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View Full Version : Any way to tell if a file used to be an MP3?



bilmar
2006-10-15, 18:46
My music collection is mostly on CD and many of those are copies.
Some of the CD's are of MP3's converted to wav files to play in my 300 disk player.
Now I have an SB3 and I have been reading all my CD's into Flac files.
I don't want to waste space keeping Flac's of MP3's when I still have the MP3's!
Is there any way to ID a file as a converted MP3?

Thanks

Bill

Mark Lanctot
2006-10-15, 19:48
I don't think so. All that you have to go on was lost in the conversion:

- the file size is now that of a WAV
- the bitrate is now that of a WAV
- the ID3 tag is gone

All that's left are the compression artifacts, and these aren't straightforward to determine by a PC - not conclusively, at least. Even human beings can sometimes hear it, sometimes not.

obstreperousness
2006-10-15, 20:48
I think a look at the file in a spectrum analyzer will show a marked cutoff at around 16kHz if it were ever crushed by an MP3 encoder. There could be other causes for a cutoff, but if you see audio data above that, you could reasonably assume that it was never compressed with MP3.

I'm not sure if MP3 encoders universally inflict the same frequency cutoff. Someone please correct me if I'm wrong.

socialxray
2006-10-15, 23:24
A spectrum analyzer is the way to go. The high frequency cutoff is determined by the bitrate. I believe the highest frequency for MP3s is around 19KHz. It is easy to see. But even then it would not be a conclusive sign that the audio was from an MP3 file. You would have to know a little history of the recording since some recordings can benefit from a relatively low high frequency cutoff.

CatBus
2006-10-18, 11:19
Worth a look, seems to operate on the CD rather than the files, but maybe that's optional.

http://www.true-audio.com/Tau_Analyzer_-_CD_Authenticity_Detector

I know nothing about the software. It was recommended in the HydrogenAudio forums.

drewe181
2006-10-23, 03:50
hi guys, can anyone tell me if, from a sound quality point of view, it's worth converting my mp3s to FLAC or whether i have the 'joy' of re-ripping my collection to get better sound.

since mp3 has been compressed, i don't know if it is uncompressed when converted -to FLAC- or not

thanks -Drewe

adamslim
2006-10-23, 05:40
hi guys, can anyone tell me if, from a sound quality point of view, it's worth converting my mp3s to FLAC or whether i have the 'joy' of re-ripping my collection to get better sound.

since mp3 has been compressed, i don't know if it is uncompressed when converted -to FLAC- or not

thanks -Drewe

Converting an mp3 into FLAC cannot improve things. You will be converting mp3 to WAV and then to FLAC; the playing process will then convert the FLAC to WAV (effectively).

This will be no different (at all) from playing the mp3 directly, except that it will take up a lot more space on your HD and take you time in converting.

If you want the better sound quality of FLAC, you need to rerip.

It may be worth trying it with 5-10 CDs and seeing if you can tell the difference. If you are comparing 320kbps mp3s with CD FLACs on a budget system, there may be no difference; if you're comparing 64ps mp3s with CD FLACs on a ninja separates system it's likely to be worth it. Only re-rip if your ears insist on it!

(Plus I guess you could consider whether there is a difference between MP3 and CD FLAC that is fairly inaudible on an A-B comparison basis, but does affect your overall enjoyment of the music. I'm not sure how much it has been analysed.)

Adam

Mark Lanctot
2006-10-23, 06:30
hi guys, can anyone tell me if, from a sound quality point of view, it's worth converting my mp3s to FLAC or whether i have the 'joy' of re-ripping my collection to get better sound.

since mp3 has been compressed, i don't know if it is uncompressed when converted -to FLAC- or not

When you encode to MP3, certain components of the music are irretrievably lost. Converting it to FLAC, WAV, AIFF, ALAC or WMA Lossless won't bring back what was discarded.

The point of a lossy encoding process is to discard musical data without it being noticed, which gets harder and harder as the bitrate drops. But at higher bitrates and with modern encoders (Ogg or the latest LAME, for example), your hearing and your audio equipment may not be able able to properly tell what was removed, so Adam's point about A/B testing is good.

That said, I always encode to FLAC. ;-)

drewe181
2006-10-23, 14:53
thanks guys. that's just what i thought. oh boy.....

bilmar
2006-10-23, 19:26
Just got back from a trip
Thanks for all the suggestions.

The Tau program looks to be just what I need except it only works on CD's - not files.

So, if a spectrum analyzer can be used to spot the absence of audio above a certain frequency, how could I use one in batch mode on Flac files?

ie analyze all flac files in myfolder ( and subdirs)
then write a logfile with filename, tagname and 'score' of that file base on the lack of audio above some frequency.
I see this running ovenight but with even a simple scoring system, I would be able to tell 'good' cd's from those that came from MP3's.

Any ideas on how to string this together?

Bill

Mark Lanctot
2006-10-23, 20:43
Just got back from a trip
Thanks for all the suggestions.

The Tau program looks to be just what I need except it only works on CD's - not files.

So, if a spectrum analyzer can be used to spot the absence of audio above a certain frequency, how could I use one in batch mode on Flac files?

ie analyze all flac files in myfolder ( and subdirs)
then write a logfile with filename, tagname and 'score' of that file base on the lack of audio above some frequency.
I see this running ovenight but with even a simple scoring system, I would be able to tell 'good' cd's from those that came from MP3's.

Any ideas on how to string this together?

Bill

I really don't think there's an automated way to do it - this is not something that's done on a routine basis, there's not an overwhelming need for it out there. Not only that, poorly mastered/recorded material could naturally have an absense of material above 16 kHz.

CatBus
2006-10-24, 08:39
Just got back from a trip
Thanks for all the suggestions.

The Tau program looks to be just what I need except it only works on CD's - not files.

So, if a spectrum analyzer can be used to spot the absence of audio above a certain frequency, how could I use one in batch mode on Flac files?

ie analyze all flac files in myfolder ( and subdirs)
then write a logfile with filename, tagname and 'score' of that file base on the lack of audio above some frequency.
I see this running ovenight but with even a simple scoring system, I would be able to tell 'good' cd's from those that came from MP3's.

Any ideas on how to string this together?

Bill

If you are running Linux, there might be a way to burn the tracks to a virtual CD via some sort of script, then analyze that virtual CD via Tau. Way over my head tho ;)

You could maybe do something with MPTrim to detect gaps, but that's only if the MP3s contain gaps, and then also only if the original source didn't also contain gaps. I don't think that's good, and you could risk screwing up your files unless you were careful.

If only flac allowed you to set a lowpass filter, and then encode everything below that cutoff losslessly. Then you could reencode it to FLAC, then to a second FLAC with a 16.5 KHz cutoff. If the two FLACs are identical, then the source contained a lowpass filter.

Just thinking out loud. None of my suggestions actually work...but maybe there's another lossless codec that does support a lowpass?

radish
2006-10-24, 08:53
MP3 doesn't have a fixed lowpass filter, it depends on the encoder and settings used, as socialxray already mentioned.

http://wiki.hydrogenaudio.org/index.php?title=LAME#Technical_information

So unless you know exactly what it was encoded with you can't really infer anything. And if you knew that, you wouldn't need to do anything anyway :)

I really don't think there's anyway to do this, and it's not due to lack of software, it's due to it just being impossible.