HELP! Is a converted AAC by iTune a real lossless audio file?

Question

Level 1

0 points

HELP! Is a converted AAC by iTune a real lossless audio file?

Veryone here has a iPod, and like its good sound playback,because it has a function to create lossless audio file by iTune. So do I.

I try to use iTune backup my own audio CDs to AAC files, use iPod & iTune to play it. But recent days, after these AAC files be converted back to wave file, I try to compare it with original wave files, I found these AAC file recoverted wave files are totally different from the original ones. But APE or FLAC files converting result don't have such a problem. What happened to AAC lossless files? Is AAC files are real lossless files?

I also try this by NeroAacEnc, by lossless switch on. Unfortunately same result on, file convert back to wave is totally different from the original one too.

Comparing software is EAC and WinHex. Result seems that bitrate totally different, from begin to end of whole file.

Is anybody here found this problem here? Could you help me to find some special optinal swtich which I don't know to convert CD wave files real lossless (no lossing) into AAC file? Thanks a lot.

The problem is that, some discs I backup at before already lost (maybe borrowed by someone but I don't remember who). That means if I cannot found these lost CDs, I lost them forever, and I cannot recover it back from the backup aac files. Because it has different quality from the original CD.

Posted on Aug 31, 2007 6:35 PM

Reply

Answer 1

StarDeb55

Level 6

19,695 points

Aug 31, 2007 6:50 PM in response to La Mer

AAC is a Lossy format, not Lossless. If you are looking to import into iTunes in a Lossless format, you might want to try Apple Lossless which will give you the equivalent quality of a WAV file but with about a 30% decrease in file size.

Reply

Answer 2

La Mer Author

Level 1

0 points

Sep 4, 2007 7:28 AM in response to StarDeb55

Thank you! StarDeb55. I'll try it under your suggestion.

If Apple Lossless format only decreases 30% of file size, there's not so much difference to APE or FLAC.

Reply

Answer 3

varjak paw

Level 10

182,668 points

Sep 4, 2007 7:52 AM in response to La Mer

If Apple Lossless format only decreases 30% of file size, there's not so much difference to APE or FLAC.

There's a limit to how much any lossless compression can save on file size, and it's going to be nowhere near what a lossy compression format like AAC or MP3 can achieve. FLAC, Apple Lossless and others all have about the same file-size savings; it's a matter of which format you find most useful and, of course, which sounds best to you (different schemes have different "flavors" to many people's ears). Apple Lossless can in some cases save up to 50%, but FLAC probably could as well.

Reply

Answer 4

Sep 4, 2007 6:04 PM in response to varjak paw

Dave Sawyer wrote:
...(different schemes have different "flavors" to many people's ears).

That doesn't make sense. If "flavoring" the sound is the result of the file being compressed using any of of the various lossless formats, then (a) they technically aren't lossless or (b) the decoder is decoding the file incorrectly on playback.

Reply

Answer 5

StarDeb55

Level 6

19,695 points

Sep 4, 2007 6:08 PM in response to thelastvoice

Dave will probably address this in the morning. In the mean time, I'm pretty sure that Dave means that how any particular decoding scheme sounds whether it's Lossless or Lossy is going to be a subjective decision based on the person's ears who are using the file. All of us who are regular posters in these forums know that each decoding scheme, whether it is Lossless or Lossy, has to follow specific set parameters when reading the file. The person who is going to use that file is the final determining factor.

PS- Dave, sorry if I put words in your mouth, but I hope I'm pretty close to what you will say in the morning.

Reply

Answer 6

Sep 4, 2007 11:46 PM in response to StarDeb55

StarDeb55 wrote:
...how any particular decoding scheme sounds whether it's Lossless or Lossy is going to be a subjective decision based on the person's ears who are using the file.

Subjectivity in regard to sound quality shouldn't really come in to play when discussing lossless formats. The whole point is that they are lossless...the decoded file is supposed to be an exact reproduction of the original. If the same audio file is compressed with various lossless codecs and differences in sound are apparent upon playback (ie. decoding), +assuming all other parameters in the decoding chain are equal+, then (a) one (or all) of the codecs are not actually compressing losslessly, (b) one (or all) of the decoders are decompressing the file incorrectly or (c) the listener is suffering from placebo effect.

All of us who are regular posters in these forums know that each decoding scheme, whether it is Lossless or Lossy, has to follow specific set parameters when reading the file.

Of course, and for every lossless codec that specific set of parameters is that the decoded file must be an exact reproduction of the original. If the decompressed file sounds different than the original file and that difference is due solely to the compression/decompression scheme being used, then logically it was not lossless.

Reply

Answer 7

varjak paw

Level 10

182,668 points

Sep 5, 2007 7:21 AM in response to thelastvoice

The whole point is that they are lossless...the decoded file is supposed to be an exact reproduction of the original. If the same audio file is compressed with various lossless codecs and differences in sound are apparent upon playback (ie. decoding), assuming all other parameters in the decoding chain are equal, then (a) one (or all) of the codecs are not actually compressing losslessly, (b) one (or all) of the decoders are decompressing the file incorrectly or (c) the listener is suffering from placebo effect.

That is the theory, yes. But theory and practice often do not equal. Even a lossless codec may, due to technical limitations or flat-out programming errors, induce unintended coloration or other errors. I have yet to see any extensive, reliable tests on the various lossless codecs to show the accuracy, though I did see one test of the Apple Lossless codec that claimed that it induced no errors from the source. I do not remember the test parameters, though, so I don't know how the test was set up and how accurate and extensive it would have been. I'm sure there are tests for the other codecs (FLAC, et al) but I haven't seen them. Regardless, some people claim that they can hear coloration differences between source and compressed material with lossless codecs. That may be true or it may be illusory; only extensive blind testing and thorough examination with accurate noise and spectrum analyzers could tell for sure.

So I suggest you do your own tests and sampling and choose according to what seems to you to be best for your needs.

Reply

Answer 8

Sep 5, 2007 2:14 PM in response to varjak paw

Not to belabor the point, but:

Dave Sawyer wrote:
Even a lossless codec may, due to technical limitations or flat-out programming errors, induce unintended coloration or other errors.

Of course all of those things are all possibilities, but then the codec wouldn't be performing lossless compression.

Reply

Answer 9

pgp566

Level 1

9 points

Sep 5, 2007 2:48 PM in response to varjak paw

That is the theory, yes. But theory and practice often do not equal. Even a lossless codec may, due to technical limitations or flat-out programming errors, induce unintended coloration or other errors.

But if the codec is genuinely lossless there will be no effect on quality.

Note to original poster: both AAC and MP3 are lossy compression formats. In the compression process information is thrown away for good; it can never be recovered. Never try to convert an AAC to Wave, modify it, then convert it back to AAC. The compression artefacts will be compounded. Always save master originals in Wave and do all processing in that format alone. You can convert to AAC at any time, but don't throw away the Wave file until you are happy that all editing and processing is complete.. preferably never throw away a Wave file, but they do take up a lot of space.

Philip
--

Reply

Answer 10

varjak paw

Level 10

182,668 points

Sep 5, 2007 3:37 PM in response to thelastvoice

"Lossless" means that no data is thrown away by the compression process as happens with "lossy" codecs such as AAC or MP3. It doesn't mean that the data won't be distorted. But we're getting into semantics. The point again is that in absence of good scientific data as to the quality of the various "lossless" codecs, you should make your own evaluation.

'Nuff said.

Reply