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Subject: Re: Trimming an MP3 file
From: spidermagnet (rokbokser_at_imtarget.cotse.net)
Date: 2003-06-21


Johan,

Well, _I_ was interested, and curious, and read the link (at least the 1st
3 questions that chris suggested) which explained the issue.

Spider's Interpretations of http://lame.sourceforge.net/tech-FAQ.txt
--------------------------------------------------------------------

In some modes, mp3 encoders "save for a rainy day". Meaning, if the sound
being encoded right now is non-complex, and doesn't need all of the
available bitrate, it can be encoded "early". That way, there is extra
bitrate available in upcoming frames to store data if the next chunk of
sound is expecially complex. (the space they save data early in is the
"bit reservoir")

Picture a railway train where all of the seats adjust on a continuous
slider throughout the railcar. Everyone takes their assigned seat, and
adjusts it according to how tall they are. If there are a group of
childern at the front of the car, they adjust all of their seats close
together, and there is more room for taller people further back in the
car. The same people are in the same train car, but the "quality" of the
ride is improved for the taller people, because they have legroom. If you
chopped the car straight across between the assigned rows, you might
injure somebody, because the person whose ticket says "row 11" may be
closer to row 9.5.

Example: At the beginning of my song is silence (childern), which takes no
bitrate to encode. The encoder will store the first part of the music
coming up (taller person) in the mp3 file _before_ the music starts,
because the silence isn't using any bitrate.

_This means that, _particularly_ when you are coming out of silence, the
beginning of the sound is likely to be stored one or more frames early.

why it causes the problem
--------------------------

So, if you cut off a file at a particular spot, some of the data required
to play that bit might have been stored in the file in a previous frames,
making it impossible to start in a particular spot.

Possible solution
-----------------

Encoders also add space at the beginning and end of files, so your best
solution is probably to edit the files as PCM wav's, into the form you
want them, _before_ ripping them to mp3.

Editorial comment
-----------------

Might I add that your response basically said "I don't want to understand
the problem, I want somebody else to figure out how and tell me". Perhaps
you meant "It's too technical for me, would someone please help me to
understand what the issue is"?

spider

Johan Vromans said:
> Chris Holt <amiga2k_at_cox.net> writes:
>
>> http://lame.sourceforge.net/tech-FAQ.txt
>> If I could add this excellent FAQ to your answer, I think Johan
>> might find it very enlightening.
>
> I only find it confusing.
> Actually, I'm not really interested in the deep internal details of MP3
> {de,en}coders. I just want to know if (and how) to produce/use MP3 files
> from CDs that have contiguous music on them.
>
> -- Johan



Page was last modified "Jan 10 2012" The Rockbox Crew
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