Jesse Kriss and DJ Axel Foley have put together an interesting collection of Visual Scratch videos:
We capture turntable velocity using Ms. Pinky and fader position using an Ecler Nuo 4. The scratching sound is generated in Max/MSP. Then we scale the control data (turntable and fader) to ±1.0, the range of digital audio signals. The control data is saved to one audio file, and scratch audio to another.
With the two sound files as input, the graphics are generated in Processing, using the incredibly helpful Ess, PDF and MovieMaker libraries. The y-axis position reflects the velocity of the turntable, color is based on the frequency content of the scratch audio, and the thickness of the line is based on the volume.
It would be interesting to see this type of work combined with generative video visualizations.