A ingesetztes frequently in signal processing approach is the sample and hold circuit, sample and hold circuit. They will allow it to scan a varying input voltage at any point in time (sample) and frozen on a signal quasi (hold) to evaluate them further. So one can digitize as a relatively slow speed AD converters and analog signals. Even in such synthesizers are used sample-hold
Here are a few links to S & H schematics

Here you will find Schematic Circuit Diagram for DIY and further information on Sample and Hold
Sponsored Ads:

Basic Sample and Hold	www.gymmelk.ac.at/the/phklass/klasse8/ad_wandler/ad-wandler.htm
Sample and Hold with PCB Layout	www.musicfromouterspace.com/analogsynth/singlechipsampleandhold/singlechipsampleandhold.html