  We introduce Composer's Assistant 2, a system for interactive human-computer
composition in the REAPER digital audio workstation. Our work upgrades the
Composer's Assistant system (which performs multi-track infilling of symbolic
music at the track-measure level) with a wide range of new controls to give
users fine-grained control over the system's outputs. Controls introduced in
this work include two types of rhythmic conditioning controls, horizontal and
vertical note onset density controls, several types of pitch controls, and a
rhythmic interest control. We train a T5-like transformer model to implement
these controls and to serve as the backbone of our system. With these controls,
we achieve a dramatic improvement in objective metrics over the original
system. We also study how well our model understands the meaning of our
controls, and we conduct a listening study that does not find a significant
difference between real music and music composed in a co-creative fashion with
our system. We release our complete system, consisting of source code,
pretrained models, and REAPER scripts.
