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View Full Version : Using TLH test for MPEG evidence?


TomB
2010-10-20, 05:40 PM
Glad I finally found you guys! This has been bugging me for a minute and I was hoping some might share their thoughts. This started in another thread so I am going share below, then follow up. Thanks for any information you might have!

Re: Trader's Little Helper
This might be a question that has been answered, but for the life of me I cannot find. If so, please forgive me!

I think I know of someone who is testing .flac files converted to .wav files for evidence of MPEG... can you speak to why a genuinely recorded .wav file that has been converted to flac, then back to .wav might produce a report that the audio came from MPEG??

Thanks !!

Re: Trader's Little Helper
Quote:
Originally Posted by Drgiggles1
Most likely a false positive. That's why it is recommended when checking for lossy you use Cool Edit Pro to check the spectral and frequency analysis preferably zoomed in on a 2 second area of the file. Also a file that has been recorded on a DAT recorder with a 16K drop off may be confused for a lossy file when indeed it is not.
Interesting... is there any information to tell what TLH actually tests?? If there is a likelihood of false-positives, then this would not necessarily be a "go-to" tool for testing if FLACs are sourced from MPEG then would it?? as you say, the only way to truly tell is to see where the freq. roll-off exists....
So in your opinion is spectral analysis the ONLY definitive way to test?

Do you think the false positive comes from the codecs involved in going wav>flac>wav and are leaving some sort of artifact that TLH is sensing??

There is a site right now that is promoting the TLH test on files that have gone wav>flac>wav to test if they are not from MPEG.... I posted a recording that went from wav to flac, using the TLH codecs, and that very recording tested positive for MPEG in the TLH test!!

I might be the only one, but I stand by my statement that flac is NOT lossless and converting it back to a wav for anything other than a qualitative exam is worthless....

This is very bizarre and I am trying to decide if I should challenge this method since it seems very flawed to be the definitive test for their overall QC....

Any thoughts??


So, I guess what I am asking is on a scale of 1 to 10, how sensible is it to use the TLH test as your overall QC screen? (1 being not, and 10 being definitely smart)


Thanks!!

TomB
2010-10-20, 06:04 PM
If these are dumb questions, since I am a noob on this board, please don't chastise me but rather just point me in the direction of the FAQ thanks !:cool:

showtaper
2010-10-20, 10:22 PM
Google is your friend:

FLAC (excerpt from wikipedia)

Free Lossless Audio Codec (FLAC) is an audio compression codec primarily authored by Josh Coalson and Ed Whitney. As its name implies, FLAC employs a lossless data compression algorithm: a digital audio recording compressed by FLAC can be decompressed into an identical copy of the original audio data. Audio sources encoded to FLAC are typically reduced to 5060% of their original size.

Lossless Data Compression (excerpt from wikipedia)

Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange for better compression rates.

Lossless data compression is used in many applications. For example, it is used in the popular ZIP file format and in the Unix tool gzip. It is also often used as a component within lossy data compression technologies (e.g. lossless mid/side joint stereo preprocessing by the LAME MP3 encoder and other lossy audio encoders).

Lossless compression is used in cases where it is important that the original and the decompressed data be identical, or where deviations from the original data could be deleterious. Typical examples are executable programs, text documents and source code. Some image file formats, like PNG or GIF, use only lossless compression, while others like TIFF and MNG may use either lossless or lossy methods. Lossless audio formats are most often used for archiving or production purposes, with smaller lossy audio files being typically used on portable players and in other cases where storage space is limited and/or exact replication of the audio is unnecessary.

<end quotes>

There are a number of lossless compression codecs, you can find a large number of more technical explanations with a search engine and a little effort.......

TomB
2010-10-21, 06:51 AM
Google is your friend:

FLAC (excerpt from wikipedia)

Free Lossless Audio Codec (FLAC) is an audio compression codec primarily authored by Josh Coalson and Ed Whitney. As its name implies, FLAC employs a lossless data compression algorithm: a digital audio recording compressed by FLAC can be decompressed into an identical copy of the original audio data. Audio sources encoded to FLAC are typically reduced to 50–60% of their original size.

Lossless Data Compression (excerpt from wikipedia)

Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange for better compression rates.

Lossless data compression is used in many applications. For example, it is used in the popular ZIP file format and in the Unix tool gzip. It is also often used as a component within lossy data compression technologies (e.g. lossless mid/side joint stereo preprocessing by the LAME MP3 encoder and other lossy audio encoders).

Lossless compression is used in cases where it is important that the original and the decompressed data be identical, or where deviations from the original data could be deleterious. Typical examples are executable programs, text documents and source code. Some image file formats, like PNG or GIF, use only lossless compression, while others like TIFF and MNG may use either lossless or lossy methods. Lossless audio formats are most often used for archiving or production purposes, with smaller lossy audio files being typically used on portable players and in other cases where storage space is limited and/or exact replication of the audio is unnecessary.

<end quotes>

There are a number of lossless compression codecs, you can find a large number of more technical explanations with a search engine and a little effort.......

There a boundless 'explanations' of the codec out there, but it seems to me that most of them are 'fluffed up' narrative, only explaining the concept... The only place I seem to be able to find any 'meat' is on the authors page. Even then it seems the critical parts like how it is more 'efficient' than most other codecs when it comes to SR errors and problems with other system's codecs decoding the signals... I get the fact that because it uses a different style math/predictors it takes less data to transfer the original information....
HOWEVER, my challenge to the concept, if you put it back into layperson terms is this:
Sure, about 98% of us could use a zip file graphic for our websites, but if we were making billboards, then we would certainly want the original, non-zipped file. If we took the zipped file to its limits, then we would certainly see evidence of it's zipping... I would think the same holds true for audio.... so, when one walks around brandishing the term lossless, it is really misleading, since in effect, no matter how you spin it, if you take a file that was one size and reduce it to a smaller size - THIS IS A LOSS, BY DEFINITION !!
I am only challenging the 'purists' conviction that flac is truly lossless ! Shouldn't it be called 'least lossy', if you are truly a 'purist' ??

U2Lynne
2010-10-21, 10:32 AM
Sometimes people compare a flac file to that of a sentence that has all it's spaces removed and there exists an application to put all those spaces back where they belong. Is the sentence really losing data if it can be put back together to be exactly that same? You have a 'compressed' file, not a 'lossy' file when you convert to flac.

TomB
2010-10-21, 04:37 PM
Sometimes people compare a flac file to that of a sentence that has all it's spaces removed and there exists an application to put all those spaces back where they belong. Is the sentence really losing data if it can be put back together to be exactly that same? You have a 'compressed' file, not a 'lossy' file when you convert to flac.

I fucking love it! You are the man!! Thank you!!

rspencer
2010-10-21, 05:25 PM
She's not a man. At least, she certainly doesn't look like one. :D

TomB
2010-10-21, 10:20 PM
She's not a man. At least, she certainly doesn't look like one. :D

My bad! But seriously, that is hands down the best analogy I've ever heard!
:clap::thumbsup:clap:

rspencer, looks like we are not far from each other on the outer limits !
(i'm off granby street)

rspencer
2010-10-21, 11:20 PM
Not too far. Out in the wilds of Isle of Wight currently. I've lived all over the area. In Norfolk, I've been on 40th street, Stockley Gardens twice (both sides of Princess Anne).

tonebloke
2010-10-22, 12:43 AM
Sometimes people compare a flac file to that of a sentence that has all it's spaces removed and there exists an application to put all those spaces back where they belong. Is the sentence really losing data if it can be put back together to be exactly that same? You have a 'compressed' file, not a 'lossy' file when you convert to flac.


Best explanation I've seen in all the forums I've ever been to. Well said Lynne. You da (wo)man !!!

U2Lynne
2010-10-22, 11:02 AM
:blush: I'd love to take credit for that, but I swear I heard it from someone else first (probably even here). It made sense to me which is why I remembered it, I guess.

tonebloke
2010-10-23, 04:13 AM
The "sentence" analogy is 100% spot on.

Praise to whomever came up with it.:thumbsup:banger2::cheers::vino::dance::kiss::angel::wolfkat: :wave:

zootype
2011-02-07, 12:47 PM
I'm finding TLH totally unreliable - I regularly record WAV files direct to my hard drive that TLH reports as "CDDA" or "source cannot be qualified" or even MPEG to some degree - and trying to read the screenshots of spectrum analysis is another way to become frustrated.

For that reason I'm hesitating to upload my recordings here lest I am falsly accused of uploading lossy material.

rspencer
2011-02-07, 05:17 PM
CDDA is fine...basically it's just saying it's redbook standard audio (lossless, 16 bits, 44.1kHz sampling rate).

Use spectrum and frequency analysis.

Any software checker, even TLH, is best for verifying losslessness...that is, if it says it's lossless, it most likely is. If it says it's possibly lossy, then you have to check by the spec/freq.

zootype
2011-02-08, 08:05 PM
CDDA is fine...basically it's just saying it's redbook standard audio (lossless, 16 bits, 44.1kHz sampling rate).

Use spectrum and frequency analysis.

Any software checker, even TLH, is best for verifying losslessness...that is, if it says it's lossless, it most likely is. If it says it's possibly lossy, then you have to check by the spec/freq.

ok thanks - I guess I'll have to get into looking at the Spects