Hello, I am planning on building a simple 5 octave hybrid analogue synthesiser, which is controlled via MIDI. My question is in regards to the exponential conversion required to control the VCO. my original plan was to use a microcontroller to DAC to then a pair of matching transistors to convert a linear voltage to expo current. However, would it be possible to avoid using a exponential converter and instead include this in the microcontroller? Would it be capable of accurately supplying accurate exponential currents capable of 5 octaves. I know that the octave range would be limited by the voltage by some microcontrollers use 5v supply. If so, could anyone share their opinion on how this could be achieved, in terms of code and circuitry. Thanks : )

 

I should think achieving 5 octaves would be an almost trivial task; that's only 5 steps of 2:1 ratio, analogous to a 5-bit DAC. Combining a microcontroller with a 12-bit multiplying DAC like the LTC 8043, which has a current output, would give you ample resolution. You could either feed the output current into a current-to-frequency converter circuit, or use the DAC the "normal" way to generate a voltage (see the circuits in the "Typical Applications" section of the data sheet), and then use that voltage to drive a voltage-to-frequency converter circuit.