Telling the difference between CD/MP3 using audacity

[daggerlee]

Hi all,

I'd like to tell if the Nirvana Unplugged CD I bought in China was pressed from CD-quality tapes or from an mp3 source. I copied one track as an AIFF file to my computer, and another from an authentic CD (Doolittle by the Pixies). Now what should I look for in audacity?

 

[infinitesymphony]

Wouldn't it make more sense to compare your copy with the waveform of an authentic copy of the same album? Different albums will have different frequency responses. There's no magical way to tell without doing a direct comparison with the original.

 

[Aman]

You have to look at a proven-legal copy of the track and compare it to your allegedly bootleg one. You can't go comparing two different tracks like that, since you'd be missing your control variable.

EDIT: Whoops, I read the question incorrectly. I thought I'd leave the info below here anyway because it may come in handy for some other fella' who uses the search feature of Head-Fi

As per your specific question, ignore what's below.
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Look at the waveforms of the bootleg's AIFF, and look at the lowest and highest peaks in pitch. Does the track go below 40Hz, or above 12kHz? Are the drums significantly louder than the rest of the instruments (look and see if, when a drum is struck, there is a sudden burst of loudness according to Audacity)?

The above questions are implying that if your album were bootleg, it may be an un-finished copy of the track, or one that hasn't gone through any sort of post-production. Basically, you're going to have to listen/observe for abnormalities that one wouldn't normally find being released from a major record company (that is, until you get yourself a real copy of the album like you always should).

 

[Davey]

Quote:

Originally Posted by daggerlee /img/forum/go_quote.gif Hi all, I'd like to tell if the Nirvana Unplugged CD I bought in China was pressed from CD-quality tapes or from an mp3 source.

Usually pretty easy to tell. Just load the wav file into Audacity, Edit -> Select All, View -> Plot Spectrum, 23.8 seconds message -> OK, and you will see the frequency spectrum. At CD quality the spectrum will usually extend strongly all the way out to the 22K brickwall limit, 256K mp3 source will be at zero by about 20K, 128K mp3 source hits zero by about 16 or 17K.

 

[daggerlee]

Thanks for the advice guys. I analyzed the plot spectrum and the info extended well beyond 20kHz, I think it's real.

 

[Zenja]

That's a good easy check, but it won't really tell you for sure. The frequency cutoff is controlled by the encoder and can even be overridden manually. I had some 192kbps mp3s that went all the way to 22kHz, or at least close to it.

It's good enough to just look at the frequency range though.

 

[trains are bad]

I'm not familiar with Audacity, but couldn't you invert the phase on one, add them together, and see if you get zero?

 

[infinitesymphony]

Quote:

Originally Posted by trains are bad /img/forum/go_quote.gif I'm not familiar with Audacity, but couldn't you invert the phase on one, add them together, and see if you get zero?

That's a good idea, assuming one has access to a confirmed legit copy.