(Not to be confused with transistor ladder or diode ladder) The circuitry installed in the keyboard of a basic analog monophonic synth, which produces the control voltage for controlling the frequency of a VCO. The ladder forms a simple voltage divider circuit for each key; when the key is pressed, a resistor inserted between a supply voltage and ground produces the desired control voltage. A resistor ladder keyboard generally also includes some mechanism for arbitrating note priority.
A sample and hold circuit is essential for a resistor ladder keyboard, because when no keys are pressed, the output control voltage will go to either the supply voltage or to zero. The sample and hold latches when no keys are pressed, and holds the control voltage of the last key that was pressed, or the last control voltage that was selected by the note priority mechanism. A synth patch will often have envelope generator release time settings that cause the note to fade out gradually when the performer lets the key up. If there were no sample and hold, the control voltage going to zero or the supply voltage would result in a large frequency jump during the release phase.