The final component of the synthesizer is finished! The exponential current source was the last component which needed to be designed and constructed. Intrinsically it’s no crucial component, in fact the synthesizer would perfectly work without it.
What does it do ?
To understand the use of the exponential current source in the global design, some knowlegde on the human ear is needed. To start, the human does not work in a linear way. Here is an example:
If a certain note is 100 Hz, the note 1 octave up will be 200 Hz. If our ears were linear, then the next few octaves would be 300 Hz, 400 Hz, etc. However, frequency doubles with each octave. Octaves are 100 Hz, 200 Hz, 400 Hz, 800 Hz, etc. This clearly is not linear.
To create an intuitive interface for the frequency control of the synth, a linear shift at a sensor should create an exponential shift in frequency at the oscillators. The distance sensor we connect to the frequency control already has an exponential response by itself. The reason we still add an exponential source to the design is to encourage creative users to experiment with other interfaces, who very often will give linear responses.
The design is based on the following circuit: