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10.4: Use Automator to mass-convert iTunes tracks
Authored by: macslut on Jul 01, '05 02:40:59AM

Baggins,

In your example you aren't doing what the author of the hint said he was doing. He was going from MP3 to AAC "mostly just to free up space" as others have suggested as the reason for the transcoding.

Transcoding from lossy to lossy always results in quality loss.

The significance of the quality loss depends on (not in any order):
1) complexity of the audio
2) source and output bit rates
3) quality of playback system
4) listening ability
5) listening knowledge

The point here is that if one is going from MP3 to AAC to reduce the size of their library they are probably doing something like going from 192kbps MP3 to 128kbps AAC, not the other way around.

Now ideally what one would do is re-rip from the source CDs where possible, but people should really understand what their results will be before permanently reducing what may be their only copies of files. Look at the post by drummer_ant. This is clearly a case of someone not understanding what takes place with transcoding (I'm not saying this is easy and don't mean to pick on anyone).

Quality only goes one way -> loss. You can't get it back, and I've seen too many cases of people thinking 128kbps was somehow CD quality, thinking it sounded ok and then once they were committed to the format realizing that their entire library sucked ass on their stereo system.

BTW: I transcode all the time. I have a 400GB library of 192kbps AAC files from my CDs. I often need to convert to MP3 for playing on various devices, and even will transcode to a lower bit rate sometimes, but this is with the acknowledgment of the quality loss and keeping the source files.



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10.4: Use Automator to mass-convert iTunes tracks
Authored by: Baggins on Jul 01, '05 10:12:34AM

Bit rate is a lot like the MHz myth: lower bit rate does not automatically correspond to lower audio quality. It all depends on the compression algorithms.

If the guy is worried, all he has to do is convert a subtle classical piece to 128 AAC and do some audio testing by playing both tracks alternatively in short clips. If he can't tell a difference, then any loss that may have occurred is irrelevant.



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10.4: Use Automator to mass-convert iTunes tracks
Authored by: macslut on Jul 01, '05 12:09:43PM

"...lower bit rate does not automatically correspond to lower audio quality. It all depends on the compression algorithms."

Correct, but the point I was making was that in your example you said you went from 128 AAC -> 192 MP3 and said you had no "discernible" loss, but did have a noticeable loss going from 128 AAC -> 128 MP3.

Your test makes no statement of what would happen to the audio quality based on what the author of the hint said he intends to do, that is go from a higher bit rate MP3 to a lower bit rate AAC.

Most everyone agrees that AAC sounds better at a lower bit rate, thus one could easily get improved quality *and* save space by re-encoding *from the source* using AAC instead of MP3. BUT... Going from MP3 to AAC inherently reduces the quality such that what may be considered an acceptable bit rate may result in a file size that is not significantly smaller.

"If the guy is worried, all he has to do is convert a subtle classical piece to 128 AAC and do some audio testing by playing both tracks alternatively in short clips. If he can't tell a difference, then any loss that may have occurred is irrelevant"

Except for when his music collection contains mostly music that is *not* subtle classical and perhaps very complex audio that doesn't compress well, or when he decides to play the music on a better/worse system, or when he learns what compression artifacts are and starts hearing them where he didn't notice them before, etc...

I would do a lot more than just checking one track on one system, and without knowing what to listen for, or knowing that you have the ability to detect the difference. This is why there is so much crap floating around out there...because people who don't know any better will make foolish claims based on flawed sampling and say things like 128kbps MP3/AAC/WMA is "CD Quality"...those people should be shot.

I'm not saying that the quality loss will always result in making the space savings or effort not worth it. I'm only pointing out that there *will be* quality loss and someone should definitely make an informed decision about this, especially if they could re-encode from the original source.

My comments are in response to comments by you and others:
"You won't lose significant quality"
"That's just audiophile snobbishness"
"as for all these people who talk about significant quality loss, ignore them"
"What a load of bull"
"99% of the population would never notice the difference"

And the most incorrect:
"Surely if you were to convert an Mp3 encoded at 192Kbps, to AAC, with the same bit rate, it would sound exactly the same?"



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