http://en.wikipedia.org/wiki/Final_Scratch
it can't be that hard. left and right are in quadrature, making clock recovery and direction easier, after that it's a freq detector on the output and using msp patches people have already done to make the scratch sound (I think). damn, this might blow my summer.Vinyl/CD time code
The most complex piece of the Final Scratch setup is the code pressed onto the vinyl. A 1200 Hertz amplitude modulated sine wave is pressed into the left and right channels with a phase difference of 90 degrees. Each channel holds one of the two bit streams required for the time code. In one cycle of either wave form, two bits are stored: one on the positive voltage peak and one on the negative voltage valley. The relative amplitudes of these peaks represent either a binary one or zero. A relatively high amplitude on either peak represents a one, a relatively low amplitude represents a zero. In each channel is a separate bitstream, the left channel is not identical to the right (disregarding the phase difference).
Finding position
The time codes themselves consist of 40 individual bits, or 20 cycles on each channel's waveform. On the right channel the bit sequence of 0, 0, 0, 1 represents the start sequence for a single time code. Those four bits along with the four corresponding bits on the left channel and the next 16 bits on each channel can be decoded as an integer position value which represents where the needle is on the record.
Finding speed
The speed at which the record is spinning can be found by comparing the frequency of the waveform being read from the record to the true frequency of the wave form on the record at normal speed. This difference represents the change from the normal speed at which the record turns.
Finding direction
The direction which the record is spinning at any given time can be found using the phase difference between the waves on the two channels. This procedure is the same as that used to determine the direction in which a ball mouse is moving.
