I studied analog and digital communications in college, and I've done a little research on USB and MIDI I/O. I wrote up a blog post (since it is too long for a forum) that explains the differences between MIDI 5-Pin and MIDI USB, as well as 7-bit MIDI from 14-bit MIDI.
This should answer most of your questions about what happens between hitting a button on your USB Midi Controller and Traktor receiving a MIDI message. Obviously the mapper maps the message to the software function.
I wrote this up to hopefully help someone who is converting to the digital world and/or for someone that just wants to know what's happening with his or her devices. Feel free to comment in the forum or in the comments of the blog. I may have left some things out. See the following URL:
http://djvandraken.blogspot.com/2010...nd-14-bit.html
Edit: This blog post does not get into the nitty gritty of tech-details, and it leaves out some crucial information on processing, A/D conversion, software, and hardware. It should still help you understand the basic differences though.
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