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M4A to MP3 320kbps
Deimuddi1955
post Jul 21 2012, 14:39
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Hello


i got some M4A audio files that i would convert in MP3's. Now my question is:


Is a M4A file with 256kbps the equivalent to a MP3 File with 320kbps. if it is so, can i convert the M4A File to a MP3 with a constant bitrate of 320 kbps? or should i convert it to the same bitrate of 256kbps??






thanks for answers rolleyes.gif rolleyes.gif rolleyes.gif
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yourlord
post Jul 21 2012, 14:49
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If the M4A is AAC (which at 256Kbps it is) it's already compressed with a lossy codec. Transcoding to mp3 will further degrade quality.

If you need the audio as mp3 then it's best to seek out the lossless source material and transcode from that.

IMO, transcoding lossy to lossy is bad form, as if/when those files make their way into the wild it pollutes the world with substandard quality files.
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db1989
post Jul 21 2012, 15:00
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Hydrogenaudio has and will continue to have nothing to do with files “in the wild”, so what is your point?

OP: There is no direct equivalence between any two formats and/or settings. The way to decide which bitrate to use is the same as always recommended here: an ABX test of the two files, ideally with the source being uncompressed.

In case you just want someone else to decide for you, they cannot because they do not have your ears. And you could benefit from searching for the many past threads on questions like this, rather than expecting people to answer them all over again.
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Deimuddi1955
post Jul 21 2012, 15:17
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the M4A files which i have, are iTunes Exclusive Songs, they can't be found on any cd
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Porcus
post Jul 21 2012, 15:41
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Then keep the original. Write-protected.

Then you can make a copy and play around with conversion settings until you find something that sounds indistinguishable from the original, or acceptably close.
I'd store the transcoded version as e.g. filename.aac.mp3 to flag for all eternity that this was the transcoded one -- then if at a point you have devices which play all formats, you know which files to delete without extensive testing.


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mjb2006
post Jul 22 2012, 07:34
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@Deimuddi1955 - Encoding music to a lossy format will change the audio, no matter what settings you use. The m4a files are probably lossy already (if they contain AAC, not ALAC). The good news about lossy codecs is they are designed to fool you; probably you won't notice the audio is changed, unless you use some of the lowest settings. So as Porcus implied, it would be wasteful to use higher settings than the minimum needed to not notice. 320 kbps would be very wasteful. Probably you will not hear the difference even at 128 kbps. Try it and see.

Listening tests have shown that people generally stop noticing the difference between the original and the lossy version at bitrate n for MP3 and at bitrate n-minus-something for AAC. But you shouldn't infer from that "if I decode 256 kbps AAC and re-encode it as 320 kbps MP3, it will be the same quality." It's the same quality if you can't tell the difference. If you can't tell the difference between the AAC and the content transcoded to 128 kbps MP3, then it's highly likely the transcoded material at all MP3 bitrates from 128 to 320 is the same, maximum quality.

Also consider if the 256 kbps AAC has some content (or noise) that it's preserving in the 16+ KHz frequency band. This content/noise may be beyond what you can actually hear in actual music, so it's wasting space in the file. AAC also happens to be much better than MP3 at handling content in that uppermost band. The MP3 encoder at 320 kbps may try a bit too hard to keep that content without messing it up too badly, thus sucking away precious bits from the lower bands that you can hear. But at a lower bitrate, the MP3 encoder may ignore that high-frequency content (or noise) and might produce better quality for you in the more-important lower bands.

This post has been edited by mjb2006: Jul 22 2012, 07:36
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Porcus
post Jul 22 2012, 14:52
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QUOTE (mjb2006 @ Jul 22 2012, 08:34) *
Also consider if the 256 kbps AAC has some content (or noise) that it's preserving in the 16+ KHz frequency band. This content/noise may be beyond what you can actually hear in actual music, so it's wasting space in the file.


It is probably not wasting space, at least not anything to worry about. Well if it had been literally preserving the 16+ frequency band, then it would not have been spending its bits wisely, but that is not the case (imagine how a 64 kb/s AAC of a complex signal -- that's down to 5% of lossless -- would sound like if half an octave should be preserved litereally ... obviously, that is not what is going on).

If the top of the audible spectrum is audible in the sense that if you crop it off you change the (subjective) tonal balance – but yet it isn't much distinguishable (in actual music) from a mosquito, then you could just add a mosquito sound at “average level” to everything, substituting for the actual treble. That could be part of the codec itself, in which case it wouldn't cost any bits in the file.


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DevilMind
post Aug 23 2012, 10:44
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QUOTE (mjb2006 @ Jul 22 2012, 11:34) *
@Deimuddi1955 - Encoding music to a lossy format will change the audio, no matter what settings you use. The m4a files are probably lossy already (if they contain AAC, not ALAC). The good news about lossy codecs is they are designed to fool you; probably you won't notice the audio is changed, unless you use some of the lowest settings. So as Porcus implied, it would be wasteful to use higher settings than the minimum needed to not notice. 320 kbps would be very wasteful. Probably you will not hear the difference even at 128 kbps. Try it and see.

Listening tests have shown that people generally stop noticing the difference between the original and the lossy version at bitrate n for MP3 and at bitrate n-minus-something for AAC. But you shouldn't infer from that "if I decode 256 kbps AAC and re-encode it as 320 kbps MP3, it will be the same quality." It's the same quality if you can't tell the difference. If you can't tell the difference between the AAC and the content transcoded to 128 kbps MP3, then it's highly likely the transcoded material at all MP3 bitrates from 128 to 320 is the same, maximum quality.

Also consider if the 256 kbps AAC has some content (or noise) that it's preserving in the 16+ KHz frequency band. This content/noise may be beyond what you can actually hear in actual music, so it's wasting space in the file. AAC also happens to be much better than MP3 at handling content in that uppermost band. The MP3 encoder at 320 kbps may try a bit too hard to keep that content without messing it up too badly, thus sucking away precious bits from the lower bands that you can hear. But at a lower bitrate, the MP3 encoder may ignore that high-frequency content (or noise) and might produce better quality for you in the more-important lower bands.


well i want to convert my 320kbps mp3 (CBR & VBR V0 ) to 128kbps m4a Nero AAC-LC CBR to save disk space.

so will 2 pass in Nero AAC-LC will help keep the quality?

This post has been edited by DevilMind: Aug 23 2012, 10:46
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dhromed
post Aug 23 2012, 11:05
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Try, listen, and find out.
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extrabigmehdi
post Aug 23 2012, 12:09
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QUOTE (db1989 @ Jul 21 2012, 14:00) *
Hydrogenaudio has and will continue to have nothing to do with files “in the wild”, so what is your point?

I understand that the TOS is rejecting any piracy. However the somehow irrational lossless mania that many people share,
comes partly from these files "in the wild".
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extrabigmehdi
post Aug 23 2012, 12:14
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QUOTE (DevilMind @ Aug 23 2012, 09:44) *
well i want to convert my 320kbps mp3 (CBR & VBR V0 ) to 128kbps m4a Nero AAC-LC CBR to save disk space.

Will you hesitate as much to resize / crop a jpg image ? There is a implicitly transcoding from lossy to lossy too.
You need to test.
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Vietwoojagig
post Aug 23 2012, 13:07
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QUOTE (Deimuddi1955 @ Jul 21 2012, 14:39) *
Is a M4A file with 256kbps the equivalent to a MP3 File with 320kbps.
Some people say so.

QUOTE (Deimuddi1955 @ Jul 21 2012, 14:39) *
if it is so, can i convert the M4A File to a MP3 with a constant bitrate of 320 kbps? or should i convert it to the same bitrate of 256kbps??
Equivalence of AAC-256 to MP3-320 does not really help you to decide this question. The question is anyway, how much do you loose during the transcoding. With MP3-320 you likely will loose less than with MP3-256.
But I'm sure you will not be able to distinguish:
  • MP3-256 from WAV
  • MP3-256 from AAC-256
  • MP3-320 from WAV
  • MP3-320 from AAC-256

So I would propose to use MP3-256 or the settings you normally use to create MP3s
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