Signal Processing was that subject I found difficult and I found no reason to be interested in audio filtering, so I didn't find an incentive to work through that difficulty. Then I see applications of signal processing, like this, that goes beyond simple audio filtering, that makes me want to learn it again. Honestly, this looks like wizardry! It is no surprise, MIT is on the bleeding edge of signal processing and it mainly is due to Dr. Oppenheim . He wrote the textbook on DSP and runs the DSP Group at MIT.