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Subject: RE: mp3-editor?
From: Chris Holt (amiga2k_at_cox.net)
Date: 2003-03-27


On Wed, 26 Mar 2003 12:20:52 -0500, Matthew P. OReilly wrote:
>That's true - I had forgotten about that. (It's been a while since I
did
>any compressed editing.)
>
>What is it about re-compression that causes quality loss? If something
>is
>compressed, uncompressed, then compressed again, shouldn't it be a
>function
>of the quality of the recompression algorithm whether or not it loses
any
>quality? If one is going from, say, 16-bit to 24-bit and back to 16-bit
>(uncompressed), all you're losing is the 8 least significant bits in the
>24->16 conversion, which would be all zeroes anyway from the 16->24 bit
>conversion. I know that compression adds more complexity to that mix,
>but I
>don't understand why it necessitates a loss of quality... if I compress
>the
>same uncompressed data twice, shouldn't it be the same as if I
>compressed,
>uncompressed, and compressed again?
>
>Thx
>Matt

There is no "scheme" commonly known as a recompression scheme, so the
quality of such a scheme is irrelevant.
MP3, like JPG is a lossy compression scheme. Where JPG tries to discard
file information that is not easy to see, MP3 tries to discard file
information that is not easy to hear. Each generation of compression is
compressing a file with discarded information that cannot be retrieved
and so quality is lost. Even worse with JPG is that file sizes can
actually increase over the first generation of compression because
complex artifacts introduced in the initial compression have to be
analyzed and compressed. Of course MP3 filesize (for CBR files anyway)
is dependant only on length and bitrate so this is not an issue.

Lossless compression schemes, like Monkey for audio or PNG/TIFF for
images, don't suffer from multiple generations of extraction and
recompression, so your example of 16-24-16 would work out with no loss.
(but I see no potential for gain in doing this either)

Chris



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