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Bit-perfect AAC/MP3/etc decoding?, Is it an issue?
mavere
post Jan 5 2013, 00:43
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Sorry if this is easily answered elsewhere.

My question is that in a comparison of lossy-format decoders between, let's say, iTunes and Foobar and Winamp, will the output (edit: of a single source file) vary between decoding implementations?

Or is it that once a decoder follows a specified format, all output is equal assuming there are no bugs?

This post has been edited by mavere: Jan 5 2013, 00:51
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DVDdoug
post Jan 5 2013, 01:00
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Unlike encoding, I believe all decoders are supposed to be the same.

The decoder has a lot simpler task of decoding & reconstructing a wavefrom from whatever data is present. The encoder has to decide what data to throw-away.

This post has been edited by DVDdoug: Jan 5 2013, 01:06
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Arnold B. Kruege...
post Jan 7 2013, 15:45
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QUOTE (DVDdoug @ Jan 4 2013, 19:00) *
Unlike encoding, I believe all decoders are supposed to be the same.


The operation of decoders is generally well-known and well-defined.

Evens so, bit perfect can be tough because different hardware implements arithmetic with subtle differences and at the LSB level insignificant variations in things like order of operations can lead to tiny differences.
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saratoga
post Jan 7 2013, 16:49
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QUOTE (Arnold B. Krueger @ Jan 7 2013, 09:45) *
Evens so, bit perfect can be tough because different hardware implements arithmetic with subtle differences and at the LSB level insignificant variations in things like order of operations can lead to tiny differences.


For mp3 its even more complex because the specification provides filter constants to specified precisions, but does not define the precision of the actual filterbank operations nor their precise implementation. Thus there is no "exact" value to compare to, only a range of values you should not exceed.

Of course as Arnold points out, these differences are incredibly small and completely irrelevant to audio quality when implemented on modern 32 or 64 bit machines. They mostly only matter when using 16 bit DSP cores.
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