Melody Processors (sometimes also called Melody Preprocessors) take an input melody (such as the output of a Piano Roll Node), apply a transformation to the melody itself, and then output the modified melody.
The output melody is then usually connected to a synthesizer Node (such as a Melodic Oscillator Node), or to another melody processor Node for further processing.
Notable use-cases include converting single notes to chords (using the Component Extension Node), and adding automatic pauses and repeats to longer notes (using the Repeater Node). Custom arpeggiators can also be built. These Nodes are different from audio effect Nodes: while those Nodes process an actual audible audio signal (or sometimes, a control signal), melody processors work on the melody before any audio signal is created.
Melody processors (like all other Nodes on AudioNodes) don’t change their input sources directly. Instead, they create copies and apply transformations there (for example, they won’t change what you have in your Piano Roll Node, but instead copy your Piano Roll Node melody and transform the copy under the hood). This lets you wire up a single melody source to multiple melody processors at the same time.