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General Question Re: Compression


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Hello all,

I'm trying to understand something about compression.

 

I know that CD tracks are pretty big files and when they get imported into iTunes (with AAC compression) the tracks get "shrunk" to pretty small file sizes.

 

From what I read, compression is achieved by eliminating "unnecessary" bits of an audio track. To my mind if certain portions of an audio track are eliminated they're gone forever .. right? However, when I create an audio CD using AAC tracks from iTunes, on the CD the tracks become full size files again.

 

So, my question is if compression was achieved by eliminating portions of the audio track how come when the same audio track is burnt onto a CD, it becomes a full size file again? Do the eliminated bits somehow come back to life in the CD burning process?

 

Isn't compression an irreversible action?

 

Is all of the above specific to AAC and CDA or is that how compression works in general - ie unnecessary bits are not really eliminated, they're just kinda hidden?

 

This brings me to another question - when an AAC track is played back - am I listening to the compressed version or the full version?

 

I've read quite a few articles on compression but they are all so technical and talk about the algorithms of compression (whatever the heck that means) but I just can't seem to find an article that ties together in layman terms compression, playback and "uncompression" if you will.

 

Any light shed on this will be greatly appreciated.

Cheers!

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Hello all,

I'm trying to understand something about compression.

 

I know that CD tracks are pretty big files and when they get imported into iTunes (with AAC compression) the tracks get "shrunk" to pretty small file sizes.

 

From what I read, compression is achieved by eliminating "unnecessary" bits of an audio track. To my mind if certain portions of an audio track are eliminated they're gone forever .. right?

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That's right.

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However, when I create an audio CD using AAC tracks from iTunes, on the CD the tracks become full size files again.

 

So, my question is if compression was achieved by eliminating portions of the audio track how come when the same audio track is burnt onto a CD, it becomes a full size file again?

----------

A compressed file can be expanded to _simulate_ the file before it was compressed--except for.....(read on). For example, an mp3 file can be "converted" to a much larger wav file. Note the quotation marks!

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Do the eliminated bits somehow come back to life in the CD burning process?

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NO, NO, NO, a thousand times NO!

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Isn't compression an irreversible action?

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YES, with the exception of a special type of compression called "lossless" compression. Lossless compression is analogous to .zip compression, which elminates dead space but does not remove any information from the file. Typically, lossless compression of a sound file leads to a file which is about one-half the size of the original file, where as a "lossy" (normal) compression might shrink the file down to one-tenth (or even greater) of its original size. Two common types of lossless compression are FLAC and APE. Although I have used FLAC (a free, open-source compression method) in the past, I am switching to APE (a free, privately developed encoding method), since APE has more flexibility and can compress a file in much less time than is required to compress a file using FLAC. To checkout APE, go to www.monkeysaudio.com. I am really impressed with this lossless codec, which is freely available for download with no strings attached--except you will not get the source code.

 

The major use for lossless compression is in archival storage, because the file can be restored to its original size (as a wav file) without any loss in quality. This pristine file can then be edited, etc., without the loss of quality that takes place when a file compressed in a lossy manner is edited (see below). This is a BIG advantage of shrinking a file for storage purposes using a lossless compression method. I use it regularly for archival storage of all of my music files, even though I store these files in mp3 form for _playing_ them in a media player.

 

Lossy compression removes information which can be removed without a terrible degradation in the sound quality. The higher the compression, however, the more the quality is degraded. Remember; once those "extraneous" bits are removed, they are gone forever, and converting the file back to a "pseudo-wav" file will not get them back. If you recompress this pseudo-wav file, you will just degrade it more.

 

Furthermore, any time that you edit a compressed file with an editor such as WavePad, the file is first decompressed to a pseudo-wav file, without telling you that this is what is happening. So you can edit as you see fit and then save the edited pieces in the same format as was the file before you loaded it into the editor, BUT these edited pieces will be of a more degraded sound quality because they actually have been compressed TWICE by the time that the leave the editor.

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Is all of the above specific to AAC and CDA or is that how compression works in general - ie unnecessary bits are not really eliminated, they're just kinda hidden?

----------

They are not "just kinda hidden". They are GONE. Sorry about that.

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This brings me to another question - when an AAC track is played back - am I listening to the compressed version or the full version?

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By "full" version, I assume that you are talking about what you get when you "convert" a lossy, compressed file back to the original (pseudo) file. Except that it is _not_ the original file, since you have lost essential information.

 

In other words, you are listening to an audio file in which certain "dispensable" pieces of information have been removed--unless the amount of compression used is so great that these removed pieces of information have become INdispensable (to _your_ ears) . Get the picture?

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I've read quite a few articles on compression but they are all so technical and talk about the algorithms of compression (whatever the heck that means)

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Compression (when talking about a sound file) means just what it says: squeezing down the file to make it smaller, whether or not the squeezing loses any information that needs to be recovered to maintain the sound quality (i.e., lossy versus lossless).

 

If your question is about the meaning of an algorithm, this is simply a series of steps used to produce some particular result--such as compressing a sound file, for example.

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but I just can't seem to find an article that ties together in layman terms compression, playback and "uncompression" if you will.

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No I won't :-) It's "decompression".

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Any light shed on this will be greatly appreciated.

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You've sure come to the light place!

 

 

 

 

Musikone

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Hi musikone

Thanks for your response. Continuing the discussion ....

 

 

----------

However, when I create an audio CD using AAC tracks from iTunes, on the CD the tracks become full size files again.

 

So, my question is if compression was achieved by eliminating portions of the audio track how come when the same audio track is burnt onto a CD, it becomes a full size file again?

----------

A compressed file can be expanded to _simulate_ the file before it was compressed--except for.....(read on). For example, an mp3 file can be "converted" to a much larger wav file. Note the quotation marks!

----------

 

 

 

capmorgan: hmmm. So what would someone gain by converting an mp3 file to a wav file? If the track has been compressed to mp3, which is a lossy compression, I don't quite see the reason someone would want to convert an mp3 to wav. The resulting file would only be larger and there would be no gain in quality.

 

Coming back to my iTunes example. I take the AAC tracks from iTunes and burn an audio CD - do you know what format the tracks on this CD are? I originally thought they would be CDA as in the original CDs. However after reading your earlier response, I gather this can't be right since I'm burning compressed tracks onto the CD. Are they just AAC format but altered so that they can play on regular CD players?

 

 

Now, about Wav files - is this format a lossy compression, non-lossy compression or orginal (non-compressed) quality?

Wondering what happens when I convert a CD track to a Wav file.

 

 

again, thanks a lot for your help.

cheers

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Hi musikone

Thanks for your response. Continuing the discussion ....

----------

However, when I create an audio CD using AAC tracks from iTunes, on the CD the tracks become full size files again.

 

So, my question is if compression was achieved by eliminating portions of the audio track how come when the same audio track is burnt onto a CD, it becomes a full size file again?

----------

A compressed file can be expanded to _simulate_ the file before it was compressed--except for.....(read on). For example, an mp3 file can be "converted" to a much larger wav file. Note the quotation marks!

----------

capmorgan: hmmm. So what would someone gain by converting an mp3 file to a wav file? If the track has been compressed to mp3, which is a lossy compression, I don't quite see the reason someone would want to convert an mp3 to wav. The resulting file would only be larger and there would be no gain in quality.

==========

Not all players can play mp3 files, although all players (as far as I know) can play .wav files. Hence the conversion of an mp3 file to a larger .wav file will allow that file to be played on one of these less flexible players.

==========

Coming back to my iTunes example. I take the AAC tracks from iTunes and burn an audio CD - do you know what format the tracks on this CD are?

==========

These are now .wav files. However, they do not contain all of the information which was present in the .aac file _before_ it was compressed. Some of that original information is irretrievably lost.

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I originally thought they would be CDA as in the original CDs.

==========

A .cda file does not contain _audio_ information. Rather, what this file does is "point to" the audio information. This information is retrieved from the CD by following the pointer which identifies it as a particular track and then processes it as a .wav file. This process is commonly referred to as "digital audio extraction," or simply as "extraction".

==========

However after reading your earlier response, I gather this can't be right since I'm burning compressed tracks onto the CD. Are they just AAC format but altered so that they can play on regular CD players?

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I can't talk knowledgeably about .aac files, since I have not used them. However, I do not understand your question. Perhaps my response about the nature of .cda file has answered it. If not, then try again....

==========

Now, about Wav files - is this format a lossy compression, non-lossy compression or orginal (non-compressed) quality?

==========

A .wav file is not compressed; it is the highest quality available. But always keep in mind that, once the file has been compressed and then employed in some fashion or another (such as burned to a CD or edited with an editor such as WavePad) and resurrected as another .wav file purporting to be the same file that existed before the compression, it is _not_ this same file, since some of that information was lost during the compression. The moral to this story is, if you want to preserve the high quality of a .wav file, always work with this file (such as in editing or burning to a CD) in .wav form and only compress it at the very conclusion of the processing. The worst thing that you can do is to compress, restore, compress, restore, etc., etc.; you keep losing more information every time that you compress.

==========

Wondering what happens when I convert a CD track to a Wav file.

==========

Nothing--if you are referring to any loss of audio information. You are just taking the information from the CD optical storage medium and processing it into a form that is useful for storing it in a magnetic storage medium (hard drive). But this process does _not_ involve a loss of audio information, comparable to the loss of information when an audio file is compressed, such as converting a .wav file into an .mp3 file.

==========

again, thanks a lot for your help.

cheers

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