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    <p>I got three quality answers, and I thank you all (and upvoted you all) for them. I haven't chosen any as the accepted answer, because each addressed one aspect, so I wanted to write a summary.</p> <p><strong>Do you need to work in MP3?</strong></p> <ul> <li><p>Transcoding to PCM and back to MP3 is unlikely to result in a drop in quality. </p></li> <li><p>Don't optimise audio-quality prematurely; test it with a simple prototype and listen to it.</p></li> </ul> <p><strong>Working in MP3</strong></p> <ul> <li><p>Wikipedia has a summary of the <a href="http://en.wikipedia.org/wiki/MP3" rel="nofollow noreferrer">MP3 File Format</a>.</p></li> <li><p>MP3 frames are short (1152 samples, or just a few milliseconds) allowing for moderate precision at that level.</p></li> <li><p>However, <a href="http://en.wikipedia.org/wiki/MP3" rel="nofollow noreferrer">Wikipedia</a> warns that "Frames are not independent items ("byte reservoir") and therefore cannot be extracted on arbitrary frame boundaries."</p></li> <li><p>Existing libraries are unlikely to be of assistance, if I really want to avoid decoding.</p></li> </ul> <p><strong>Working in PCM</strong></p> <p>There are several libraries at this level:</p> <ul> <li><a href="http://pymedia.org/" rel="nofollow noreferrer">PyMedia</a></li> <li><a href="http://lame.sourceforge.net/" rel="nofollow noreferrer">LAME</a></li> <li><a href="http://spacepants.org/src/pymad/" rel="nofollow noreferrer">PyMad</a> (Linux only? Decoder only?)</li> </ul> <p><strong>Working at a higher level</strong></p> <ul> <li><p><a href="https://echonest.github.io/remix/" rel="nofollow noreferrer">Echo Nest Remix API</a> (Mac or Linux only, at the moment) is an API to a web-service that supports quite sophisticated operations (e.g. finding the locations of music beats and tempo, etc.)</p></li> <li><p><a href="http://mpesch3.de1.cc/mp3dc.html#dwn" rel="nofollow noreferrer">mp3DirectCut</a> (Windows only) is a GUI that apparently performs the operations I want, but as an app. It is not open-source. (I tried to run it, got an Access Denied installer error, and didn't follow up. A GUI isn't suitably for me, as I want to repeatedly run these operations on a changing library of files.) </p></li> </ul> <p>My plan is now to start out in PyMedia, using PCM. Thank you all for your assistance.</p>
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