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How much audio quality is lost converting 320kbps MP3 to 128kbps AAC?

K

kuykee

macrumors newbie
Original poster
May 20, 2010
10
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How much audio quality is lost converting 320kbps MP3 to 128kbps AAC?

I'd like to know so that I can store more files on my iPhone and iPod classic (also to make the iPod classic hard drive spin less). Also to save space on my Macbook hard disk
 
My guess would be that it depends on what kind of headphones you are using. If you are using something along the lines of Apple's earphones, my guess would be not that much. But if you have a $200+ set you might notice the decrease in quality.
 
Why not test it out yourself

Oh and why not try iTunes Match... Then nothing is on your MB at all and your Classic is fine!
 
jamesjingyi said:
Why not test it out yourself

Oh and why not try iTunes Match... Then nothing is on your MB at all and your Classic is fine!
Click to expand...
I use 192kpps compression when syncing my IPhone with ITunes. Best trade off in quality vs size for me. Your listening environment is more important than compression for sound quality.
 
there is an option to down convert to 128kbps when syncing to iPods and iOS devices, so you might want to try that. If you don't like it, you can just uncheck the box and resync.
 
Why would you buy a classic to save space? :confused:


The loss in quality cannot be quantified. You need to physically listen to each song, and it's possible if you have poor headphones, or even a bad hearing, you can't tell the difference.
 
kuykee said:
How much audio quality is lost converting 320kbps MP3 to 128kbps AAC?

I'd like to know so that I can store more files on my iPhone and iPod classic (also to make the iPod classic hard drive spin less). Also to save space on my Macbook hard disk
Click to expand...

It won't be nearly as bad as converting MP3 to MP3 using LAME, which can produce obvious artifacts after just one generation of transcoding. In fact, high bitrate MP3 to AAC will probably be pretty good. Let your ears be the judge, but please perform valid comparisons using foobar2000's ABX comparator or similar. There are apps that will let you do the comparison right on the Touch.
 
kuykee said:
How much audio quality is lost converting 320kbps MP3 to 128kbps AAC?

I'd like to know so that I can store more files on my iPhone and iPod classic (also to make the iPod classic hard drive spin less). Also to save space on my Macbook hard disk
Click to expand...

Plug in your iPod. In iTunes, click on the iPod. Click on "Summary", and "Convert higher bitrate to 128KBit". That option is non-destructive. Nothing on your hard drive is changed, so you don't have to worry about any loss. iTunes converts the music while it gets downloaded to the iPod. If you don't like it, you just change the option. I cannot distinguish AAC 192KBit from originals. 128KBit loses something. It's not the same.
 
foobar2000 comparator looked interesting, but it's still only listening by ear according to this video http://www.youtube.com/watch?v=jt7GyFW4hOI

Is there something else that can give you a graph or more statistical view of what is being lost from or changed in the file or in which parts there have been changes?
 
depends on what earbuds/headphones you're using and how good your hearing is. Personally, I would never rip in 128 because it sounds so awful.
 
kuykee said:
foobar2000 comparator looked interesting, but it's still only listening by ear according to this video http://www.youtube.com/watch?v=jt7GyFW4hOI

Is there something else that can give you a graph or more statistical view of what is being lost from or changed in the file or in which parts there have been changes?
Click to expand...

This question and discussion has been ongoing since the 70s. (only the formats have changed) Good luck on receiving an answer.
 
kuykee said:
foobar2000 comparator looked interesting, but it's still only listening by ear according to this video http://www.youtube.com/watch?v=jt7GyFW4hOI

Is there something else that can give you a graph or more statistical view of what is being lost from or changed in the file or in which parts there have been changes?
Click to expand...

Viewing things like waveform difference files is not particularly relevant. The only thing that matters is your own subjective perception and performing a valid blind test. Besides using foobar ABX or equivalent, the two files to be compared must be derived from the same mastering, and preferably the exact same source (one from the other obviously counts); otherwise, you may be testing for differences in mastering, not encoding, and differences in mastering can be genuinely profound and trivial for anyone to ABX.

There is an infinite amount of nonsense in discussions about these things. Here is a fairly detailed slideshow of a scholarly presentation that used high-end equipment under ideal conditions:

http://www.music.mcgill.ca/~hockman/documents/Pras_presentation2009.pdf

Its conclusion were:

  • Trained listeners can hear differences between CD quality and mp3 compression (96-192 kb/s) and prefer CD quality.
  • Trained listeners can not discriminate between CD quality and mp3 compression (256-320 kb/s) while expert listeners could.
  • Ability to discriminate depends on listeners’ expertise and musical genre.
  • Artifacts can be verbalized and do not depend on musical genre.

I think AAC is a lot better than MP3 at low bitrates, judging by my threshold for hearing artifacts in various "killer samples", which disappear for me at AAC 128 Kbps but persist in LAME 3.98 MP3 up to 192 Kbps and a little beyond. I've also found transcoding high bitrate MP3s to AAC to be much more transparent than going MP3->MP3, which introduces obvious artifacts after one generation. If you want to read a lot of subjective crazy talk including things like cable directionality, try the stevehoffman.tv forums. For people who value blind listening tests, try hydrogenaudio.org.
 
For me, where I listen to my iPhone, I don't see that it matters. In my truck, there is enough wind noise that I don't notice.
 
kuykee said:
How much audio quality is lost converting 320kbps MP3 to 128kbps AAC?

I'd like to know so that I can store more files on my iPhone and iPod classic (also to make the iPod classic hard drive spin less). Also to save space on my Macbook hard disk
Click to expand...

Well, the real sequence of this conversion is the following:

WAV 44.1khz/16bits -> MP3 320kbps -> WAV 44.1khz/16bits -> AAC 128kbps

In the first transition (from wav to mp3) you are losing audio information, that information is missing when you convert it back to WAV and it cannot be recovered.

Then you lose even more audio information when you convert that "incomplete" WAV to AAC 128kbps.

So you will get pretty crappy audio files, worse than regular AAC 128kbps (they usually came directly form CDs). So you can think of them as 96kbps AAC or maybe worse because different lossy codecs cut different things.

Going from one lossy format to another lossy format is not a good idea.
 
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