2810: An ARP Odyssey part 2

Like the Moog examples, the ARP Odyssey keyboard current will go through the keyboard resistor string. However, if two notes are held down at the same time the current will bypass the resistors between the notes since they are shorted out by the buss.

Up to this point we've been concerned about the buss voltage, which is responsible for pitch CV. But what about the voltage supplied by the current source to the high end of the resistor string? Let's figure out what happens to that using the Odyssey as an example. When one note (or none) is held down the high end of the keyboard voltage is 3-I*Rall where I is the current and Rall is the resistance of all resistors in the chain. For the Odyssey it's 3-.00083*36*100 = 0 V. But if two keys are held down the resistors between them are shorted out. So the voltage is 3-I*(Rall-Rbetween). But since we know 3-I*Rall = 0, this simplifies to I*Rbetween. This is how the Odyssey gets it's duophonic capability.

This voltage I*Rbetween is the CV difference between the two notes. Remember a normal CV is still generated by the keyboard voltage buss and corresponding S+H circuit. At oscillator 2 this voltage can just be added to the normal pitch CV, giving a CV corresponding to the high note.