Scratching .. with a mouse?
What the hell?
I was playing around with scratching wave files on the computer via "SoundCraft" (the beer in the image ought to help in answering the obvious question, that being why I was doing this..) when one of my roommates suggested the obvious - why not put the mouse on the turntable, and scratch it that way?
The result is shown in this video clip. By holding the optical mouse roughly perpendicular to the turntable's slipmat, the mouse would detect movement by the nature of the turntable spinning. Because of the angle, it would move horizontally on the computer - giving the exact motion needed for the scratch effect in SoundCraft.
Explanation
SoundCraft is a nifty "vinyl scratching emulation engine". It plays a wave file called "sound.wav" when started, the interesting part being when you click and drag on the program window. You can move left-to-right to simulate scratching the sound from the point when you clicked. When you let go of the button, the sound continues playing. Quite a cool effect, and according to the author this isn't exactly rocket science to program if your math is up to snuff.
The same effect can be achieved by taking apart a serial mouse with a ball in it, (serial is reported to perform better than PS/2 for this application), and using the X-axis wheel to scratch. You can tie down the left mouse button contacts (so it's always "down") after getting SoundCraft running, and then go to town with the audio.
With the serial example, you can also rest it on the turntable at the right angle so the X-axis wheel will come in contact with the slipmat; then you'll get the same effect by spinning the slipmat, and it's practically guaranteed to be more accurate than holding the optical sensor over top. Speed might be an issue though, I had to offset the pitch on the turntable to compensate for the difference in speed.
The speed of the sound playback depends on how fast the mouse is moving; when being used on the turntable, the faster surface is of course the outside area (high-school trig, ie. circumference, kicking in yet?) Moving the mouse toward the center will conversely slow down the playback.