-02 22:33:45
Another example, with music. Here is a lot easier to see the difference, but, as I said before, we must study the WORST SCENARIO, and that's Pink Noise.

This time I've used my 1994 Disque Americ (Canada) print of... Dark Side Of The Moon.
Mr. Alan Parsons did a nice mastering job...  8)

EAC, no errors, and the wav looks like this:

(http://i.imgur.com/GtXZYZG.jpg)

And now the Lame V0
(http://i.imgur.com/BVQYOue.jpg)
Code: [Select]
This is the script if you want to use it:
#! /bin/bash
mkdir MP3 && for f in *.wav; do lame -V0 --noreplaygain "$f" ./MP3/"${f%.wav}.mp3";
done


As we can see, in a "cleaner environment" is easier to see the difference, even in a V0 (no lowpass) compression. Around 16KHz there's an abrupt cutoff on the low level content.

And, very interesting, the wav seems to show a cutoff around 20KHz... And we know that is a perfect rip of an original print.


So, in conclusion, an apparent cutoff (like in this wav) or a full frequency spectrum (like the pink noise mp3 V0) are ABOSLUTELY USELESS AND IRRELEVANT to determine if it's a fake or not.

In the best case, when you see a big cutoff at 15-16 KHz you can suspect it's a lossy file, but a 20 KHz cutoff proves NOTHING.

EDIT: Lossless Audio Checker said CUTOFF to my rip. So, you can't trust this software.

EDIT2: I know it may sound too fundamentalist when I say "proves nothing". I mean, this is like the "for home use, no professional"... You can "suspect" is fake or real, but no more than that.
Title: Re: True FLAC vs. Fake FLAC
Post by: j7n on 2016-04-04 19:15:33
I don't think the static noise sample tells a whole lot. Music would be more closely approximated by a mixture noises varying in level over time, sometimes sinking below where the encoder has set the threshold, other times remaining above it. Then you'd analyze how abrupt those transitions are.

A lot of CDs, in fact most in my collection, have a lowpass between 20.5 and 21.5 kHz. The rolloff is usually smoother than the one made by an MP3 encoder. To see it better, keep the time and frequency resolutions balanced so that there are not elongated pixels, and zoom in close enough. SoX defaults of 256 frequency bands and its natural colors is a good choice. An extra 10 dB of footroom can hilight artifacts better, but isn't necessary.

The Spek spectrogram isn't very clear, and has been saved with chroma subsampling, which washes out blue detail.

Check the L-R channel for blocking. With joint stereo, this part is heavily quantized.

I can't get the Lossless Audio Checker to finish processing. It has hung with 100% cpu usage for half an hour. AuCDtect gives a rather 'binary' yes/no answer, and is easily tripped by a lowpass without any encoding. A spectrogram can give more detailed picture.

• Source "Another Brick in the Wall (Part I)".
(https://i.imgur.com/wmJ5xd9.png) (http://i.imgur.com/TByIjWY.png)

• MP3 CBR 320
(https://i.imgur.com/n0woHlq.png) (http://i.imgur.com/bjuV2Xo.png)

Densest possible spectrum for MP3. Some parts have no obvious artifacting. I added stereo pink noise fading from -inf to -36 dB RMS. Blocking can be clearly seen up to the 18th second, where noise power is -54 dB.

The noise you demonstrated would have the power of -24 dB and is very loud, and drowns out most of the music.

• MP3 CBR 320, dither, High, Ultra Shaping
(https://i.imgur.com/LHTGpSk.png) (http://i.imgur.com/z59YP8X.png)

More realistic usage than the pink noise. Blocking can be seen more easily.

• Vorbis, Q8, 247 kbit/s.
(https://i.imgur.com/83UbpoR.png) (http://i.imgur.com/hHMvA26.png)

The threshold rises smoothly without a jump at 16 kHz. "Piano roll" style blocking can be seen nevertheless, up to the 15th second (-60 dB noise).
Title: Re: True FLAC vs. Fake FLAC
Post by: herkow on 2016-04-04 19:23:50
The Pink Noise is only Pink Noise. There's no music.
Title: Re: True FLAC vs. Fake FLAC
Post by: j7n on 2016-04-05 05:15:50
LAC.exe worked, whose download link I missed; only the GUI version didn't work.
It finished very quickly, but only reported "cutoff" for both cases.

AuCDtect found the original 99% CDDA, while the transcode was 100% CDDA, same result with normal/medium dither.

Another authentic track, which sounds great, with the following smoothly rolled off spectrum, was given a score of "cutoff" / 54% CDDA. (Paul Mauriat - "My House and the River" (1970 / 2012))

(https://i.imgur.com/PAvvu3H.png) (http://i.imgur.com/evmUwpA.png)

Both tools appear to be of no use. Even if they got the scores approximately right, the percentage would still not tell us anything about the distribution of the problematic areas. (Such as if some lossy coded or filtered samples had been used.)

If I doubt the lineage of an album, I set my computer to generate maximum resolution spectrograms using SoX / Frontah, walk away, and later quickly flick through them.